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LABORATORY MNNuAL OF VERTEBRATE ZOOLOGY LABORATORY MANUAL OF VERTEBRATE . ZOOLOGY (For B. Sc. Students) BY S. N. Lecturer 1n PRASAD, M. Sc., D. PHIL. Zoology, University of Allahabad. AND P. V. RAJAMANNAR, M. Sc. Zoology Department, qniversi~y of Dellji. With a foreword by D. R. BHATTACHARYA, PH. D., D. Sc .• F. N. I. Vice-Chancellor, University of Allahabad. Formerly Professor of Zoology ALLAHABAD. UNIVERSAL BOOK COMPANY 20, MAI{ATMA GANDHI MARG. 1951 Publislid by Sri Asanand for the rJ.lliversal Book Co. Mahatma Gandhi Marg, Allahabad Priltted by D. N. Bhargava at the l'zrthraj Press, 93, Chak, Allahalad • FOREWORD. The method of teaching comparative .ariatomy to students preparing for the B~chelor's degree has much to recommend it. 'fhe student in the practical class, however, &uffers from a great disadvantage. The theory work cannot be co-ordinated with the -practical. For the Indian student the .difficulty is more pronounced as there is no practical manual describing all the Indian types of animals for laboratory work. In the present book the authors ha ve tried to overcome this difficulty. They havenot only described all the Indian types, but. _have covered the entire requirements of the syllabus for the degree. Now that our Universities are introducing Hindi as a medium of instruction, the description of .the Indian types will be very help~ul in preparing text books in Hindi. D. R. Senate House, University of Allahabad August 1951. BHATTACHARYA. PREFACE Practical courses in vertebrate anatomy vary in scope and choice of material used. There are admirable books of proved merits, 'dealing with, the laboratory requirements,; but they are not' satisfactory as help books for an Iridian student. None J-nCQrpofates aU the vertebrate typ'?s in one volume, ,and,. aborve all, none describ61$ the Indian types tl}at are ,used' in our'laboraI'r·, ' " , ' ". ., ,r tories.' In preparing the· presel~t guide book 'two' things hit ve a weighty influence-firstly, the requirement.~ of an Indian student prep~rillg' for ,the,', degree' of Ba,chelor 'Of Science, and 'secon(Uy 'the"lnccirporation of Indian types used in our laboratories: ,The primary purpose of the· present volume is to pr.Jvide.'a manual which o)1r students may rise~ to advantage and' with· the help of'which they may carryon work without supervision.' ,,' < All the dissections described in the book were carried out in the laboratory p~rs9n~l\y by th~ a\lthors. Special efforts were made to study ,and note down tne normal, difficulties that the stu:le,nts aftf.n faoe, during the progress of their work, and attempts have been made to solve them. ' As most of the animals used in Otlr laboratories are undescribed often interesting and new features have been reported. For instance, one student pointed out; the prtlsence of a prominent branch of the carotid artery arising just opposite to the origin of the vertebral artery in the pigeon. This blood vess,cl is lllore prominent than the "ertebml and the oesophageal, arising from the same place, natUIally the curiosity of a stude:lt could not be PREFACE viii satisfied. The presence of the bloG<! vessel was checkf'd in a large number of specimens during class work; and quite a number were dissected by us. The same has been nampd the cl)taneous. Lil<ewise the presence of an eitra division of the coraco-brachialj~ brevis muscle in pigeons ~pecially kept for flight, a.nd the preSem(l of b )th the external jugular veins in the wall-li:~ard etr, have been noted. It was pointed out to us by some of our senior colleagues that the nomenclature used while describing the arteries of the head of tbe dog. fish is a bit confusing. We agree with thtl above criticism, but, a.s we arc not in position to check the same and changf'. we have used the nomenclatur€'. as adopted by E. M. ThillaYl:lmpalam in hf:>r Memoir on ScollOdon, edited Itnd revised by Professor .[{. N. Bahl, of Lucknow Unh-el ~ity. a It is a pleasure to ackno,vlil'dge the cordial cooperation of a nnmber of senior members of the depltrtment in discusaing the qis.sections and finally in t,he preliminary preparation of the .manllscript. W £' art< indebted t.o Prof. D. R. Bhattacharya, Ph. D, D. Sc. for writing the foreword. 'Department of Zoology Uni'versity of Allahabarl August, 1!J51. S. N. Prasad. CONTENTS. Page ,. vii Preface. Introdu ction. Notes on Dissections-Notes on Drawing CHAPTER I The Dog-fish. External-'lharacters-The Coelom and viscera-Dissection of the Digestive system-Dissection of Respiratory system-Dissection of Circulatory system-Dissection of Renal and Reproductive systems-Dissection of Sense organs-Dissection of the Cranial nerves-The Skeletal system CHAPTER 1 II The Frog Dissection of the Cranial nerve~-The Brain CH.HTER 43 III The Lizard (Varanus). External features-Dissection of the Coelom and viscera-Dissectiori of the Digestive system -Dissection of the Respiratory organs-Dissection of the Oirculatory system-The Renal and Reproductiv{' system-The SkeHal system 49 CONTENTS :x IV CHAPTER The Pigeon. External characters-Dissection of the Muscles of flight-Dissection of the Coelom-Dissection of the Digestive organs--Disspction of the Circulatory system-Renal and Reproductive systems-DiE!section of the Brain-The Skeleton of the fowl. CHAPTER Page 81 V The Rat. External features-Dissection of the Abdosystem--The minal viscera-The Digestive Thoracio viscera-Dissection of the Circulatory system-Dissection of the Heart-The Urinogenital system-Dissection of the Neck--'I.'he Brain-The Skeletal system CHAPTER 121 VI Histology. Whole Mounts~Permanent preparations-Fixation-Staining etc.-Preparation of tissues. CHAPTE:R 170 VII. Histology tContd). Examination of tissues-The Skin-Tooth -Stomach-lntestine- LiverPancreasKS,.ney-Testis- Ovary. 184 CONTENTS CHAPTER Xl VIII. Page The Development of the Chick. t The Egg-Fertilisation-Cleavage-Gastrutation Primitive streak-NotochordaL Process- . Head fold-Coelom formation-Gut formationHeart forMation , ( 195 CHAPTER ! . IX. Main Groups of Vertebrates. - Class Pisces-Class Amphibia-Class Class Aves-Olass MarpmEJ.lia. Reptilia~ 209 Appendix. Protochordata-Balanoglossus,' Ascid_ian ana Amphioxus " 240 LIST OF ILLUSTRA.TIONS. Page (l) The Efferent blood vessels and the arteries of the head of the. dog-fish .. (2) The cranial nerves of the dog-fish the dog-fish and its visceral skeleton 15 29 (3) Lateral view of the skull of 35 (4) The fifth; sAventh. ninth and tenth cranial nerves of the frog 44 anterior view of atlas, B, lateral view of axis, 0, lateral view of .a typical cervical and D, posterior view of a caudal vertebra. of the va.ranUB 67 (6) Lateral view of the sacral vertebra of the varanus 68 (7) The sternum of the varanus 69 (8) A, the dorsal view and B, the ventral view of the skull of the vara.nus .. 71 (5) A, (9) One ramus of the lower jaw of the 75 varanus (10) The circulatory system of the pigeon .. 93 (11) The circulatory system of the rat 132 (12) The male the ra.t 142 urinogenital organs of LIST OF ILLUSTRATIONS Xl11 Page (13) The urinogenital organs of the fem::lle rat. 144 (14) Diagram showing t,he arrangement of bones of a mammalian skull (15~ 154 Sagittal section of t.he skull of the dog 155 (161 The side vie"w of the skull of the d~ (17) The egg of a hen the shell removed. l~ with half of 195 (18) Median section of c hick blastoderm segmentation cavity. 197 (19) Vertical section of blastoderm show. ing formation of the endoderm 198 (20) Surface view of the blastoderm showing the primitive streak and the formation of the notochordal process ~21) Transverse section of the embryo showing the migration of mesoderm. . (22) Surface view of 10·somite embryo 199 200 " 202 (23) Longitudinal section of a young embryo showing the head fold 203 :24) Surface view of the embryo showing head fold and formation of two somites 204 LIST OE ILLUSTRATIONS XIV Page (251 Transverse section of the lO·somite embryo showing the forma.tion of splanchnocoel. 205 (26) Transverse section through a seven· somite embryo showing the formation of foregut 206 (27) Transverse section of the embryo showing the formation of the heart .. 207 INTRODUCTION. Success in practical work depends upon siilCere approach of the studoot. Presence of mind and common sense add considerably to it. To perform any practical well the student must, in some sense, get inside its details and know what he has 'to do before starting. A good student knows the tracks along which to proceed, and he is constantly on the watch for the mistakes, which he often tries to delude. The main spirit of his work is to help himself to his best advantage and capacity. The following pages contain all that is needed to guide such a student-all the laboratory methods required by the syllabus, along with notes on the important features of the development of the chick and a brief summary of animal classification. Figures in such books do more harm than good because of the student temptation to copy them. Therefore, only a few illustrations have been given, where indispensable, and those too are diagrammatic. On Dissection, Dissection means to cut open an animal to ascertain tbe structure of its parts. For the purpose of the study of gross anatomy it is necessary to spoarato the structures from each other so 9S to give prominence to their outlines and relationships. This is done by cutting away the connectJive tissue binding the different parts. For a good dissection a number of precautions are necessary. (1) The most important of them all is the possession of a good dissecting set. Some of our present day students come without sets and att£lmpt to do their work by borrowing instruments from their neighbours. This is a uicidal habit which must be avoided at any cost. INTRODUCTION XVI (2) So far as possible dissect the animals under water in wax-trays. But large animals like the dog-fish and the rabbit can be fixed to large wooden boards for dissectiqn. Keep the dissec~ion moist. (3) Follow the instructions carefully, make a mental picture of what you have to do before starting and proceed with the dissection carefnlly and slowly. Do not wait for the teachers to ~'p'ush" you through different stages, or do not get impatient to see every thing Soon. Haste alway!!. spoils ·dissections. (4) Do not be afraid of breaking any thing Breaking is bad but not so bad that it may stop your progress completely . On Drawing. Skt'tching is neither art nor scienc"'. it is l'll exercise in (lommon-sense. It is something all students can- do well if only they spend some time thinking how best to catch all the characteristic point'> in one simple sketch. The mistake with the majority is that the figures are drawn just to satisfy them at the moment-later on the same fig'llre may appear meaningless even to the authors. The important point is that a good sketch should explain its contents' even to a layman. The object of the student should be to devise figures which represent his main results at a glance, yet so far as possible diagrammatic and semidiagrammatic figures should be avoided. (1) The drawing should always he a faithful representation of the object or dissection, as such the object must be before the student. (2, The drawing should be large enougl• .to show all the parts in actual proportion. Proportion is more importanl tha1J minute details. XVll I;\TRODUCTro~ t3) The lines must b~ clear and uniform. A broken or confused line manifests confused bont of mind. (4) Shading of any type shonId be avoidei so far as possible. When extremely necessar.v pencils of different hardness should be used but never different colours. (5) The figures must be labelled just when drawn. (6) The guide lines should not be scrawled across hurriedly and without order. If so done they become -either unduly obvious, or inconspicuous, and thus stand out at the cost of the drawing. The guide lines should be of uniform thickness with both ends without any a.rrow my,rks. '(7) The labelling should be done by "printing" the names in block le~ters always, parallel to the base of the page. CHAPTER I THE DOG·FISH The common dog·fish is a marine animal, ai such iu the universities and colleges away from the sea-shore only formalin preserved specimens are available for study The characteristic feature of the dog.fish is that its entire internal skeleton is cartilaginous, hence, included in the class Chondroptery!!,ia. There are several varieties of dog·fishes found off our coasts. The description below is taken from one of the abundant species of the Indian seas, the Scoliodon. 1. EXTERNAL CHARACTERS. (A) Shape of the body. Long laterally compressed body, tapering at both the ends, is the broadest in the region of the petoral fins. The dorso-ventrally compressed head produced into a flat pointed snout anteriorly. The head bears two large circular eyes at its sides. The eyelids are immovable and can 'hardly be distinguished from the surrounding integument, but each eye is provided with a. special eyelid, the nictitating membrane, which lies along the antero. ventral border and can be drawn over the eye in time of danger. The pupil is narrow and v~rtical. The trunk passes on into tail compressed from side to side and stlightly bent upwards. At the junction of the trpnk and tail there are two shallow pits-one dorsally and the other ventrally. These are the caudal pits. 'l'he colour of the preserved specimen is dark grey dorsally, whereas, the ventral surface is whitish. 1 THE DOG-FISH A mere touch reveals that the entire body is covered with small scales. On either side of the body there is a faint line, the lateral line. extending from the head toG th:2 posterior end of the hody. (B) The Fins. The fins are flattened expansions of the skin, usually triangular in shape, and supported by cartilaginous rods and horny fin-rays. (1). Median Fins. (i) The first dorsal fin is laqer, triangular in shape situated a little infront of middle of the body. {ii) The second dorsal fin is situated a little distance behind the first and is similar in shape though smaller in size. {iii) The caudal fin forms a vertical fringe round the posterior portion of the tail. It is divided into twO\ lobes-dorsal and ventral. The dorsal lobe forms a low ridge along the grea tel' part of the upper surface of the tail in the median line. The ventral lobe is ·well developed divided into two parts, a la.rger anterior and the other posterior which is smaller. {iv) The ventral median fin (anal fin) is situated in the mid-ventral line slightly infront of the caudal fin. This is almost opposite the second dorsal fin. THE DOG-FISH (2). Paired fins (comparable with the limbs of higher 'Vertebrates.) (i) The pectoral fins are the largest among fins, originating from the ventro-Iateral margins of the body immediately behind the gill-clefts. Note their triangular shape and the space covered by them. (ii) The pelvic fins are smaller in size and similar in shape; they arise close together from the ventral surface of the body a little infront of the middle of its length. Their inner borders touch each other, and in the male are fused together behind. They enclose the cloacal chamber. In the male a part of the fin on each side forms a copulatory organ, the clasper. Each clasper is a stiff rod-like appendage grooved dorsally. The groove leads into a cavity, ; the siphon, beginning at the base of the clasper. (0) External apertures. (1). Median Apertures. (i) The mouth situated on is bound by bearing one is a wide crescentic opening the ventral side of snout. It the upper and lower jaws each or two rows of oblique teeth. {Ii) The- cloacal aperture is an elongated opening on the ventral surface of the body between the pelvic fins. 4- THE DOQ-FISH (2). The paired apertttr.es. (i) The nostrils are a pair of obliquely placed crescentic apertures on the under surfaee' of the head in front of the mouth. A small fold of skin from the anterior edge covers each nostril partially. The nostrils are only olfactory Eitructures and not respiratory, as such, they do not open internally into the mouth cavity. (ii) The gill-clefts are a series of five vertical "lits on each side infront of and slightly above the pectoral fins. Pass a seeker through the gill. clefts into the mouth and note that they open obliquely backwards into the giU.pouches and pharynx. (iii) 'l'he abdominal pores are a pair of openings situated on elevated ptotpillae, the cloacal papillae, on either side of the cloaca. Through these minute pores the coelom communicates with the exterior. (3). Scattered apert1tres. , Examine the head carefully and note numerous groups. of small openings on the, head and snou~. These are the apertures of the ampullary canals. To make these openings more obvious squeeze the head and note minutedroplets pouring out of the openings. D. THE EXOSKELETON. The skin is covered with minute dermal denticles orplacoid scales which are arranged in regular oblique rows and form the exoskeleton of the shark. These sca.les. are derived from the skin. THE DOG-FISH 5 Cttt a small piece oj skin and boil it Jor a few minutes in 20% solution of caustic potash. Allow it to stand. If the scales have isolated they will settle down at the bottom of the tube. Decant the fluid, add sonte water and again decant. After washing in water, thus, mount the scales in glycerine and examine. Each scale consists of a diamond-shaped basal-plate from \vhich a flat trident spine projects out. The spines are directed backwards2. THE COELOM AND VISCERA. Lay the animal on its back on the dissecting board and pin dow1~ throngh the fins. Determine the positions of the pectoral and pelvzc girdles by feeli1tg them through the skin, the pectoral girdle being a little infroni of the pectoral fins, and the pelvic girdle a short distance in/ront of the cloacal aperture. Now give a mid-ventral inctszon extending from the pelvic girdle to the pectoral girdle. Cttt through the body-wall tl'ansversely just behind the pectoral girdle and pin the two fiaps down. A~ The Coelom. 'fhe spacious coelom is divided into two unequal cavities, the pericardial and the abdominal, separated from each other by a membranolls partition, the septum trallsversum. The pericardial cavity is a small triangular. space lying beneath the pharynx and surrounding the heart. Thp abdominal cavity is quite large surrounding the viscera. and communicating with the exterior through the abdominal pore. B. The Viscera. I dentijy the positions and relations of the fullowing organs before proceeding further with the dissection. 6 THE DOG-FISH (1). The heart is almost triangular in shape lying ill' the pericardial cavity. (2). The liver is a large solid elongated gland. It haa two lobes uniting anteriorly. A median suspensory ligament connects the liver lobes with the anterior waH of the abdominal cavity. Embedded in the right lobe is found a V.shaped thin-walled sac, the gall bladder. (3). The stomach, a wide muscular tube, occupies the major portion of the body cavity lying between the liver lobes. (4). The spleen is a large brownish-red body attached to the loop of the stomach. (5). 'The pancreas is a whitish, laterally compressed body lying in the loop ~f the stomach and t'"e intestine. (6). The intestine is a nearly straight tube running upto thQ cloaca. ('7). The Gonads. (i) Female : The ovary is a median organ lying on the ventral side of thE' vertebral column just behind thG base of the liver. The tubular oviducts extend through the entire length of the body cavity uniting behind to form the vagina which opens into the cloaca. Between the ovary infront, and the coecal gland behind extends a long tubular strand of tissue, the epigonal oI'gan. (ii) Male: The testes are a pair of elongate structures, whitish in colour, lying along the dorsal region of anterior two-third of the abdominal cavity. THE DOG-FISH 7 (8). The kidneys are a pair of long ribbon-like glandular structures lying dorsally to the peritoneum. and extending from the root of the liver to the cloaca. (9). The peritoneum is the smooth membrane lining the body cavity. In the mid·dorsal line it is reflected beneath the vertebral column to form a double fold of membrane, the mesentery, which suspends the gut. 3. DISSECTION OF THE DIGESTIVE SYSTEM A. Alimentary Canal. (I). The mouth, ventral and crescentic, opens into dorso-ventrally compressed spacious buccal cavity which is lined with thick mucous membrane. The mucous membrane is raised ventrally into a thick fold forming thc non·muscular and non.glandular "tongue". (2~. The teeth are oblique and have sharp, more or less, compressed cusps, the edges of which are smooth and non-serrated. The teeth are all similar in shape, homodont, and are borne in several parallel rows on the inner margins of the upper and lower jaws. (3). The pharynx: The buccal cavity opens into the pharynx on either side of which lie the internal openings of the spiracle and the gill-pouches. The spiracle is vestigial (not visible externally in S::diod:m) rcpresfmted hy an inconspicuous oval pit. The gill-pouches are .arge. Internally the pharynx is lined with mucous membrane containing numerous dermal denticles. Note: The study of the buccal cavity and the pharynx should be attempted after completmg the dissection of blood vessels. 8 THE DOG-FISH (4). The oesophagus: The pharynx narrows posteriorly to form a short oesophagus, the inner lining of which is thrown ipto longitudinal folds. Cttt open the oesophagus to see the folds. (5). The, stomach is a. horse-shoe-shaped tube, the long proximal limb of which is called the cardiac stomach, whereas, the short distal limb is thE' pyloric stomach. At the junction of the two limbs is a blind outgrowth, the "blind sac". The inner mucous lining of the stomach is also thrown into longitudinal folds that end in the depression of the "blind sac". 'fhe lining of pylorus is quite smooth proximally but slightly folded distally. Give a median longitudinal incision to the stomach and exami1te it~ internal make·1~p. (6). The bursa entiana is a thick. walled muscular chamber into which the pylorus opens anteriorly. The opening of the pylorus i8 guarded by a. pyloric valve. The bursa entiana continues into t,he intestine. (7). The intestine is a wide tube running straight backwards into the' abdoniinal cavity and opens posteriorly into the rectum. Cut open the intestine by a longitudinal incision along iis right side and along its whole length. . Wash Ollt its contents and examine. The internal surface of the intestine is increar:Jed by a characteristic fold of mucous membrane, the scroll valve, having one edge attached to the inner wall of the intestine and the other rolled up longitudinally on itself into a scroll, making an anti-clock-wise spiral of about two and a half turns. THE DOG-FISH (8) The rectum is the last part of the gut. coecal or rectal glands open into it dorsally. 9 The (9). The cloaca is the chamber into which the rectum .as well as the urinogenital ducts open. B. The glands of the digestive sytem. (i) The liver has I already been described earlier. The right lobe of it carries the gall bladder. The bile duct leaves the ga.ll bladder between the two lobes of the liver, almost in the median plane. ] t is about half an inch on the whole. It receives ducts from the lobes of the liver. (ii) The pancreas as described above lies in the fold of the stomach amI intestine. The pancreatic duct runs forward through the pancreas, close to its Vf>ntral border, enters the ventral wall of the intestine, runs along it for about half an inch, and opens just to the outer side of the line of attachment of the spiral valve. (iii) The rectal gland is a short thick-walled tube lying in the abdominal cavity dorsal to the rectum. . 4. DISSECTION OF RESPIRATORY SYSTEM. Expose the gills on one side by prolonging the external, ajertures upwards and downwards with scissors. The Scoliodon breathes by lpeans of gills borne in a series of gill-pouches. There are five pairs 'of such gill-pouches, each of which is compressed antero- posteriorly 10 THE DOG-FISH and communicates with the pharynx by a large inter-branchial aperture ann with the exterior by a narrow external branchial aperture. The gills are a series of closely set, soft and highly vascular folds of a mucous membrane radiating outwards from the cartilaginous branchial arches and supported by gill-rays. Each gill-pouch has two row~ of giJl-lamellae one on its anterior and the other on its posterior wall. Successive gill-pouches are separated from one another by stout fibro-muscular partitions, the inter-branchial septa. (i) Tho holobranch or complete gill consists of two sets of lamellae atta":led to a visceral arch and its inter-branchial septum. (ii) The demibranch or of only a single set. a half-gill c0nsist& (iii) The pseudobranch is a rudimentary gill, consisting of about half a dozen small parallel folds, on the anterior wall of the spiracle but in Scoliodon tho spiracle bears no gill-lamellae. 5. DISSECTION OF THE OIRCULATORY SYSTEM (1). Pericardiai Cavity and Heart. Pin the dog- fish on its back, on the dissectillg board. Remove the middle portion of the pe(;toral girdle alld caref1tlly open the pericardial camty. CZtt away the ventral wall oj the cavity so as to expose it fttlly. The Pericardial Cavity. THE DOG-FISH II It is a median triangular cavity lying between the gills with the apex directed forwards, and is almost completely occupied by the heart. Tpe heart is a single muscular tube bent on itl'ielf forming four chambers, each separated from the other by a transverse constriction. (i) The sinus venosus is a triangular thinwalled chamber elongated transversely, attached along its whole length to the, posterior and dorsal wall of the pericardial cavity. Laterally it rec~ives two large veins, the ducti Cuvieri and two hepatic sinuses open into it in the postero.median line. Anteriorly the sinus opens into the atrium by a median aperture. Expose the sinus venosus, press the 'ventricles slightly forwards. (ii) The atrium (auricle) is a large sac occupying the dorsal half of the pericardial cavity. lts \\ aIls are thicker than those of the sinus venosus. It is triangular in shape, with its posterior angles produced into processes which project laterally at the sides of thE' ventricle in its natural position. Push the globular ventricle to one side to expose the auricle. (iii) The ventricle is the most conspicuous chamber of the heart. It has thick muscula~ walls, the inner surface of which is produced into numerous muscular strands, giving it a spongy tpxture. Ventrally the ventrIcle is supported by the coracoid cartilages. 12 THE DOG-FISH (iv) The conus arteriosus is a stout muscular tube running forwards from the ventricle to the anterior end of pericardial cavity. The inner wa.ll of the conus arteriosus is provided with two transverse rows of semi-lunar valves, one dorsal and two ventro-Iateral in position. (2). The Arterial System. A. The Afferent Branchial Arteries. The aif erent branchial vessels are almost su perficial. Trace the continuation of the conus arteriostts towards the anterior side, upto the posterior border of the hyoid ilrclz where it bifttrcates. Follow other branches arisil:g from it on both the sides to gills. Expose as much of thE' blood vessels as pO$sible by removing the tissues coverintZ them, and also the gills. (i) The ventral aorta is the name given to the anterior prolongation of the conus artf riosus. Passing through the wall of pericardium it extends along the ventral surface of the pharynx right upto the posterior border of the hyoid arch, where it bIfurcates into two branches, the innominate arteries, each of which again divides into two forming the first and second afferent branchial arteries. first afferent branchial, the anterior bra-nch of the innominate artery. runs along the posterior border of thf' hyoid arch and supplies the gill-lame lae of hyoid.ean demibranch. (ii) The (iii) The second afferent branchial supplies arterial branches to the anterior and posterior gill-Iam<lllae of the first branchial arch. THE DOG-FISH 13- (iv) The third afferent branchial arises a little behind the bifurcation of the ventral aorta and supplies the second branchial arelh. (VI The fourth and fifth afferent branchials arise, almost equidistant from one another, from the ventral aorta and run along the outer borders of the third and fourth branchial arches. B. The Efferent Branchial vessels. Insert 01'1e of the blades of a strong pair of scissors into the mouth, and cut back horizontally along the left side through the gill-arches to the last one_ Cut transversely across the floor of the pharynx behind the heart from the hinder end of the above inclsion. Continue to cut to the correspondtng pomt on the opposite side. Turn the floor of the mOllth w1th the heart over to the right stde. Wash the floor thoroughly and carefully remove the mucous membrane from the rooJ oj the mouth. Find the efferent branchtal vessels rtmning inwards and backwards from the gill-slits, and follow them, on one side, outwards to the gills and inwards to the dorsal aorta in the middle line. The blood from the gills is collected by four loops of blood vessels around the four branchial clefts on each side Besides these a single vessel collects blood from the fifth branchial cleft. The four loops are connected with one another by short longitudinal connectives running across the inter-branchial septa. Likewise the last vessel of th~ half-Ioop-, is also connected with the blood vessels of its side (a part of the loop surrounding the fourth gill-cleft.) The loops are further connected with each other by a network of longitudinal commissural vessels between their ventral extremities. :1.4 THE DOG-I~ISH (i) The efferent branchial arteries are nine in number (eight formed by 4 collector loops and the independent ninth). They run along the anterior and posterior borders of the five gill-clefts. (ii) The epibranchial arteries are four on each side. Each of the four collector loops is continued into an epibranchial artery which runs backwards and inwards to the mid-dorsal line. The half-loop has no epibranchial of its own. tiii) The dorsal aorta is formed by the union of the four pairs of epibranehials, (md runs backwards along the w'hole lengf,h of the body. Anteriorly it runs a Iittl' distance and them bifurcates into two branches. ·c. Th~ Arteries Of The Head. The first efferent bra-nchial supplies blood to the head, though a little is also contributed by the anterior end of the dorsal aorta. Clean the first ejJ erent bra1tchial careful! y and ty y to locate its following branches. Trace them to the head region. (i) 'J' he external carotid artery is a largo vessel arising at antero-yentral corner of hyoidean efferent. Anteriorly it travels to the hyoid arch and divides into two branc'tes (a) the ventral mandibular supplying the mandibular region and (b) the superficial hyoid supplying the skin and sub-cutaneous tissues over the ventral part of the hyoid arch. 15 THE D 00 -FISH !lut'RkQ~e'TA\" \ \ flllSl EPleRA I'lt 1111\1-. __ _ SEtONO E PI!?R.AMC.llli1L---- ttONO 'j II III l- e: d; TIURO IZ III a: III ....U101 Fig. I.-The Efferent bloorl vesselfl and the artetiev of the head of the dog-fish_ The external carotid artery that is shown at the extreme right, is actually on the ventral surface, that is why it is often removed while cleaning th~ent arteries unless the student is careful. 16 THE DOG·,FISH (ii) The afferent spiracular artery a.rises a.t about the middle of the hyoidean efferent. It runs forwards on the outer side of the hyomandibular and epihyal cartilages, and then bends inwards surrounding the spiracle. It continues forward as the spiracular epibranchial artery across the floor of the orbit and enters the cranial cavity through a small foramen. Before entering the cranium it gives a branch to eyeball, tho great ophthalmic artery. (iii) The hyoidean epibranchial artery arises from the other end of the first efferent, a little before its fusion with the second efferent to form the first epibranchial. It run:, forwards· and inwards to the posterior border of the orbit where it is joined by th;> anterior branch of the dorsal aorta and immediately divides into two branches: the stapedial and' the internal carotid. The stapedial artery runs forwards and outwards and ellters theorbit where it gives branches, one supplying the eye muscles, and the superficial tissues, in the region above the auditory capsules, and other supplying the anterior boundary of the orbit. The former is called the inferior orbital while the latter is the superior orbital. The internal carotid passes inwards to the buccal cavity and enters the cranium, where it divides into two branchE'S, one of which unites with its fellow of tl e opposite side, while tbe other unites with the stapedial to form the anterior and posterior cerebraL. arteries. THE DOG-FISH 17 D. The Dorsal Aorta and its Branches. The dorsal aorta is formed by the union of the 4 pairs of the epibranchial arteries. It runS backwards along the whole length of the body, lying bene&th the vertebral column in the trunk. In the tail region it continues into the haemal canal of the tail.vertebrae as the caudal artery. The following are the principal branches of the dorsal aorta: (i) The subclavian arteries are a pair of vessels arising close to the union of the fourth cpibranchial arteries, and run outwards and backwards in the body wall and along th{' hinder border of the pectoral girdle to the pectoral fins, which they supply. (ii) The coeliaco-mesenteric is a large median artery arising from the dorsal aorta behind the junction of the fourth pair of epibranchial arteries. It divides into two unequal branches, (a) the coeliac that supplies the stomach and liver etc., and tb) the anterior mesenteric supplying the pancreas, the intestine, and the rectum. (iii) The lieno-~astric artery arises a short distance behind the origin of the coeliacomesenteric. It is also a median blood vessel giving off branches supplying the genital organs, posterior part of the intestine and the spleen etc. ' 18 THE DOG-FISH (iv) r£he posterior mesenteric artery is a small median vessel arising from the aorta about an inch and a half behind the Heno-gastric. It runs backwards to the mesentery and the rectal gland. (v) The parietal arteries are a series of paired vessels arising at iatervals along the whole length of the aorta and supplying the bodywalls. (vi) The renal arteries are small paired vessels arising from the parietal arteriE's and supplying the kidneys which they enter from the dorsal surface. (vii) The iliac arteries are a pair of vessels similar to the parietals, each of them extend into the pelvic fin. Note: If only one specimen has to be used for the study of all the systems the dissection of the posterior part of the aorta and its branches should be postponed until after the dissection of the renal-reproductive organs and the cloaca. 3. The Venous System. The blood in the dog-fish iB not returned by narrow tubular veins but by large blood spaces or sinuses. (i) The anterior cardinal sinuses collect blood from the oui,er sidE' of the head and the branchial region. Each is a large sinus running backwards between the dorsal ends of the gill-pouches and the muscles of the body wall. Posteriorly it ent€'l'S the Cuvierian THE DOG-FISH sinus. It collects blood from the orbit, the nasal region and the hyoidean region. (ii) The inferior jugular sinuses are a pair running parallel and ventrally to the anterior cardinal sinuses, and each collects blood from the sides of the lower jaw and the ventral region of the gill-pouches. It commences just infront of the postero-ventral margin of the first gill-cleft and is situated beneath the floor of the buccal cavity and pharynx. Posteriorly it opens into the Cuvierian sinus. ('iii) The posterior cardinal sinuses lie close together along the roof of the abdominal cavity. They originate in between the kidneys, along the posterior region of which only a median inter-renal vein represents these, but in the anterior region of the kidneys the dght and the left posterior cardinal sinuses are distinct. In the region of the oesophagus each of them expands into a wide thin-walled sac opening finally into t,he Cuvierian sinus. t(iv) The hepatic sinuses a.re two l~rge thinwalled sinuses collecting blood from the lobes of the liver. After traversing the whole length of the right and left lobes of the liver they open anteriorly into the sinus venosus by two apertllres in the median line. ,{VI The Cuvierian sinuses are a pair, each of which runs transversely, like its fellow of the opposite side, passes through the pericardium and opens into the basal angle of the sinus venosus. I THE DOG-FISH 20 6. The portal represented. systems are two and quite weIr (i) The hepatic portal vein is formed by the union of {lhe veins from the various· parts of the alimentary canal. It divides into three branches before entering into the liver lobes. (ii) The renal portal system consists of two renal porial veins formed by the bifurcation of the caudaJ vein. which running on the inner side of the kidneys, break up into branches in the substance of the kidneys. The blood is then collected by the renal veins which open into the hinder portion of the posterior cardinal :,inus. 6. A. DISSECTION OF THE RENAL AND REP·· RODUCrrVE SYSTEMS. The Male: After removi1tg the viscera dissect 0 if the peritoneum' from the ventral. surface and expos~ the whole lengt~ of the' kidneys. Examme. Cttt the W 01ifzan duct of one stde away from the ventral surface of the mesonephros and trace it to the cloaca. Also follow the vasa efferentia opening into the) Wolifian duct. (i) The kidney is mesonephric fully differentiated into genital (anterior) and r~nal (posterior) portions. The functional kidneys lie, one on each side, closely adposl'd to one another in the hinder end of the coelom and covered by toughish peritoneum. The genital portion, which is almost devoid of excre, THE DOG-FISH 21 tory tubulf>s, extends forward to the front end of the body cavity beneath the peritoneum. (ii) The testes are elongatE'd extending more than half way back wards from the front end of the roe 10m and covered by peritoneum. The' fold of peritoneum suspending the testis in the body cavity is called the mesorchium. (iii) The Wolffian duct is well developed and is thrown into an intricate series of coils overlying the genital portion of the kidney and forming the vas deferens on each side. ~iv) The vasa efferentia are minute tubules which open into the anterior end of the vas deferens. (v) The seminal vesicles are simply enlarged portions of the vasa deferentia on each side before they open into a large triangular chamber, the urinogenital sinus, which finally opens into the cloaca on an elevated urinogenital papilla. (vi) The sperm sacs are two blind elongated outgrowths of the urinogenital sinus, whose function seems obscure. Dissect away the sperm sac from vesicula seminalis and follow it back to the urinogenital sinus. Cut open the ventral wall of the ttrinogenital sinus and expose the r:avity. Examine its parts. 22 THE DOG-F_[SH (vii) The "ureters" are spcdal collecting ducts developed in connection with the functional l<idneys. From each kidney arise five collectmg ducts into which the renal tubules open. The ducts join to form a large channel, the "ureter", on each side, which opens into the urinogenital sinus, alongside the opening of the vesicula seminalis. (viii) The Mullerian ducts are not functional, but vestiges of them are found in the anterior part of the coelom on the ventral side of th~ oesophagus. (ix) The claspers are grooved elongations of the pelvic fins which are inserted into the cloaca of the female during copulation a'ld, thus, the sperms are transferred to the femr.le. (x) The siphons are muscular sacs lying beneath the skin of the ventral surface in the pelvicregion. Posteriorly these sacs extend as siphontubes, each of which opens into the groove of the clasper of its side. The use of the siphons seems to be obscure, though it has been ascribed the function, by squirting out the contained sea water, of flushing spermatozoa. accumulated in the claspers into the femalecloaca. B. The Female. Remove the viscera and Observe the ovary and the opening of the oviducts near liver, immediately beltznd lite of the ovrdttcts to the cloaca. dissect off the pentoneu11t. kidneys. Locate the anterior tlte suspensory ligament of the pericardzal wall. Follow one THE DOG-FISH (i; The kidneys are paired each showing the same differentiations into an anterior and posterior portion. (ii) The ureters commence in the anterior part and continue backwards and in the region of the functional kidney each dilates and joins its fellow of the opposite side forming a common duct opening into the urinary sinus, a triangul&r chamber. This sinus opens into the cloaca at the tip of the urinary papilla. (iii) The ovary is only single in the mature fish. It is apparently median in position suspended in the body cavity by a fold of peritoneum, the mesovarium. It is actually the right ovary. (iv) The oviducts are the well developed Mullerian ducts, each of which is a stout tub,:,. The anterior coelomic opening, of these have coalesced to from a single wide aperture, the oviducal funnel. From the common aperture each oviduct narrows slightly and passes backwards at the sides of the coelom to the cloaca. At about one third of the way down its anterior end pach oviduct swells out to form the oviducal gland which is responsible for the secretion of the egg case. Behind the glands the oviducts are large and dilatable with longitudinally folded walls. Posteriorly they unite and open by a large median aperture into the dorsal wall of the cloaca. Expose the cloacal chamber by cutting in between the pelvic fins and note. THE DOG-FISH (v) The cloaca is a shallow depression between the pelvic fins. In the female there are three openings into it, the rectum infront, the genital aperture in the middle and the urinary sinus behind. 7. DIS.::lEOTION OF THE SENSE ORGANS. A. The Orbit and the Eye Muscles. Dissect away the eyelids of one of the eyes and remove the skin surroundillg it. Examine the followillg : 1. The eye ball is almost hemispherical in shape. Only its outer surface is flat. It is held in position by si'l: eye muscles, and a cartilaginous stalk, the optic pedicle. The stout optic nerve mayalso be seen crossing the orbit and entering the eye ball. 2. The eye muscles consist of six narrow muscular j bands which arise from the wall of the skull and are inserted into the eye ball. (a) The recti muscles are four in number arising from the posterior end of the or bit. (i) The superior rectus rnns outwards and upwards and is insertcci on the dorsal surface of the eye ball. (ii) The inferior rectus runs outwardE and downwards to be inserted on the ventral surface of the eye ball. (iii) The anterior rectus runs forwards and is inserted on the anterior surface. THE DOG-l!'ISH (iv) The posterior rectus runs backwards and is inserted on the posterior surface. (b) The oblique muscles are two and arise from the anterior angle of the orbit close together. (i) The superior oblique is inserted on the dorsal surface of the orbit infront of the insertion of the superior rectus. (ii) The inferior oblique is inserted on the ventral surface of the eye ball. 3. The Nervous Supply of the Eye Muscles. (i) The oculo-motor or the third cranial nerve enters the orbit and divides into branches that supply the antE'rior rectus, the superior rectus, the inferior rectus muscles as well as the inferior oblique muscle of the eye ball. (ii) The trochlear or the fourth cranial nerve arisE'S from the dorso·lateral surface of the mesencephalon, and on entering the orbit supplies exclusively the superior oblique muscle of the eye ball. (iii) The abducens or the sixth nerve is a slender nerve arising from the medulla oblongata, and it innervates the posterior rectus muscle on entering the orbit. THE DOG-FISH B. Dissection of the Ear. Clean the hinder end of the skull so as to expose the auditory capsule fully which can be easily located by three prominent ridges on the dorsal smface. These ridges lodge the three canals of the internal ear. They can be even sem through the cartilage. Having located these hold the specimen in your left hand and carefully scrape away the cartilage with a sharp scalpel 1tllt~l the several parts of the auditory orgnas are exposed. The dissection is not at all difficult because the parts of the internal ear can be seen from outside owing to the transpareilcy of the cartilage. The thing needed is absolttfe care. After exposing the canals and the vestibule study it in situ. Then remove the entire organ, keep in a watch glass, study and sketch. 1. The vestibule is a laterally compressed sac differentiated by a constriction into a dorsal ch,p,mber, the utriculus and a lower chamber, the sacculus, the posterior end of which forms an out-growth, the lagena. From the sacculus arisE's a narrow tube, the ductus endolymphaticus which runs upwards and pierces the cranium. 2. The semi-circular canals are three tubular loops opE'ning into the utriculus. These canals are mutuaUy at right angles one being horizontal, the horizontal canal, and the other two vertical, the anterior vertical canal and the posterior vertical canal. At one end where it joins the utriculus each semi-circular canal is swollen out to form an ampUlla. C. The Sensory Ampulla. At the anterior end of the snout there are a number of small openings, as mentioned earlier.. Squeeze the nead of the dog-fish provided so as to make water exude out, and thus render the openings vis~ble. Cut off a rectangluar bit of the tissue along with the skin and muscles almost a ce1Z~ THE DOG-FISH timeter deep. Make sure before wtting that the area contains the openings of the canals. Carefully identify the masses of the ampullae and the nerves in connection with them and then tease them out so as to separate the ampullae, mount ana examine under microscope. Sketch. The ampullae are specialized neuromasts sunk below the skin and found in groups on the dorsal and ventral side of the head. Each ampulla is an elongated tube' opening externally by a pore in the skin, and ends in radially separate ampullary sacs, lying deeply beneath the integument. The ampullary sacs are innervated below. 8. DISSECTION OF THE CRANIAL NERVES. The first cranial nerve, the olfactory nerve, consists of a group of separate fibres, which arise from the olfactory cells in the olfactory organ and direct]y pass into the olfactory lobes of the brain. The second or the optic nerve, as described earlier, innervates the retina. Before entering the orbit it forms the optic chiasma. The third or the oculo-motor, the fourth or the trochlear and the sixth or the abducens, supply the muscles of the eye (see page 25). The eighth or the auditory is also a small nerve directly passing from the medulla obiongata to the auditory capsule. The fJIlowing is the description of the fifth or the trigeminal, the seventh or the facial, the ninth or the glossopharyngeal and the ter.th or the vagus. Remove the skin from the 1tpper surface of the head; now remove the eye ball clean tlte tissues carefully and note a prominent nerve running across the dorsal margin of the orbit. Tlzzs is the ophthalmic nerve formed by the ophthalmic branches of both the fifth and the seventh. Trace it bothways anteriorly as well as posteriorly to the places of its supply and origin respectively. THE DOG-FISH 28 The fifth, seventh and eighth nerves arise very close together from the side of the medulla oblongata, at its widest part, opposite the posterior part of the lcrebellum. The hinder-most is the eighth and it passes straight to the auditory capsule, while the fifth and the seventh nerves ;pass outwards through the skull-wall by a foraman at the posterior and inner angle of the orbit,. (i) The fifth or trigeminal branches. nerve has three main (a) The ophthalmic branch, as already noticed, arises from the anterior border of the root of the fifth nerve, close to th'l brain. It runs forwards and backwards for about a quarter of an inch ",nd then ,'merges out into the orbit, where it runs along with the branch of the seventh. The main stem of the fifth nerve appears as a broad ribbon-like band entering the orbi<:;, near the outer margin of which it separates into the maxillary and mandibular bfl1nches. Dissect the t~ssues aif and trace the nerves across the floor of the orbit, and follow its branches to their distribution. (b) The maxillary branch is the anterior of the two. At the anterior border of the orbit it turns over the upper j ,tW and divides into branches which innervate the region of the upper jaw. (c) The mandibular crosses the upper jaw, curves round the angle of the mouth and 29 THE DOG-FISH runs along the lower jaw. It is very superficial, just below the skin, at theplace where it crosses the upper jaw. (ii) The seventh or facial nerve. The root of theseventh arisE's immediately behind the fifth and divides into four main branches. ('a) The ophthalmic branch pentratcs the orbit close to th(' similar branch of the fifth :o.nd runs along its side to the cutaneous sense organs on the snout. -m OPHTHALMIC Yf1 m ~ V&vn: IX X LATERAL X \ \ ~!!~:-:;';f-~f'--::::~:::::::::~-::::::'::::: \ \' , \ \, • , I I ' I .- , eUCCAl'iII'" ... MANDIBULAR V, ... I HYOID PALATINE 'al I Fig. 2.-The cranial nerves of the dog-fish. The main stem of the seventh nerve enters the orbit immediately bchtnd the fifth and divides into three branches. Dissect ottt each of them to the place of its innervation. (b) The buccal branch arises from the main· root and runs along the maxillary and mandibular on the floor of the orbit. (c) The palatine branch is a slender nerve which crosses the floor of the orbit. .30 THE DOG-FISH (d) The hyomandibular branch which lies in the posterior wall of the orbit and passes in the direction of the spiracle. Looping around the spiracle the main ramus of the nerve continues closely beneath the skin to the lower jaw as the external mandibular branch and another branch runs down the hyoid arch as the hyoidean. (iii) The ninth or glossopharyngeal nerve arises from -the medulla oblongata closely behind the eighth and innervates the first gill-cleft over which it divides into a pre-trematic branch and a post-trematic, the former passing up and the latter passing down the first branchial arch. (iv) The vagus arises from the side of the medulla oblongata behind the ninth by a series of rootlet,s. The nerve has three main branches: (a) The lateralis branch, quite prominent and long, runs posteriorly supplying the lateral line receptor organs of the trunk. (b) The visceralis branch continues backwards into the body cavity where it gives off branches to th~ heart and viscera. (c) The branchialis branch consists of four su19sidiary nerves supplying second to the fifth gill-slits, each nerve dividing into a pre- and a post-trematic branch, the latter running right down the gill arch. 9. THE DISSECTION OF THE BRAIN". The dog-fish lives in deep seas and as such depends more ,upon the sense of smell than upon the sense of sight. Also, the powerful movements during swimming neces- THE DOG-FISH sitate efficient control of muscular activity. The bmin, therefore, is specially modified to accommodate these activities. The olfactory lobes and the cerebellum are well developed, whereas, the optic lobes are not enlarged. For convenience the brain should be dissected after the dissection of the cranial nerves. Remove the skin of the head and slice off the clrtilaginous covering of the cranium exposing the brain fttlly. The brmn ocwpies the cranial cavity ~llmost fully. Observe the following parts. (1) The fore brain is represented by (a) The relatively enormous olfactory lobes which are closely applied to the olfactory organs. The pallium merely consists of small paired protuberances on the dorsal surface of this region. (b) The thalamencephalon narrows from the wide anterior portion. Dorsally it carries the slender pineal stalk terminating in the pineal body lying closely against th~ roof of the cranium. The greater part of its dorsal surface is occupied by the anterior choroid plexus. Ventrally the thalamencephalon has the optic chiasma and a relatively large pituitary body, the latter consisting of the infundibulum, hypophysis and the accessory lobi inferiores of the infundibulum. (2) The mid-brain is very moderately developed. Dorsally aI:e two rounded prominences which project somewhat beyond the sides of the brain. These are the ,optic lobes (corpora bigemina). Ventrally the crura cerebri are hidden by thb posterior part of the pituitary >body. (3) The hind-orain has a very prominent cerebellum .that projects forwards, to some extent covering the optic THE DOG-FISH lobes, and backwards over the roof of the fourth ventricfer The cereb"Uum continues in the medulla oblongata that. is very well developed. (4) The Ventricles. Cut a median longztudt1lal section of the enttre brain and examine the cavities of the brain that are spaciolts. The cavity of each olfactory lobe is known as rhinocoel which communicates wi\h the lateral ventricles behind. ThE' lateral ventrIcles open into the large third ventricle behind, each by a foramen of Monro. The cavity of the third ventricle extends into the infundibulum of the pituitary body and also into the base of the pineal stalk. The optic lobes have optocoels within them. The fourth ventricle is the cavity of the medulla into which also opens the cavity of the cerebellum. A common space connecting the third and fourth ventricles, into which the optocoels also· open, is called the iter. 10. 'THE SKELETAL SYSTEM. Preparation of the Skeleton. Take a fairly large sized dog- fish well preserved in formalin. The skeleton zs cartilagin01tS as such is ltable to be damaged easily. Remove the skzn and muscles etc. carefully. Dip the specimen in hot water once or twice to loosen the connective tissues and then scrape hghtly. An lld tooth brush will be of immense helP in bmshing away the Jissues. A great care should be taken particularly when cleaning the skull and visceral skeleton. Whw the skeleton is thoroug-My cleaned -up, keep it in week formalin. THE DOG-FISH 33 t> A. The Vertebral Column. AJter cleaning the vertebral column thoroughly note the attachment oj median fins with it, detach the med~an fins later: Study a portion oj the column' Jrom olte side specially noting the arrangement oj the vertebrae and sketch: Thn~ cut transverse and sagittal sections oj ihe same 'and complete your study and sketch. . 1. The centra surround the notoohor}l, and are short cylindrical bodies of cartilag~ hollowed oU,t at either ends, ie. the vertebrae are amphicoelous. The notochord is constricted in the middle of the centrwp.),mt ~s. greatly dilated in the intervertebral spaces. . . I The centra are strengthened by ca:Icified fibro-cartilage developed as four wedges which traverse the body of 'the centrum from its periphery almost to the centre t':!us giving rlse to a curciforum figure, "Maltflse cross," which can be seen in a trans ver3e sectiu-n of the cen tru m through the middle. 2. The neural- plates are a series -of hexagonal of cartilages forming the sides of neural arches;the spaces between the . consecutive vertebral neural plates in contiguous vertebrae are occupied by the intervertebral neural plates. ~lates 3. The neural arches lie dorsal to the centrum enclosing the spinal cord. Each arch is formed of elements of two kinds viz. (i) the- neural processes, the blunt bases of the neural arches projeoting upwards from the sides of each centrnm, and (iij the neural spines, a series of median nQdul@s of cartilage completing the n:mral arch above. 4. The transverse processes are a pair of blunt 'lOrizontal projections on each side of the lower surface of 3 34 THE DOG-FISH (j 'Jentrum, in the trunk region which bear at their anterior ends movably articulated cartilaginous ribs about half a):l inch in length. .•• 3 5. The haemal arch is found in the vertebra of the tail region. Here the transverse processes, instead of projecting laterally, are bent inwards beneath the centrum and meet and fuse in the mid-ventral line to form an arch, the haemal arch, the ribs, as such, are absent. Each haemal arch enclo8ing the haemal canal is produced into a backwardly directed and flattened haemal spine. B. The Skull The skull in the dog-fish (lonsists of a brain hox with an incomplete roof, a pair of thin olfactory capsules anteriorly, a pair of stout auditory capsules po:;teriorly, and at the sides are the shallow orbits. Because the cartilaginous condition is retained into adult life it is called a chondrocranium. The brain box or cranium is a simple cartilaginous cylinder open infront and behind. The roof is arched and the floor flat. Anteriorly it gives off three cartilages which are prolonged forward to fOJrm the rostruOl. Examine the various parts of the cranium in the following order. Sketch. 1. The occipital region forms the posterior part of the cranium enclosing the large median opening, the formen magnum on its posterior side. On eit,her ,side of the formen magnum there is a prominent cccipital condyle. On the roof of the oecipital region there is a prominent median ridge, the ocdpi~al crest. External to the occipital condyles on either side lies a large foramen for the exit of the tenth nerve, the vagus. \ 'l.'HE DOG·Fl; ",,>Ji• :::: ....0CIl ~ ~ <T.I <6... <lJ <:.> til '> ~~ :i<l\ ,.._.. ,,'" ._ II" ~Z <T.I ;-=: \ "'d - ~ .," ... 01 ~ cil ~ <T.I <:;::I bo 0 0:1 "0 !:; ...c: ..., CD .....0 ~ .!<I <T.I Q.> .... ~ .....0 e:: <lJ -'>...,... cil <lJ cil ....:: \ C'6 bO ri; THE DOG-FISH 2. The auditory region consists of the auditory capsulefS and the pa rt of the cranium they are fused with. 'rhe lsrge latE'rally projected auditory capsules lodge the auditory organs, as such, the outer surface ::"hows three prominenJ ndges lodging the three semi-circular canals of the ear, which ean usually be seen through the cartilages. On the roof of the cranium lying between the two capsules, there is a marked depression, the parietal fossa, with a pair of apertures on each "ide of its posterior part. The posterior pair are the fenestrae or the openings of the perilymphatic spaces of the two capsules, whereas, the an~erior are a pair of apertures through which the endolymphatic ducts (aqueduct>us vebtibnli) of each internal ear piprces the cramum . . 3. The orbit lodges the eye ball and its lliJscles. The dorsal bound a ries of the orbits are formed "Y the supra-orbital crests, which are prominent cl;rved ridges of cartilage running along the sHes of the skull from thp olfactory to thp auditory cap:.uli's. Fnrther the boundaries on thp othpr sides are marked by t~e pre-orbital cartilages. which are slender cartilag~s arising frolll the roofs of each olfactory capsule Ju"t infront of the orbit', and curvcd backward partially t'ncircling the orbit, and the post-orbital cartilage on cal' h SIde arit:les from the a udi tory capsule and cun es forward along the uppl:'r border of the orbit. The Foramina of the Orbit. (i) The orbito-nasal foramen is a small circular aperture at the anterior inferior angle of the orbit. ~ii) The optic foramen is a large apertlll'e for the optic nerve at about tIll:' middle of the orlits length near its ventral border. 1'RE DOG-FISH 37 iii I The foramen for the third nerve is a small hole in the inner wall of the orbit behind and above the optic foramen. lV) The foramen of the fourth nerve is a small hole vertically above the optic foramen often pushed a bit pogteriorly. (v) The foramen of the fifth and seventh nerves is a large hole immediat(jly infront of the auditory capsule. The sixth nerve also enters the orbit through the '>ame opening. (vi) The aperture of the inter-orbital canal is a cireular opening infront of the large aperture for the fifth and sE'venth nerves. The inV'r-orbital canal traverses the base of the skull and places the orbital blood sinuses of the two Hides in communication. (vi.) The foramen for the spiracular epibranchial artery into the ('mnium lies slightly anterior to the- (vi). <viii) The foramen of the hyoidean a'rtery lies a little infront of and a little below the aperture of the inter-orbital canal. I)X) The foramina for the ophthalmic branches of the fifth and seventh nerves are two .. eparatp apertures near the posterior end of the orbit just mfront of the auditory f'apsule and above the foramE'n for the main hranches of the fIfth and seventh nerves. 38 THE DOG-FISH 4. The olfactory capsules are two large oval cartilaginous cups at the anterior end of the skull, firmly united to the cranium in the adult condition. A thin median cartilage, the internasal septum separates the two olfactory capsules from each other. The cranial roof in this region is complete, there being a large anterior fontenelle covered over by a sh!'et of connective tissue. Within each olfactory capsule there is a large opening leading into the cranial cavity for the entrance of the olfactory nerve . . 5. The rostrum is formed by three cartilaginous bars. In front of the anterior fontenelle arises a pairof dorso-Iateral cartilages one from the roof of each olfactory capsule; these run forwards to converge and meet in the front with a median ventral canilage which projects from the base of the cranium. 6. The floor of the cranium is broad and flat and bears towards its hinder end two obliquely transverse grooves, the carotid canals. It narrows considerably at its anterior end where the olfactory capsules are attached. Immediately behind the olf!tctory capsules lie two large and prominent articular surfaces for the attachment of the ethmo-palatine ligaments of the upper jaw while infront of these articular surfaees lie the anterior opening of the orbito-nasal canals. 7. The articular surface for the hyomandibular cartilage is a concave depression on the side of the hinder end of the skull, below the auditory capsule. C. The Visceral Skeleton. The visceral skeleton is made up of a series of visceral arches which develop in the pharyngeal wall between the visceral clefts. Typically each visceral arch is an incomplete ring of cartilage consisting of 8 basal piece in the mid-yentral line, with which articulate a. THE DOG-'FISH 39 series of pieces on each side extending upwards almost to the mid-dorsal line. It' is clear, therefore, that ths skeletal arrangement is independent of the chondrocranium and strictly speaking is not even part of the axial skeleton, though for convenience it is usual to consider it to be so. Tn the dog-fish seven visceral arches are developed, and it is the first two which warrant special attention. (1) The mandibular arch is the first pair bordering the mouth and modified to form the jaws. It has lost its original arch .form and consists of four pieces of cartilage, two on each side of thp. mouth and all joined by ligaments. Laterallyan upper jaw, plato-petrygoqpadrate bar, above and lower jaw, Meckel's cartilage, below meet behind the mouth. The two rami of the upper and lower jaws meet anteriorly in the mid-line. The jaws have no direct connection with the chondrocranium, which is provided by the second or hyoid arch. (2) The hyoid arch is a loop of, cartilage made up of five pieces. (i) The basihyal is a median piece of cartilage lying between the rami of thf' lower jaw. It is rounded anteriorly and produced behind into two horns which are attached by ligaments to the dorsal surfaces of the ceratohyals. (in The ceratohyals are two cartilages each of which is long and slender and runs forwards and inwards in the floor of the mouth articulating with the basihyal at its lower end. Each carries gill-rays along its posterior border and lies in the anterior walls of the gill clefts. , THE DOG-FISH (iii) The hyomandibulars are two cartilage::. , dorsal to the ceratohyals. Each hyomandibular is a short stout rod fitting at its upper end into depressions in the auditory capsules. of the chondrocranium to whirh they" are attached by ligaments. These" cartilages are the sole direct skeletal connections between the jaws and the cranium forming on each side the suspensorium of the' upper and lower jaws. (3) The branchial arches are the remallllllg five, visceml arches lying in relation with the branchial clefts.· They ,gradually diminish in size antero-posteriorly. Each arch is ma~8; up of a variable uumber of cartilaginous pieces. Observe their numbers and note. 0" (4) The labial c~rtil~ges are two pairs" ~lender , cartilaginous rods in the folds o£. the skin at t 11(' sides of the mouth.' ." D. 1. The Skeleton of the Fins. The Median fins. The skeleton of the two dQrsal and the ll,ledian ventral fins consists of a seril's of cartilaginous rods called somactidia or pterygiophores, bearing dista~ly a double ..cries of horny fin-rays, or c;erat6trichia. A wide strip of ligamentous tissue connects the somactidia of the fjn with the vertebral column. Count the number of somactidia an{/, note. The other median fins are built on the same plan as the above described. Only in the c<1u,dIt1 fin, somactidia are absent. The noma1 and haemal sp~pes of the vertebral column are elonga,tet;l and flattened to support the dorsal lmd ventral lob{'" of the fin. THE DOG-FISH 2. The Paired Fins. -\. The Pectoral region. -t-l (i) The pectoral girdle consists of two halfloops of cartilage fused in the mid· ventral line and free at thc dorsal ends. Each haH of the girdle is a scapulo-coracoid-the dor,,;al one is the tapering scapular region and the ventral one is the broaa.er coracoid region In the mid-ventral portion of the girdle, at the junction of two' coracoids, is a well defined depression, the pericardial depression lodging the perieardium and th£' heart in life. (Ji) The pectoral fin articulates, with the girdlt by the basal cartilages the pro, meso, and metapterygium. Arising distally from these are the radial cartilages or the cartilaginous fin.ruys. Each consists typically of three pieces, thus, forming three rows, the proximal contains pieces of varying sizes but uniform arrangement, the second and third rows are not uniform because the pieces forming these ttl.ke the form of interfitting, polygonal plates. The periphery of the fin is formed by an upper and lower series of horny fin.rays or dermotrichia. B. The Pelvic region. [i] Thf' pelvic girdle consists of it simple bal of cartilage, the ischia-pubic bar embedded transversely in the ventral body muscles, infront of the cloaca. 42 THE DOO - FISH (ii) The pelvic fins articulate with the ends of the pelvic girdle. Each is made up of a,. single basal cartilage, the basi pterygium, supporting a fairly uniform series of radials. As in the pectoral fins the peripheral portions of the fins are formed of dermotrichia. In the malf" the two fins are joined together posteriorly and in connection with each basipterygium there is a grooved, backwardly directed cartilaginous prolongation which forms the axis of the intromittent organ or clasper. CHAPTER II THE FROG. Dissection of the Cranial Nerves. There are ten pairs of cranial nerves in the frog. Of these the fifth, the seventh, the ninth and the tenth are branched and have long courses and take some time to dissect the rest have short courses and are very easy to dissect. Remove the skin from the head. Remove the nasal by scraping 0 if and expose the olfactory cavity. Remove theeyelids and cut along the dorsal margin of the eye very carefully. Press the eye ventrally and examine. (i) The olfactory nerves are small nerves emerging out of small foramina in theethmoid bone. By removing the ethmoid bone they can be traced directly to theolfactory lobes of the brain, from which they arise. (ii) The optic nerve will be visible on pressing the eye ventrally, as a stout nerve entering the eye ball. After forming the chiasma it emerges out of the cranium and enters theeye ball. (iii) The oculo-motor is the third cranial nerve and supplies four of lhe muscles of the eye ball, the rectus superior, rectus inferior, rectUfr medialis and obliques inferior. It emerges: from the cranium through a small foramen near the opening for the optic nerve. (iv) The trochlearis or the fourth nerve is a small nerve leaving tbe skull through a 43 ...1.4 THE·FROG special foramen a little above the optic nerve. It innervates the superior oblique muscle ofth(> eye. , v) The trigeminal 'or the fifth nerve is one of the largest cranial nerves of the frog. It is divided into two branches, the ophthalmic and the maxillo-mandibular which divides mto two branch(>s bhe maxillary and fhl mandibular " I V OPHTHALMIC V TRIGEMINAL VllMANDIB\.,LAR- - -~/ - ----..,L_ _ _~ VII HYOIDEf\N- - - -IX.GLOSSOPHARYNGEAL Fig. 4.-The fifth, seventh, ninth and tenth cranial nerves of the frog. As soon as the eye ball is pressed towards the 1·entral side a very delicate rtave 'It:ill be seen running across the dorsal margin of the orbit. ThIS is the ophthalmic branch of -the fifth. While dissecting this ,special care should be taken as it is easily removed along with other tissues. NO'll} trace it wwards to its place I)f origin. Now remove the eye ball completely. Tlti'i will expose the maxillary brarch of the fifth crossing the orbit alung its 1'entro-posterior margin. Locate the mandibular branch of the fifth that lies posterior to the maxillary branch. Dissect it up'wards to where both meet. Trar:e tlie maxilla-mandibular upto its Place of 'lvigin.. THE :FROG 45 (a) The ophthalmic arises from the anterior border of the' root of the nerve close to thE' brain and runs along the dorsal side of the orbit, passes into the nasal eapsule of its side and innervates the skin of the anterior side of the head. (b) The maxillo-mandibular runs outward and backwards behind the eye. It soon divides into the maxillary and mandibular. The maxillary enters the orbit, crosses it. and innervates the region of the upper jaw. Each mandibular turns posteriorly, runs a little distance behind and loops anteriorly crossingthe af\gle of the jaws and runs along the mandibles and. supplies .the skin of thelower jaw, To expose and to trace the course of the mandibular thefree end of the squamosal has to be removed. (vi) The abducens or the sixth nerve emerges' through the same opening as the fifth 'and innervates the external rectus muscle of' the eye. (vii) The facial or the seventh nerve arises from the medulla closely behind the fifth and leaves the cranium along with the same. It soon divides into two branches the palatine' and the hyomandibular. Remove the muscles of the eye complet,ely. (a) The palatine will be seen crossing the floorof the orbit just above the mucous membraneof the mouth. The main nerve passes on to the nasal chamber and the anteriorportion of the roof (If the mouth. THE FB,OG (b) The hyomandibular runs outwards and then backwards around the auditory capsule and just at the angle of the jaw it divides into a hyoidean supplying the hyoid apparatus, and internal mandibular supplying the skin and muscles of the region of the lower jaw. (viii) The auditory or the ei~hth nerve is a short nerve entering directly the auditory capsule. (ix) The ~lossopharyn~e:al or the ninth nerve arises from the side of the medulla from the group of roots in common with the tenth. These emerge from the skull through a foramen in the exoccipital, external to the condyle. To expose the glossopharyngeal and the vagus the supra:scapula has to be removed. Cut 0 if the muscles binding the suprascapula and cut it from its junction with the scapula Observe the stozel second spinal nerve, brachial, going ..to the arm. A short distance after emergence the t,runk of the lossopharyngeal forms a ganglionic swelling where it , ives a delicate branch that joins the hyomandib11lar of the seventh. Later the glossopharyngeal runs downwards .and backwards turning towards the anterior side, a little behind the angle of the jaw. It runs ventrally along the lower jaw and innervates the to~gue and the pharynx. (x) Tht> va~us or thf' tenth arises al,mg with the ninth and on emergence passes blLckwards and gives, branches to the larynx, oesophagus, stomach, lungs and l).eart etc. (Note: The hypoglossal or the first spinal nerve that runs almost parallel to the glossopharyngeal in the region .of the lower jaw and supplies the tongue and several THE FROG 47 muscles attached to the hyoid often confuses a studam. If so, care must be taken to trace the hypoglossal, upto the place of it!:! spinal origin). 2. THE BRAIN. The dissection of the brain of the frog, though already done in the intermediate class, is included here only by way of revision. Structur.1lIy it is quite simple and without much specialisation. It will be of immense help to examine the brain and its various cavities. Scrape 0 if the fronto-parietal bones of the cranium carefully and expose the &r.ain as fully as possible. As the brain is quite soft and pulPY take care not to injure it. Do you know the following parts? 1. The fore-brain. (i) The olfactory lobes. (ii) The cerebral hemispheres. <iii) The median furrow. (iv) The thalamencephalon. (v) The pineal body. (vi) The optic chiasma. (vii) The pituitary body. 2. (viii) The anterior choroid plexus. , The mid-brain. (i) The optic lobes. (ii) The crura cerebri. lb THE FHOG 3. The hind-brain. (i) The cerebellum. (ii) The posterior choroid plexus. (iii) l'he medulla oblongata. 4. The Ventricles. Cut the brain t'ttto two halves, by a sharp !mire or a; 'yazor blade, alO1i g the lateralltne. attd note the following. ), The rhinocoel. (ii) The lateral ventricles. (iii) The third ventride. (iv) The foramen of Monro. (v) The optocoel. (vi) The fourth ventricle. (vii) The iter. CHAPTER III THE LIZARD (V ARANUS) The living reptiles include the lizards and chamaeIeons ; the snakes, tortoises and turtles; the tuataras ; and the crocodiles and alligators; of these some lizards such as the varanus (varanidae), the uromastix (agamidae) or gecko (geckonidae) are examined as representative of the class. The following is the description of the varanus, but it can safely be used for the study of uromastix or ordinary wall lizard (gecko), as the case may be. 1. EXTERNAL FEATURES. Examine the specimen placed before you. Notice that the body is covered with scales like that of the dog-fish and that the body has a distinct neck supporting a comparatively smaller head. What is the difference between the scales of a dog-fish and those of a reptile? What type of a neck, if at all, was present in the frog or the fish? The caudal region in the lizard is quite long. The body is· elongated supported by two pairs of short limbs, of approximately equal siz e. a. The head is triangular in shape with apex directed forwards. (i) The mouth is a wide slit·like aperture running around the anterior border of the head. 49 50 THE LIZARD (ii) The nostrils are minute apertures situated above the anterior ~xtremity of the head. What types of nostrils are met with in the fishes? The eyes are large, at the sides of the head, provided with two-the upper and the lowermovable and opaque eyelids. A transparent nictitating membrane is also present which lies foldt'd at the anterior angle of the orbit when withdrawn. (In gecko the eyelids are' fused forming a transparent covering as in snakes). (iv) The tympanic membrane is a circular, brownish patch of skin on each side, behind the eye slightly sunk below the general surface of the skin. b. The trunk is elongated flatter ventrillIy and ('onvex dorsally. It is supported by two pairs r,f pentadactyle limbs. (i) The fore limbs arise from the trunk just behind the neck. Each consists of the upper arm (brachium), the fore arm (antibrachium), and the hand having five clawed digits, the first digit (pollex) is the smallest. (ii) The hind limbs are situated towards the end of the trunk more towards the ventral side. Each has a thigh (femur), shank (crus) and r. foot (pes) having five digits with horny claws, the first or hallux is the smallest. (On the under side of the manus and pes in gecko, leaf-like adhesive pads are present. The surface of the pads on each is <::ivided by a. series of transverse plates. They help them clim b smooth vert,ical surfaces. In the uromastix and var~he limbs are usedfor digging). 51 THE LIZARD (iii) The cloacal aperture io; a slit-like transverse aperture on the ventral surface of the tail. c. The tail is dorso-ventrally compressed structure, thick in front and gradually tapermg to a narrow posterior extremity_ (Some lizards break off their tails very easily to keep the attention of their enemie::. engaged to it while they escape. The tails regenerate later on. Is the process painful to the arumal '! If not, how does it happen ?) Draw neat diagrams showing the dorsal and ventral views. 2. DISSECTION OF THE COELO}I A~D VISCERA Lay the animal 01t its back and Cltt open the body .cavity by a mid-ventral incLsion. .Vote the pQsition of the organs of the body cavity. (i) The coelom or the body cavity is a cylindrical ,cavity lined by thin glistening membrane, the peritoneum .and lodging the viscera. (ii) The heart is triangular in sh:1pe, situ:1ted in the middle of the anterior portion of the body cavity. It is enclosed in a transparent, pericardium. (iii) The lungs arc enlongate fu.,iform sacs of oranae ,colour lying on either side of the heart, dorsal to it, a~d .extending posteriorly along the outer margins of the liver lobes, extending upto two-ttlird of their lenO'ths. (The posterior parts of the lungs in 'lome geckos'" are 'produced into narrow diverticula). (iv) The liver is a bi-Iobed structure of dark-red colour having a more or less, rounded gall bladder on Its right lobe, towards its lower margin. f;2 THE I .. IZARD (v) The stomach is elongate tubular st.ructuH', the anterior portion of which is covered over by t.he liver lobes and lungs. It is attached to the body wall by a fold of' peritoneum, the mesogaster. (vi) The duodenum is the first part of the intestine that forms a fold with the stomad}. (vii) The pancreas is a diffuse gland lying in the fold of the stomach and duodenum. Disentangle the mtestilles, spread them and measure t],Cl1,1'I (viii) The intestine is a coiled continuation of the duodenum attached to the body wall by a fold of peritoneum,. the mesentery. (ix) The rectum is the last portion of th~ 'tIimentary canal opf:'ning into the cloacal chamber. It i." wider and short tube. At it,s antf'rior end, or sometimes at the junction of the intestine and the rectum, there is a short coecum. It is attached to the body "all by a fold of peritoneum, the masorectum. (In some of the ordinary wall lizards coecum is quite small ancl in others absent.) (x) The cloacal chamber is an elongate tubula" chamber divided into an anterior and a posterior part. Into the anterior part tIle rectum opens anteriorly. a tbm walled urinary bladder ventrally and the genik. I ducts open into the posterior part of the cloaca from the dorsal side. (xi) The kidneys are two dark.red bodies, e[.ell> consisting of two lobes lying in the posterior part of the body cavity, dorsal to the cloacal chamber, and conr,:,dI by peritoneum only ventrally. Thl:'ir tapering posteriOD" dortions lie almost in contact with one another. THE LIZARD 53 (In the gecko the posterior portion of the kidneys extend into the caudal region. The pelvic girdle has to be cut in the middle to expose them fully. Their colour is pinkish and not dark-red). (xii) The gonads are not situated besides the kidney .as in other animals. In the males the testes are oval white bodies placed much anterior to the kidney. They are not at the same level on both sides. In the female the ovaries are irregularly oval bodies situated slightly ,behind the position taken by the testes in the male. In the female two oviducts may also be found, one on either side. Draw a neat diagram showing the organs of the body -cavity. 3. DISSECTION OF THE DIGESTIVE SYSTEM. A. The Buccal Cavity. OPen the mouth cav~ty as fully as possible and study the following salient feat1,tres. (i) The teeth are conical and similar found on both the jaws, The teeth ar13 fused directly with the jaw bones. A patch of teeth is also found on the roof of the mouth cavity, the palatine teeth. (ii) The tongue is long, narrow' structure forked at tho tip. It is attached to tho floor of the mouth. At the point of its attachment it is provided with a sheath into which it can be retracted. (In the geckos it is short and bifid posteriorly drawn out mto pcocesses 0.:1 either side of the glottis). THE LIZARD LocatE' the glottis, gullet and the openings of theeustachian tubes. Note.-Salivary glands are absent. Labial glands are' pr('sent. The poison glands of the snakes are upper labiltl' glands. B. The Alimentary Canal . . (i) The pharynx is that part of the alimentary canal where the respiratory and digestivepassap:es cross one another. (ii) The oesophagus is a short muscular tube lying dorsal to the ,vind pipe and connecting the gullet with the stomach. (iii) 1'he stomach is wider than thf' oesophagus and has thick muscular walls. ~ nternnJly thestomach is lined by glandular tissues. Cut open the stomach, wash its contents ani examine. (iv) The duodenum has been described already. Note its shape locate the pancreatic duct and see where it opens in the duodenum. tv) The intestine is re1atively short and a<; mu~J is an organ of dIgestion (also see page 52). and a&similation (vi) The rectum or large intestine is also short and opens into cloacal chamber. C. The Glands Associat~d with Dige'3tion. (i) The liver has already been descr bed on page 51. (ii) The pancreas is a diffuse gland producing thepancreatic juice and transferring the same to· THE LIZARD 55 the duodenum through the pancreatic ducts (also see page 52). Sketch the organs of digestion. 4. THE DISSECTION OF THE RESPIRATOB,Y ORGANS (i) The glottis is a slit-like aperture situated behind t,he tongue. (ii) The larynx is a short chamber into which the glottis opens. Its walls are supported by the cricoid (cricothyroid) and arytenoid cartilages. (In geckos vocal cords are also present). (iii' The trachea is an elongated cylindrical structure arising from the larynx. The walls of the tube ~re supported by complete tracheal rings. Posteriorly the trachea bifurcates into two short bronchi each entering the lung of its side. (iv) The lungs, as described earlier, are tubular structures. Cut open one of the l1tngs and examine its internal make-up. The inner lining of the lung is thrown into a network (9f delicate ridges. It thus presents the appearance of a honey-comb. 5. DISSECTION OF THE CIRCULAR ATORY SYSTEM A. The Heart. Remove the pericardium covering the heart, and careftilly clean the base of the heart and roots of important vessels. avoiding injury to them. THE LIZARD The heart is a triangular organ consisting of a sinus venosus on the dorsal side, two auricles anteriorly and a ventricle on the posterior side. B. The Dissection of the Heart. Important.-The dzssectwn of the heart shottld always be attempted after· completing the dissection of the blood vessels. Remove the heart along with small lengths of the important blood vessels, and examine. (i) The sinus venosus is a triangular thin-walled chamber on the dorsal side of the heart. At the two anterior ends of the sinus the precavals enter. Posterior apex of the sinus receives the large postcaval vein. Cut open the sinus venoS2ts dorsally by a VeHtCaZ incision and note: (a) The sinu-auricular opening is oval and opens into the right auricle. (b) The sinu-auricular valve is a two lipped valve guarding the opening. Do yV1t know the functwn of such a valve? (ii) The auricles are two-right and left-thin-walled chambers externally separated by a faint groove. Externally the auricles are separated from the ventricle also by a coronary sulcus. Rcmove the ventral wall of the auricles carefully. This reveals that internally the two aurides ale separated into two chambers by a complete interauricu ~ar septum. The interauricular septum reaches the auriculo-ventricular ap'erture dividing it into right and left, divisions' This opening is guarded by the aurlculo-ventricular THE LIZARD 57 'valve, attached to the posterior free end of the inter.auricular septum, and consisting of two semilunar flaps, Dne guarding the right opening and the other the left. Note the nature of the auricular walls. (iii) The ventricle is a conical muscular portion of the .'heart posterior to the auricles. Cut open the ventricle and note the spongy nature of its wall. The cavity is broken up by muscular trabeculae with a relatively small central space. The dorsal and ventral wall of the auriculo-ventricular valves are attached by muscular bands to the dorsal and ventral walls of the ventricle, thus, forming an incomplete .septum dividing the ventricle into two chambers right .and left. Of these the larger right IS the cavum venosum whereas, the left one is the cavum arteriosum. Trace the origin of blood vessels from these cavities. , {i) The pulmonary arteries arise from the right of the partition. (ii) The aortic arches arise from the left of the partition. Draw a neat figure showing the various parts of the 'heart. Draw separate figures of the dissection. C. The Veins. WIth the exception of the blood coming from the lungs, all the blood is retuI;Ilpd to thp heart through the three large vpnous trunks which enter the sinus velJ-osus. 58 THE LIZA.RD Impc;>rtant.-Tho veil1'l are relatively therefore, they should be dissected first. superfeciaf,. Immedwtely on re11lIJv11lg the pericardi1t1n, clean the roots of vartous blood vessels, and push the heart towards one side. This wtll expose the precaval. H avzng loc~ted it trace the branches opening mto it and dissect them to the places where from they collect blood. (i) The right precaval receives blood through three prominent veins the internal jugUlar. the external jugular, and the subclavian. The subclavian receives the blood from the region of the fore limb and pectoraf girdles etc, the internal jugular from the brain etc., where as, the external jugular passes forward by the side of the trachea and receives blood from the tongt'e etc. (ii) The left precaval receives blood onh from theinternal jugular and the subclavian. The external' jug"!llar is absent in this case. (I n the gecko we found the external jugular veins' on both the sides running on either side of the trachea and posteriorly opening in the precavals of both the sides.) . Remove the peritonelt1n covering the posterior vena cava. Pttsh the 1'iscera towards one side. Be carefttl not to break tfte hepatic portal and the anterior abdominal ~'eills while doing so. (iii) The posterior vena cava. The renal yeins collecting blood from the substancp- of the kidneys uniteto form a pair of large trunks, each running along the inner border of kIdneys. Both these unite a little distance ahead of the kidneys to form the median postcav.1l, which· runs forwards and opens into the sinus venosus. The two efferent renals forming the postcaval anteriorly are continued behind the posterior t'dge of the kidneys and THE LIZARD 59 meet in the median line forming a sort of loop. The postcaval receives two hepatic veins from the liver before entering the sinus. (iv) The Portal Systems. a. The Hepatic Porta] System. 5 pread the fold of mesentery supporting the duodenum and the stomach as fully as possible, without injuring the delicate vessels. The main trunk of this system takes its oflgm· posteriorly from two very prominent veins lying one on· each ,side of the rectum. These two originate directly from the posterior end of the renal portal vein. A number of small veins, such as the gastric,. pancreatic, splenic and mesenteric collect blood from thE' various parts of the alimentary canal and open lnto the main trunk to form a prominent vein, thehepatic portal vein which opens into the liver. (In the uromastix and geckos the hepatic portal is formed only by the union of a number of blood vessels· from the various parts of the alimentary canals.) Do you know what is a portal system ? b. The Renal Portal System. Expose the caudal vein and trace its bifurcation jast at the base of the tail. Dissect out the femoral and sciatic veins. from the leg and trace thezr connectio1ls with the pelvic. 5 pecial care is needed to keep the anterior abd'lminal intact THE LIZARD (i) The caudal vein collects blood from the tail and bifurcates into two renal portal veins immediately on emerging from the tail region. (ii) Each renal portal vein continues forward into the kidneys. Close behind thf' kidney the veins from the hind legs, the femoral and the sciatic, join the rf'nal portal. Still further backwards arises the root of the hepatic portal vein. {iii) The anterior abdominal vein is formed by the union of the two pelvics. After that it bends upwards then forwards, runs over the viscera and opens into the left Jj'Ter lobe. The pelvics do not arise directly from the caudal 01' the renal portal vein, but ar,' formed by a number of small vessels from tlwir side. They do have indirect connections with the renal portal. (In geckos and the uromastix the pelvics directly arise from the C'1udal and they receive the femoral and the sciatic veins from the leg before curving upwards to form the anterior abdominal .vein.) Make a neat diagram of the venous system. D. The Arteries. Both the right and left systemic aortic arches persist in the lizards, but the important one is the right arch as it gives rise to ~he carotid and subclvaian arteri ~S. Locate the arches and trace them forwards. Some of the veins may prove bothersome. They can be safely removed to expose the artenes as they h:we bem examined ani sketched. THE LIZARD Gf .(i) Tho right systemic arch arises along with the left systemic arch from the left. part of the ventricle. (:1) The carotid arterips arise from the right aorta and each runs parallel with the corresponding aort,ic arch for a little distance, tllen curves forwards and runs along the sIde of the trachea. Before entering the head It divides into the internal and external carotids. lIn geckos and other lIzards the carotid is connectl:!d with the systemic behind by means of ducts caroticus tHst where it turns towards the anterIOr side). (b) The subclavian artpries arise from the right aortIC arch hefore it meets the left arch. Both the fIght and left subclavian take opposite courses and supply respective fore-limbs. (ii) The left systemic arch does not giverise to corresponding blood-vessels. (iii) The Dorsal Aorta. Both the aortic arches curve backwards around the oesophagus then run dorsally and meet each other in the middle forming a median blood vessel, the dorsal aorta. The dorsal aorta runs straight behind giving a number of branches, and goes up to the tail rpgion where it is known as the caudal artery. Other brll,llches of thedorsal aorta are as foll\nvs: (a) The coeliaco-mesenteric is the first median artery arising from the aorta just after it~ formation. It divides into an 62 THE LIZARD anterior branch or the coeliac artery and a posterior branch, the anterior-mesenteric. The coeliac supplies, by means of narrow branches, the stomach, the pa,ncreas and the liver. The anterIOr mesenteric supplies the intestine, spleen and cloaca etc. (b) The posterior mesenteric is the second median artery arising from the dorsal aorta a little distanC'e behind the origin of the coeliaco-mesenteric. It supplies the large intestme. genital arteries are a pair of blood vessel'! arising from the aorta and supplying the genital organs. In the mal.~s they are called the spermatic arteries tmd in the female the ovarian arteries. (e) The The renal branches arise from the aorta as a pail' of delicate vessels just before the aorta enters the region of the kidneys and supply the kidneys. (e) The iliac arteries are two arising from the aorta in the pelvic region and supplying hind-limbs. Each iliac subdivides into a femoral artery and another sciatic artery supplying the outer and inner margins of the leg. (f) The intercostal branches are 11, series of pairs of delicatf' vessels arisinf from the aorta that supply the intercostal muscles in the region of the ribs. Sketch the arterial system. THE LIZARD 6. THE RENAL AND REPRODUOTIVE SYSTEMS. A. The Urinary System. Remove the peritoneum covering the kidneys and cut the pefvic g~rdle in the middle to expose the kidneys Jully. (i) The kidneys a,re two dark-red bodies situated in close contact with the dorsal wall of the posterior portion of the body cavity. Each is roughly divided ink) two lobes. Their posterior tapering portion is almost in contact with each other. (ii) The ureters are delicate ducts arising from the kidneys and opening posteriorly in the cloaca. (iii) The urinary bladder is a median thin_walled sac opening into the oloara ventrally. _E. The Reproductive System. (a) The Male. (i) The testes are two oval white bodies situated in the body cavity quite' ahead of the kidneys. Note that both the testes are not on the same level. The right testis is anterior, situated just behind the right lobe of the liver; whereas, the left testis IS situatei posterior to this on the left side. ,(ii) The mesorchium is the fold of peritoneum that k3eps each testis attached to the body wall. -(iii) The epididymis is on the inner side of ea ch testis and is formed of a convoluted mass of narrow tube'_ Each continues 64 THE LIZARD behind as a narrow convoluted tube, thevas deferens which runs posteriorly and joins the ureter of its side before openinginto the cloacal chamber. Thus, the ureter and vas deferens open by a common' aperture. (iv) The copulatory sacs are two eversible hollow sacs opening into the posterior wall of ,the cloaca. They are made l1p of erectile' tissue when everted they become cylindrical. A groove runs to them from the opening of the vas deferens for the conveyance of the spcrm. (v) The Cloaca. ,Cut open the wall oj the cloaca Jrom one sid!' and turn the ventral wall towards the other slde. Internally the cloaca is divided into two chambers by a ridge of the dorsal waH. The ractum and the urinary bladder open into the anterior division of thl) cloaca, whereas, in the posterior division are the openings of the urino-genital ducts. The copulatory sacs ca.n also be seen a330ciated wit~ the posterior wall of the cloaca. (b) The Female. (i) The ovaries are irregular shaped organs. situated in the posterior part of t e body ca.vityanterior to the kidneys (not as much. as the testes'. Their upper snrface seems studded with rounded protuberances representing the ova. (ii) The mesovarium is a fold of peritonenm that keeps the ovaries attachedl to the dorsalt wall of the coelom. 65 THE LIZARD (iii) The oviducts are two thin-walled tubes whose walls are mnch folded internally. Anteriorly the tube is wider, much 'plaited and opens by a longitudinal opening facing sideways. They run posteriorly and open, independent of the opening of the ureters, into the posterior division of the cloaca. (iv) The broad ligament is a fold of peritoneum that keeps the oviduct of each side attached to the body-wall. $ (v) The cloacal chamber is similar to that of the male but for the fact that the openings of the urinary and genital ducts are sepa. rate, and that the copulatory sacs are not developed at all (as in .gecko) or are rudimentary. Draw sketches of the male and the female generative organs separa.tely. 7. THE SKELETAL SYSTEM. Preparation of the Skeleton. Thc most prevall:'nt method of preparing a skeleton is by boiling, but the method has it,s disadvantages. Boil. ing separates most of the small bones particularly those of the carpus, digits etc., and oftl'n bones of the skull, and besides by this method the skeleton is not perfectly white. The best method to be recommended for student work is by maceration. Take a fairly large-sized animal, after skinning cut 0 if the flesh covering the bones. This should be done very carefully in thf; region of tlte ribs, sternum, hand and foot. After cleaning as much as possible transfer it to a small trough containing lime water and leave it overnight. Take it out 5 66 THE LIZARD next morning, dry the bones with a duster and scrape off the bones slowly and carejully remove all the traces oj mzescles and tiss~tes. Again steep them in lime water, overnight, to sojten the tisszees, and clean the bones as much as possible next morning. For this and jurther cleaning it is better to 1tSe an old tooth brush. Repeat the process till the bones are completely cleaned and jree oj the offensive smell. For final dipping a clear solution of bleaching powder can be 1tsed. Bleaching in this case is done by dryi1'g the thus cleaned skeleton in the sun, for a lIulnber of days. This is definitely a long process but a better one. A. The Axial Skeleton. The vertebral column consists of a large number of vertebrae usually divided into cervical. thoracic (thoraco-lumbar), sacral and caudal regions. (a) The Cervical Region. (i) A typical cervical consists of a stout cen· trum which is hollow in front and convex: behind i.e. procoelous. The neural arch bears a prominent crest·like neural spine. Anteriorly it bears articular facets, the prezygapophyses that are directed upwards, and posteriorly other set of articular facets, the post zygapopbyses, that are directed downwards. Posteriorly the centrum bears a backwardly directed process, the hypapophysis on the ventral surface. All the vertebrae behmd the tbird have aJt;l!ular sur· faces for the cervical ribs. (ii) The atlas or the first cervical is ring-like having no distinct centrum. Tnfact the ring is made up of three bony pieces, one ventral occupying the place of the centrum and two THE 67 LIZARD dorso-lateral arching over the Yl'ntral. The latter two do not meet in middle though they appear so, as the space in between them is filled by membrane. A transverse ligamentous band is seen dividing the ring of the atlas int) two. Ventrally on its anterior face is found an articular facet for the occipital condyle. c D Fig. 5.-A, anterior view of atlas; B, lateral view of axis; C, lateral view of 3 typicnl cerviraJ.; and D, posterior view of a caudal vertebrae of the varanus. (iii) The axis or the second cervical vertebra is just like a typical cervical il} structure, but it is slightly larger and bears a hook like l)rooe88, the odontoid process, on the ventral surface of the centrum towards the anterior end. The pointed ~ip of the process is directpd inwards towards the hypapophysis). The neural spine is a large plate like structure. The pre-zygapophyses are represented by mere notches, whereas, the Ipost..zygapophy8es are as usual. 68 THE LIl~ARD (b) The thoraco·lumbar vertebrae are like thetyPical cervical, as described above, but for the fact that they are somewhat larger and that each bears prominent articular facets, the capitular facets, at the junction of tne centrum and the neUTaf arch, for the articulation of the ribs. ThE' first few thoraco.lumbar vertE'brae also bear hypapophysE's ventrally which go on dwindling in size, and finally disappear. (C) The sacral vertebrae are two in number and articulate with the ilia of the pelvic girdle. They are, therefore, very stont. The c{'ntrum in each case is stout and short and 'bears strong expanded transverse proces· ses The tips of the transverse processes of the anterior vertebra are deeply notched to provide articulation space for the ilia. • Fig. 6.-Lateral view of the sacral vertebraE' of the varanus. Note the nature of zygapophysE's. (d) The caudal vertebrae are numerous and becomegradualy smaller towards the posterior side. so much so, that at the end of the .tail the whole vertebra is just a bony red. The anterior caudal vertebrae hav(· relatively longer centra, slender transverse processes and longerneural spines. These vertebrae have Y shaped chevron bones attached to the ventral' Burfaces of the centra. Twoupper limbs of Y articulate' with the' centrum, thu8 r enclosing the haemal canal .. THE LIZARD {.e) The Ribs and Sternum. (i) The ribs are slender curved rods. They are singlA headed and are attached to '-the vertebrae between the centrum and the neurqI arch. Excepting the firilt three vertebrae, all preeaudal vertebrae bear ribs. The cervical ribs are short rods not conneded with the sternum. The thoracic ribs are relatively larger and are connected with the sternum by cartilaginous sternal ribs in the first three thoracic vertebrae. The fourth carries a little cartilaginous rod attached to its free -end representing the dwindling sternal rib. In other vertebrae the ribs are short and free ventrally (not connected with the sternum). {ii) The sternum consists of a rhomboidal plate of cartilage from the posterior end of whicl). articulate two sternal ribs. From its posterolateral edges articulate two sternal ribs on each side . • Anteriorly the apex i'l drawn out into a long narrow rod of cartilage, the inter-clavicle which ends forming a Tshaped structure. Tho: vertical limb of T continues on the ventral Fig. 7.-The sternum of the varanus. 4) 70 THE LIZARD surface of the sternum; on the small anterior arms of Trest two clavicles of eithersides. The bones of the pectoral girdle articulate with thenutero-Iateral borders of the rhomboidal plate. B. The 81\:ul1. The skeleton of the head, like that of the dog-fish, consists of the cranium proper, the olfactory and auditory capsules, and the visceral skeleton, but the viscl:'ral skeleton, including jaws, hyoid bone and auditory ossicles is so closely associated with other parts that it is convenient to describe all these parts under 'skull'. (1) The cranium proper forill8 the posterior part of the skull that lodges the brain. [n the varanll::l, as in othE'l" lizards, some important bones of the craniuUl, such as the Itlisphenoids, orbitosphenoids and the pl'esphenoids, are missing. In th.;;) birds and mammals these bones form a. major part of the cranium ventrally. ThitJ part of the brain box is formed by membrane8 in the varanus. Pick up the skull in your left hand. Examine the bOlles of the branl-case starting from the posterior side. (a) 'rhe occipital se~ment forms thp po;;tel'iormost part of tIl(> brain ('USl around the foramen magnum. It ('ollsists of foUl' boneR which fuse inseparabI~'. Of theRe the baSi-occipital fOfIllti the base, the exoccipital the sides and the supra-ocdpital forms t,h(> roof. ThE' ba~i-o('('ipital carrieR the occipital condyle mainly fornwd by it. (b) The parietal segment ('olltihts of: (i) a broad basisphenoid with two prominent 71 THE LIZARD « ..J ...J i:. X ::s ~ c:( c:;J ;..; c:;J ~ ;;.. <l) ...=: .." '+-< 0 ~ ..;:: '" <l) ...=: .." '+-< 0 , i!; '\~~ iit.!S -' ..J ~ ~ o III c:( - 0. III ~ 0 .t( '<\, Z. .., ~ <l) ~ a. I- ,_~ Wo uJ« ~ ;J If) c( 0-1/) :E:I/) ~~ 0- ~ '" :;, 1 \ I \ \ II) Oo! UJ ..., , ,;:) "'~" ...I .( d. .J a: 4. ] If) ex 1 z-Z o!:Z: J "" ~ 0.. ::> o! 0 4. « ,.J ii5 ex: .( .;;dO '" i,... ...I ::E. ...=: ..JZ "'" «Ill ~ ~rt: "0 >= c:;J 4,0 e-u. ~ .J .:; a. ~ Q) .« !: OJ jj ;..; 0 \,) 2 « "0 Q) ...=: -+" -<tl I .c: 2 0 tJ! "" 00 tl() 1£ 72 THE LIZARD processes at its antero-Iateral sides, forming the base, and; (ii) the parietals forming the roof. The two parietals are fuscd completely in the adult even a suture is not visible. The parietals are narrow posteriorly and broad anteriorly. A pineal foramen is found on the parietal bituated a little interior to the sut,ure of the parietal with the frontals on the mid-dorsal line. The sides of the parietal segment are not covered by bones, as in higher fl.nimals, because the alisphenoids are ab!:!ent and the membranous tissue that takes their place is removed while cleaning the skull. Nete: A careful examination reveals that tIle parietals are not joined with the supra-occipital by "uture but there is a distinct gap which is filled up by fibrous tissue that is removed during cleaning. The fronto-parietal part of the skull that can be seen separate is slightly moveable upon the occipita-sphenoidal part. se~ment is incomplete. It is without a base as the presphenoid is absent, and without sides even, as the orbitosphenoids are absent. These are replaced by membranous tissues. The roof, however, is formed by the paired frontals that are suturally fused in the middle line. Posteriorly the frontals are s6parated from the parietals by a distinct suture, the coronal suture. (c) The frontal (2) The olfactory capsule is dorsally covered by the nasals and floored by the vomers. The nasals are flat triangular bones fused in the middle. The rod-shaped THE LIZARD 73 vomers are two separated posteriorly but fused with each other anteriorly. They form the inner margin of the posterior nares. (3) The auditory capsule usually consists of the opisthotic, epiotic and pro-otic, of which, in this animal, the epiotlc is fused with the supra-occipital the opisthotic is fused with the ex-occipitals, only pro-otic remains separate. (4) The suspensorium. (i) The parotic process is formed by the prolongation of the exoccipitals and pro-otic. Each is a horizontal process directed outwards and slightly backwards, on either side of th\' foramen magnum, and articula.tes with the squamosal and the quadrate. \(ii) The squamosal is a short bony rod bending forwards and upwards, attached to the parotic process posteriorly. Anteriorly it extends forwards to the post-frontal (except in geckos) forming the supra-temporal arcade. (iii) The quadrate is rod-like bone attached to the anterior aspect of the parotic process directed forwards and downwards articulating with the pterygoid. (5) The temporal region. (i) The posterior temporal fossa is a vacuity guardI'Jd ventrally by the parotic and dorsally by paricto-squamosal or posterior tempora! arcade. The parietal sends out a proctlss posteriorly which fuses with t he squamosal forming the parieto-squamosal. 74 THE LIZARD (ii) The supra-temporal fossa is the cavity between the supra-temporal arcade (formed by the union of the sq uamosal and the post-frontal) and the parietals. (iii) The lateral temporal fossa is bridged over by the supra-temporal and its posterior" margin is formed by the quadrate. Ventrally it is open as the quadrato-jugal is absent. and no infra-temporal arcade is formed, and anteriorly, also, the fossa is hot closed as the jugal is not joined with the post-frontal. Thus, the orbit and the fossa communicate. (6) The orbits are large and closely approximatedl being separated by only a thin vertical interorbital septum. Dorsally it is roofed over by the fronl.als and' anteriorly it is bound by a small bone pre-fJ"orl.tal and another perforated bone, the lacrymal. Posteriorly the orbit is incompletely bound by the post-frontall (dorsally) and jugal (from ventral sidl'). Another triangular bone, supra orbital over-hangs thl' orbit. Its flat base is attached to the anterior angle of the orbit formed' by the pre-frolltals and the lacrymal, while the pointed free end is directed towards the post-frontal. (7) The bones of the maxillo-palatine appamtus (i) The premaxillae are fused in the middle-" forming the antenor extremity of the snout. The median bone, thus formed, bears six to eight small te('th. (ii) Th(' septomaxillary is a small bone OIl! each side just above the vomer in the anterior part of the lGasul region. (iii~ The maxillae are a pair of irregular bone~ forming a large part of the skeleton of the 75· THE LIZARD upper jaw. Its body is called alveolar' portion as it bears teeth. The palatine process is not well developed in this' case, thus, leaving space between the maxilla and palatine. The maxilla bpars conical teeth ankylosed to the bone by their sides just, inside the edgp.s of the jaw (pleurodont). (iv) The palatines are irregular bones and pass. backwards from the vomers and thfr maxillaries to the pterygoids. (t) The pterygoids are again irregular bones, which, diverging from one another, extend backward to tIle quadrate articulating in the way with the lateral processes of the basi-sphenoids. Extending between the pterygoids (below) and the parietal above of each side there is a rod shaped bone called the epipterygoid which is also called 'Columella cranii. (To avoid confusion with the columella auris it is better to call it epipterygoid). (vi) The transpalatine is a small bone pllssing from the pda.tine and pterygoid to meet at the junction of maxilla and juga]. (8) The lower jaw consists of two rami each made up of six bones the dentary, angular, supra~angular, artiCORONOID 0'":""' A.::;HNr.UL•• ~"~b ,. ... ANGULAR AP.TICUL.A~ SPLf.NIAL Fig. 9--0ne ramus of the lower jaw of the Varanus. THE LIZARD ... eular, coronoid and the splenial. Of these the dentary is the largest, constitutlllg almost the anterior half of one of the rami of the lower jaw bea~; the pleurodont teeth. The posterior half of the ramus consists of a triangular ,coronoid with a backwardly direeted process, at the top; the supra-angular, a flat elongated bone in the middle, .and the articular and angular below; the angular is a. small bone wedged in betweep. the articular and dentary. The s'plenial is found lodged in a groove on the inner face of the dentary, from outside only a narrow slip is seen below the dentary. (9) The hyoid apparatus consigts of an elongated body or the basihyal and two long cornua on each side. The-", anterior cornua are elongated cartilaginous rods that curve inwards and flatten at their free ends. The posterior cornua are cartilaginous rodB arising from the posterior end of the basihyal, each consisting of two '''ieces. B. The Apendicular Skeleton. a. The pectoral girdle consists of two similar halve~ <lying one on either side of the inter-clavicle. Each half {:onsists of a supra-scapula, scapula and coracoid. (i) The supra-scapula is a flat distally expanded plato made of cartilage articulating internally with the scapula. (ii) Th.e scapula is oblong bone constricted in the middle. Its end articulating with the supra-scapula is broader The posterior side of the lower end forms part of the glenoid fossa. From the anterior side of the scapula, just opposite to the glenoid cavity arises a completely ossified prO( ess, the mesoscapula. ,(iii) The coracoid is a stout bone constricted in the middle and broadly expa.nded at its THE LIZARD 77 inner end. At its out,er extremity the cora-coid also bears a flat process, the meso-coracoid. (iv) The epicoracoid, an irregular shaped cartilage starting from the proxi~al end of the coracoid, runs along the inter-clavicleupto its end, whence it bends back and, runs to the mesoscapula. Before turning back it also gives a process to the mesoco-racoid. Thus, the whole of the epicoracoid appears fenestrated. b. The Fore Limbs. (i) The arm consists of the humerus both theends of which are greatly expanded. Thehead lies at the proximal end and the distal end has pulley like articular surface, the trochlea that articulates with the radiuS' and ulna. A prominent crest like deltoid, ridge L"I also present. (li) The fore-arm consists of two bones of' which the radius is slender consisting of" a shaft and two epiphyses. Distally it has a concave articular facet for the carpus. The proximal end of the ulna is produced into a. upwardly directed olecranon process, while the distal end bears a convex articular surface for the carpus. (iii) The carpus consists of ten polyhedral carpal bones arranged in three rows. 'The proximal row consisting of three, (the radiale, ulnare, and intermedium), the central row 6f one (centrale) and the distal row of five- THE LIZARD smail pieces. Besides these, the pisiform is attached to the distal epiphysis of the ulna on Its post-axial side. (iv) .The di~its are five each consisting of a metacarpal bone a,nd varying number of phalanges. The first digit (pollex) has two phalanges, the second three, thfl third four, the fourth five, and the fifth hl1.s three only. c. The pelvic girdle consist,s of the ilia, pubes a.nd Ischia which form two triradiate bones, the ossa Jnnominata. (i) The ilium is a compres'led rod pl~ssing upwards and backwards to 9.rticdate with the sacral vertebrae. \ii) The pubis on each side passes downwards and forwards to meet its fellow in the middle line at tho pubic symphysis. Between the anterior ends, of the pubis, in front, is a nodule of calcified cartilage, the epipubis. On the other end, near its fusion with the ilium and ischium each pubis has an oval foramen for the obturator nerve. Just external and slightly posterior to the foramen arises the pre-pubis which is a small rod like process directed outward. ,(iii) The ischium runs inwards and meets its fellow at the ischiatic symphysis. Hypo-ischium is a small piece of calcifi('d cartilage in between the two ischia, at the anterior face. THE LIZA.RD 79 r(iv) The obtut'ator foramen is (1 wide space between the pubes and ischia. This is divided into two by a median ligament. The ligament is often removed while cleaning. (v) The acetabulum is a concave articular surface for the hC'ad of the femur. It is situated on the outer side at the point where the three bones fuse. At the base ot the depression the' lines of fUf'ion can be seen ell-arly. ,d. The Hind Limbs. (i) The femur is a stout bone of the thigh. Its anterior epiphysis develops a rounded head that fits into the acetabulum. Neltr the head rather a bit posteriorly on the preaxial side is the prominent lesser trochanter. The greater trochanter on the postaxial side is ncarly obsolete. The distal extremity is pulley-shaped having two condyles for the articulation of the tibia and fibula. ,(ii) The shank or crus consists of tibia and a fibula. The tibia is stout, bone bent inwards (towards fibula), having a longitudinal ridge, the cnemial ridge along the dorsal edge. The fibula is slender bone the proximal end of which articulates with the femur and the distal with the tarsus. ,!iii) The pes or tarsus consists of only five tarsal bones in the adult. Two of the proximal row are suturally fused and articulate with the tibia and fibula. The distal row consists so THE LIZARD of three cuboid bones of different sizes: articulfl.ting with the metatarsals of thetoes. The number of phalanges in each toe is variable; the first has two, the.second three, the third four, the fourth five and the fifth three. CHAPTER ~ 1. A. IV PIGEON EXTERNAL CHARACTERS. The Divisions of the body. (i) 1'he head is elongated antero-posteriorly with it~ rounded cranium and prominent beak formed by the upper and lower jaws covered by horny sheets. At the base of the beak is a nakEd swollen patch of skin, the cere. , The eyes are large and provided with the upper and lower eyelids and a semi-transparent nictitating membr:.:tne. (ii) The neck is long cylindrical and very mobile sharply marked out from the head and the trunk. (iii) The trunk is plump and somewhat compressed from side to side. The' boat-shaped carina or keel of the sternum is raised into a prominent ridge along the midventral line. Posteriorly the trunk is drawn out into a short conical projection, the true tail or uropygium, from wliich originate a group of large feathers, to which the term "tail" is generally applied. B. The Limbs. (i) The wings show three typical divisions of the fore-limbs, the upper arm, fore-arm and 81 6 82 THE PIGEON the hand. The parts of the hand are closely bound together by skin. The several bones can be identified by feeling them through the skin. Between the upper-arm and the fore-arm, on the anterior side extends a fold of skin the alar membrane or prepatagium. A similar fold exists between the posterior portion of the upper-arm and the trunk, this is the post-patagium. fii) The hind limb has a short thigh closely bound to the trunk, a long shank extending from the knee downwards and backwards and a foot clearly divisible into a proximal portion, the tarso-metatarsus, and four digits. / C. The External Apertures. (a) Median A PerillreS. (i) The mouth is terminal guarded by the upper and lower jaws covered by horny beak. It has a wide gape. _ (ii) The cloacal aperture is a large transverse slit situated ventrally at the junction of the uropygium and the trunk. (b) Paired Apertures. (i) The nostrils are a pair of slit-like apertures between the beak and the cere. (ii) The external auditory apertures are two circular openings below and slightly behind the posterior or outer angles of the eyes. THE PIGEON .v;;: 83 The Feathers. Excepting the lower joints of the legs and the toes, nearly the whole surface of the body is covered with feathers, which are specially modified epidermal structures. (Compare with the scaly exoskeleton of fishes and reptiles). To study ·the structure of a typical feather get any, large feather and note the following parts. 'f. The Structure of a Feather. (i) The calamus or quill is the hollow stalk of the f{'ather having at its base a small aperture, the inferior umbilicus. Into this umbilicus fits a small conical prolongation of the skin, the feather papilla. A second, extremely minute aperture is situated at the junction of the quill and the vane on the ventral surface. This is the superior umbilicus. Important: In the pigeon, as in many other birds, the superior umbilicus appears to be covered by numerous hairy outgrowths, the aftershaft, which has to Ibe scrapped off by a scalpel, to see the superior umbilicus situated in a pit. (ii) The rachis or shaft is the distal solid continuation of the quill tapering towards the tip. It is often grooved along its ventral side. (iii) The vane or vexillum is the expanded portion of the feather developed around the rachis as its longitudinal axis. The vane is apparently like a continuous sheetlike structure formed by barbs and .barbules. 84 THE PIGEON Exami1tc the vane under a microscope or strong magniJying lens and note: (a) The barbs are delice~te thread-like structures attached to each side of the rachis and extend obliquely outwards. (b) The barbules are extremely delicate oblique filaments attached to the sides of the barb. They bear hooklets; which hold the adjacent barbs together, and so give the feather t,he power of resistance to the air needed for flight. II. Kinds of Feather. (a) The quill feather are the largest feathers of the wings and tail. The wing -luills are known a,';; remiges and the tail quills are rectrices. (i) The remiges have inner or posterior half of the vane much broader than the outer or anterior half. (ii) The rectrices have the two sidei.' vane almost equal in size. of the Count the number oj these and note. (b) The coverts arc structurally similar to the quill feather but smaller in size. They cover the bases of the quill feathers of the tail and the wings. (c) The contours arc short, and wooly feathersforming a general covering of the body. Tn THE PIGEON 85 these, the barbules are much less developed and the barbs are loose and can be easily separated. (d) The filoplumes are delicate feathers having a hair-like stalk with fine barbs at the top. There is no interlocking arrangement of the barbs. Tht'y are seen scattered on the body surface after the bird has been plucked. (e) The down feathers are soft, wooly and transitory, found only in the newly hatched bird. Each has a short stalk bearing barbs at the tip but no interlocking arrangement is present. '2. DIS~EOTION OF THE MUSOLES OF FLIGHT. Plnck the bird thoroughly particularly the ventral aspect -of the thorax and wings. Lay it on tlte back on. the dissection board and pin keeping at least one wing fully stretched. Remove the skin from the ventral surface of the thorax and from one shoulder and the wing. The bird has one depressor, one elevator, two acce3sory .depressors and four tensor muscles. A. The depressor and elevator muscles. (i) The pectoralis is the large3t of all the muscles, triangular in shape occupying the whole space of the ventral half of the keel of the sternum. Its broad flat tendon is inserted into the deltoid ridge of the humerus. It is the great depressor muscle i.e it pulls the wing to thc lower side. 86 THE PIGEON Cut inwards with a fine scalpel along the mid·ventrcrt line. Go on cutting along the latero-ventral aspect of th' keel to the anterwr side. As yDtt proceed towards the anterior side, be careful to locate certain blood vessels that emerge from the anterwr side of the keel and tztY1t backwards and sideways to suPPly the pectoral muscles. Clean these blood' vessels and ligature carefully. Tum the muscle tozwards the anterior side keeping the tendon in its normal position. (ii) The subclavius is a much smaller muscleand more deeply placed. It is similar in shape to the first and completely covered by it. It arises from the posterior part of the side of the keel of the sternum, on its ventral surface. Its long narrow tendon passes through the foramen triosseuw. (openmg formed by three bones i.e. the scapula, coracoid, and clavicle) and is inserf,.'d into the dorsal surfacl' of the humerus, ncar thedeltoid ridg0. It is the only muscle that elevates the wing, hence, called elevator. Being attached just ltke the pectoralis and being of the same shape how 1S that it flt11ctlO'HS m entirely reverse manner 1 Important: To see the insertion of tIle tendon of th(' subclavius, push aside the deltoid mUbele that covers t,lw back of the shoulder-joint. Remove the pin from the 'Irillg and pull the I1lZtscles Olle: after another to test the actton of the J1ttlscles. B. The Accessory Depressors. (i) The coraco-brachialis longus appears as a triangular muscle immediately on clearing the pectoralis and the subclavius. It arises THE PIGEON 87 rom the posterior two· third portion of the coracoid, and from the costal process of the sternum. Its narrow tendon is inserted to the post· axial aspect of the head of the humerus. (ii) The coraco-brachialis brevis is a narrower muscle infront ot the coraco·brachialis longus. Cut the anterior one-third portion of the coracoid to expose these muscles fully. . It arises from the middle of the antero· dorsal aspect of the sternum just in between the junction of the two coracoids. The COl aco· brachialis brevis muscles of both the sides anse from the sallle place and take opposite courses. Note: In some pigeons, that are specially kept for flight, we found two divisions of the muscle--one going to the dorsal aspect, as described and the other turning round the comer internal to the foramen triosseum to be attached on the antprior aspect of the hUlllerus. C. The Tensor Muscles. (i) The tensor longus is the longest among all the tensor muscles. It arises from the anterior side of carpometacarpus, runs along the outer border of pre.patagium, and is inserted to the front of the head of the humerus. It is very delicate muscle in the beginning and, thus, it often becomes difficult to distinguish. (ii) The tensor brevis is a thin sheet of muscles which spreads between the biceps muscles 88 THE PIGEON and the posterior margin of the proximal portion of the tensor longus. (iii) The tensor accessorius is comparatively broader muscle lying across the prepatagium. All these three muscles help to keep the pre-patagium stretched while flying. (iv) 'rhe tensor posterious is a thin strip of muscles that runs along the outer edge of post-patagium and keeps it fully stretched during flight. 3. DISSECTION OF THE COELOM. The Abdominal Viscera. Lift Ztp the end of the sternum, and cut through its sides about mid-way between the dorsal and ventral surface. Cut through the coracoids about the middle of their length. Try 10 detach the sternum and note. (i) The falciform ligamcmt will be visible as soon as an attempt is made to lift t,he sternum. It is a median vertical sheet of peritoneum connecting the dorsal surface of the sternum with the viscera. (ii) The oblique septum is a fibrous membrane attached to the pericardium infront and separates the air-sacs from the general body cavity. -' After observing these, remove the sternum entirelY. (iii) The heart is comparatively larger; it is triangular in shape with the apex directed backwards and is enclosed in the pericardium. THE PIGEON 89 (iv) The lungs arc spongy bodies lying at the sides of and dorsal to the heart and very largely concealed by it. (They are not Sll.Clike as found in the amphibia and certain reptiles.) (v) The liver is a solid bilobed organ of darkred colour lying behind and at the sides of the heart. The right lobe is bigger than the left. Thorl? is no gall-bladder. (vi) The great omentum is a fold of mesentery loaded with fat covering the viscera behind the liver. It is continuous with the falciform ligament infront. Remove the great omentum and examine the organs lying ilelow. (vii) The gizzard is a large, hard, round body lying behind the left lobe of the liver. (viii) The duodenum is a loop of intestine lying along the right side of the abdominal cavity. (ix) The intestiue forms a convoluted mass in the hinder part of the abdomen . .Push the intestine to one side and examine. (x) The kidneys are dark-red fiat bodies, each divided into three lobes, lying in the posterior portion of the body cavity on the dorsal surface. (xi) The spleen is a dark-red coloured elongated body, of variable sizes lying dorsally to the proventriculus. 90 THE PIGEOJ',[ (xii) The gonads, or organs of reproduction, found associated with the kidneys, will be' described later. 4- THE DISSECTION OF THE DIGESTIVE ORGANS. A. The Buccal Cavity. OPen the mouth and cut about half an inch deep at the angle of the jaws on both sides, turn away the jaws so as to expose the buccal cavity fully. ~i) The jaws are withoJ.1t. teet.h. The posterior naria! apertures are a pair of long narrow apertures lying, side by side, in the roof of the mouth. The anterior two· third of their length are hidden by a pair of prominent folds of the mucous membrane of the mouth. (ii) The apterture of the eustachian tube is a median opening posterior to the nari,11 apertures (iii) The tongue is marow and trianguhu' "ith a sharply pointed tip. Posteriorly it is produced into two lobes fringed with fine horny processes. (iv) The glottis is an oval aperture in t 1e floorof the mouth Just behind the tongue. (v) The gullet is a large wide opening behind the glottis. THE PIGEON B. 91~ Tbe Alimentary Canal. . Unravel the intestine by cutting through the mesentery- close to it, but taking care not to touch tlte dztadenal loop. (i\ The oesophagus- The gullet opens into a long tube, the oesophagus, that runs back through the nf'ck to the stomach. In the neck region it enlarges to form a thin-walled sac, the crop. It again becomes thickwalled as it enters the thorax. (ii) The stomach or proventriculus is the continuation of the oesophagus lying dorsally to the left liver lobe. It is comparatively wider and has thick glandular inner lming that produces the gastric juice. To its right side is attached the spleen. (iii) The gizzard is a large hard body flattened laterally forming a biconvex structure. Its walls are thick and muscular and its inner lining is horny. The cavity is very p,mall and often contains small pieces of stones. Why? The oesophagus opens into the gizzard dorsally, slightly to the left side, while the duodenum leaves it a little tothe right side of the same spot. e1ft open the gizzard along its ventral edge and examine. (iyJ The duodenum is a V-shaped loop formE'd by the beginning of the intestine. Between its two limbs lies the pancreas. (v) The small intestine is a convoluted tube of uniform diameter. It is the longest part of the alimentary canal. Exie1ld it ampletel)" 11'eaSZlre and nete tis length. THE PIGJiJON (vi) The rectum is the terminal, portion of the intestine. At its junction with the intestine are situated two coeeal processes, the rectal diverticula. The rectum opens in the cloaca posteriorly. {J. The Gland Associated with the Digestive System. (i) The liver has been described earlier (page 89). Of t4e two bile ducts, the left opens into the proximal limb of the duodenum and the right opens into the distal limb . "\ The pancreas is It compact pinkish gland (II, lying in the loop of the duodemlm. It:> three ducts, the pancrel}tic ducts, 0pen into the distal limb of the duodenum. Pin out the dttodenum on a dissecting bourd, and expose the bile and pancreatic ducts. For better results do it ttnder water. 5. THE DISSECTION OF THE CIRCULATORY SYSTEM. A. The Heart. The large heart of the bird is four chambered-the aerated blood is completely separated from the nonaerated. The dissection of the heart will be deferred to a later stage. The course of the blood shows that there is complete .double circulation. The blood from the entin' 1)odv comes to the right side of the ht'art, and thence driw to the lungs to be aerated; lungs send blood to the left side of the heart which derives it to all parts of the body. n OPen the pericardial cavity and expose tlte heart completely. THE PIGEON VEINS ARTERIES JUGULA R----- -- INTERNAlILlA(_(OCCYGto- MESENTERIC _______ ~~~:...._ CAUDAL~----- ____ ~ -- _- - - - CAUDAL Fig. JO.-Thc circulatory systt'm of the pigeon. B. _The v:.~il1s. As the veins lie, marc or less, superficially it is convenient to study them first. Veins are also thin-walled as such, great care sholllcl be taken in clearing them. '94 THE PIGEON I. The venae cavae are three, all of which open into -the right auricle, where they bring all the venous blood .of the body. (a) The right anterior vena cava is a small thin-walled vessel that collects blood from the right side of the head, the neck, the right wing and the right side of the breast through: ,(i) The right jugular vein that runs a.long the right side of the neck, close to the vertebral column and alm08t behind the oesophagus, On the under surface of the base of the skull, it un\tes with its fellow of the left side forming a loop. During its course it receives veins from the sides of the neck, the crop and the vert~bral colnmn of the region by the oesophageal and tl.c vertebral veins. (ii) The right brachial vein returns blood from the right wing. -(iii) The right pectoral vein returns blood from the large pectoral muscle. It also receives a delicate internal mammary vein, that runs along the inner surface of th~ thoracic wall, before entering the precavaL . (b) The left anterior vena cava is a corresponding vein of the left side receiving blood through the blood vessels corresponding to those of the right side. ,(c) The posterior vena cava is formed by the union of the two iliac veins a little behind the liver. In the region of the liver it receiv{'s the hepatic veins. 'IRE PIGEON 95 To expose this blood vessel the heart has to be tltmed ;upwards. II. The V ei~s of the Posterior Part. (i) The caudal vein is 11 small narrow blood vessel collecting blood from the tail region. (ii) The hypogastric veins are formed by the bifurcation of the caudal vein. Each passes onwards over the lddneys, sending off a few brunches to the kidneys (carrying blood to that organ), 8nd receiving a delicate sciatic vein between the second and third lobes of the kidney, n,nd a prominent, femoral vein, returning blood from the leg, between the first and sec-ond lobes of the kidney. ,(iii) The iliac vein, formed by the union of the femoral and the hypogastric, leaves the kidney at its inner border. between its anterior and middle lobes, and then runs forwards, joins its fellow of the other side and forms the posterior vena cava. ~iv) The internal iliac veins are a pair of small vessels, each joining the hypogastric coming from the inner surface of the pelvic cavity. (v) The coccygeo-mesenteric vein arises from the caudal vein before it bifurcates into the hypogastrics. 1t run\) parallel to the rectum and intestine, to a little distance and opens into the hepatic portal. {vi) The posterior mesenteric vein returns blood from the hinder part of the body 96 THE PIGEON cavity and mesentery etc. into the coccygeomesenteric just near its root. (vii) The hepatic portal vein. Spread the fold of mesentery, the great omentum, supporting the hinder part of the mtestine carefully and note the fine blood vessels. The hepatic porta.l vein is formed by a number of narrow blood vessels coming from tlle various organs of the digestive system On reaching the liver it bifurcates into two branches, one going to eaClh lobe. The blood. from the greater part of the alimentary canal is conveyed to the liver whence it goes to the heart. Ill. Veins openin~ into the left auricle. The pulmonary veins are the only two very ",hort blood. vessels that bring blood back from the lungs and open into the posterior surface of the left auricle. What type of blood tS returned by these? C. The Arteries. I. The rigbt aortic arch is the only arch arising from the .left ventricle. It curves· over the right, branches and passes backwards as the dorsal aorta. Immediately after origin it gives rise to two, right and left, innominate arteries which appear more prominent than the aorta itself. (a) The left innominate artery is a prominent, though short, art!:'!'? and during its course it gives ['iee to the following branches. (i) The left carotid al'trry arises after about a quarter of an inch of its course. It runs- 97 THE PIGEON anteriorly bending towards the mid-ventral side and finally it enters a groove on the ventral side of the vertebral column of the neck. In the same groove, lies, close to its side, tl:c carotid of the right side. About an inch after its origin below the thyroid gland it gives rise to a short vertebral artery supplying the vertebral column, a oesophageal artery supplying the crop and the oesophagus; and a long prominent cutaneous artery supplying the skin and muscles of the region of the neck. About an inch posterior to the head the carotids leave the groove and move forwards and sideways, and opposite the angle of the jaws each divid~s into two. The ou~er branch, the external carotid supplies the tongue, the muscles of the jaws and other parts of the head; whereas the internal carotid artery supplies the br~in. ,ii) The subclavian artery is the largest to arise from the innominate' it runs forward just a little and divides into two: (1) The brachial runs straight outwards to the :wing giving a distinct, though delicate, branch to the shoulder girdle. (2) 'l'he pectoral artery is a large and' -'prominent vessel that turns towards the posterior side and looping round the antero-lateral region of the sternum, enters the great pectoral muscle, which it supplies. Ib) The right innominate gives out similar branches. exactly 7 THE PIGEON (c) The dorsal aorta. Push tlte viscera to one side and cut off the peritoneal covering to expose the aorta. Be careful ?wt to C1t~ ale arteries supPlying tlze mesentery. The aorta arches over to the right side and rUIl>! backwards crossing the right pulmonary. Then it runs backwards in the mId-dorsal line giving off the following branches : (i) The coeliac artery, a median vessel, arises just as the aorta emerges below the ventricle. It supplies the stoma ch, gizzard and part of the intestine. (ii) The a!1terior mesenteric arises about a quarter of an inch posterior to the coeliac artcry, nms back into the mesentery and supplies tllP intestine. (iii) The anterior renal arteries are a pair of small vessels arising from the aorta, a little anterior to the first lobe of the kidneys. Before entering the substance of the anterior lobe of the kidney each gives off a branch going to the testes in the male, and in the femate only the one on the left side gives off a branch supplying the ovary. (iv) The femoral arteries are a pair of prominent blood vessels arising opposite the anterior lobes of the kidneys. Each passes outwards in between the anterior and middle lobes of the kidney, and supplies the muscles of the thigh. Before emerging through the pelvic girdle each gives rise to a delicate, though quite long blood vessel, the pubic THE PIGEON 99 artery running along the ventral borner of the pubic bone. .(v) The sciatic arteries are a pair of prominent vessels arising opposite the middle lobes of the kidneys and passing outwards between the middle and the last lobe of the kidneys. After passing through thO":l ilio-sciatic foramen each supplies the muscle of the thigh on the inner side. ./ (vi) The middle and posterior renal arteries are two blood vessels arising from the sciatic on each side. The middle renal supplies the middle lobe of the kidney and the posterior renal supplies the posterior lobe. ",'vii) The posterior mesenteric artery is again a median artery supplying the rectum and duaca and arising from the aorta opposite the hil}der end of the kidney. (viii) The internal iliac arteries arise almost at the same place as the posterior mesenteric and runs behind to the hinder part of the pelvis. ~ix) U. The caudal artery is the terminal portion of the aorta going to the caudal vertebrae and the pygostyle. Dissection of the Heart. Remove the heart by cutting 0if the vessels about a Dissect it under water. First slit ztp the outer walt of the auricles with scissors, wash out the contents and examine the following : 1Juarter of an inch from it. 100 A THE PIGEON The ri~ht auricle. (i) The openings of the venae cavae are threelying close to each other. (ii) The eustachian valve is a muscular fold on the right side of the opening of the pos· terior vena cava. (iii) The septum auricularium, the interauricular septum, is the thin muscular partition between the two auricles. It bears an oval' patch, the fossa ovalis, at the position. occupied by the foramen ovale in the em· bryonic stages. (iv) The ri~ht-auriculo-ventricular aperture is the crescentic opening of the auricle into' the ventricle. B. The left auricle_ (i) The openings of the pulmonary veins are in a small recess of the dorsal wall of the auricle. (ii) The left-auriculo-ventricular aperture is the circular opening of the auricle into the left \-eutricle. C. The Ventricles. Cttt transversely across the ventricles, abozlt the tiP, and examine. Expose tlte ventricles fttlly later on. (a) The right ventricle is thin-walled and wraps around the left ventricle. In transverse section it appears crescentic, lodging the left ventricle in its concavity. THE PIGEON 101: {i) The auriculo-ventricular valve formed of two muscular flaps is connected to the ventricular wall at the base of the origin of the pulmonary artery. {ii) The aperture of the pulmonary artery is a. t the extreme anterior end towards the left side of the ventricle. Guarding the entrance 01 the artery are three semi-lunar valves. (b) The left ventricle has thick spongy walls enclosing a somewhat circular cavity. It carries: (i) The left auriculo-ventricular valve {the mitral valve) consisting of three muscular flaps projecting into the ventricle. (ii) The aperture of the aorta sitnq,ted at its anterior end slightly towarris its right side. This aperture is also guarded by three semi-lunar valves. ,6. RENAL AND REPRODUCTIVE SYSTEM. A. The Male Pigeon. T. The Renal System. (i) The kidneys are two, each of which is divided into three lobes. They lie in the body cavity behind the lungs a,nd :He covered over by the peritoneum. (ii) The ureters are straight narrow tubes each of which arises from the anterior lobe and emerges between the anterior and middle 102 THE PIGEON lobes. Thence, it runs backwards along th0' inner side and ventral surface of the middl0' and posterior lobes to the dorsal wall of thfr cloaca. (iii) The adrenals arE' a pair of elongated yellowish bodies attached to the anterior ends of the kidnE'Ys. II. The Re productive System. (i) The testes are a pair of oval bodies lying in cont~ct with the ventral surface of the anterior lobes of the kidneys. (ii) The vasa deferentia arc a pair of tubes with opaque walls emerging from the inner sides of the posterior ends of the testes. They run back along the outer edges of the ureters. In the breeding season their hinder ends are swollen to form the seminal vesicles, just before they open into the cloaca. (iii) Thc cloaca is a tubular portion divided internally into three compartments. Cut open the ventral 22'all of the cloaca and examinlJ carefully. (a) The coprodaeum is the anterior portion that is pushed towards the ventral side. Tho rectum opens into this port,ion. (b) The urodaeum is the next compartments into which oprn the urinary and genital ducts. c) The proctodaeum is the last portion opening externally. A thick-walled glandular THE PIGEON 103 vouch, the bursa Fabricii, that lies against the dorsfi,l wall of the cloaca (in young birds only), opens into the pro0todaeum. R. I. IT. The Female Pigeon. The .renal system is the same as described above. The Reproductive System. (a) rrhe ovary is only one on the left side in the adult. (In the embryonic stage both the ovarks are present but in the adult the right one atrophies). It is an irregular tlh!L}led body lying intI'ont of the anterior lobe of the left kidney. It is loaded with ova. (b) The oviducts are two, the left of which, being functional, is wide convoluted tube. Anterior·y it becomes, t,hin-walled and opens , by a long oblique mouth, forming 'a sort of funnel lying in close contact with the ovary. Posteriorly it opens into the cloaca. The right oviduct is abortive. 7. DISSECTION OF THE BRAIN. Remove the skht covering the head completely, with a sharp scalpel, scrape off the bones nf the r60f and slowly slice off the bones bit by bit. Good care must be taken as -",'ou proceed with the dissection. The soft brain lies just below the bmy roof. It may be injured by slight carelessness. After exposing the brain fully expose about half an £nch of the spinal cord by removing the neural arches of the first few vertebrae. Cut 0if the nerve roots and pic~' 11 p the brain. Place it in a watch glass and examine. .tQ4 THE 1'1OEO.N If the brain is soft enough for study place it in spirit or a mixture of absolute alcohol and acetic acid for at least twenty four hours or till it becomes com pletely hardened. A. The Brain. T. The dorsal view. (a) The cerebral hemispheres are a pair of large bodies occupying the median position. Their surface' is smooth. They extend backwards to meet the cerebellum and due to their growth press the optic lobes outwards, which becoma lateral instead of dorsal. (b) The olfactory bulbs are extremely small bodies projecting forwards from the anterior end of tho hemispheres. (c) The optic lobes are two smooth ovoid bodies at the sides of the brain. (d) The pineal body is a small oval body immediately behind the hemispheres lying in a space between their median posterior angles. (e) The cerebellum is of great size and has a large median portion and two small lateral lobes, the flocculi. II. (f) The medulla oblongata is thick ani wide and shows ventral flexure. I t continues into the spinal cord. The ventral view. (a) The optic chiasma is a crossing formed by the optic nerves lying between the optic lobes, slightly towards the anterior side. THE PIGEON 105 (b) The infundibulum is a small median process immediately behind the optic chi~sma. B. The Ventricles of the Brain. Dissect the brain into two along the mid-ventralliue by :means oj a sharp blade. The various cavities are very .narrow and represented by almost simple grooves. (a) The diocoel or the third ventricle is the cavity of the diencephalon, which does not appear externally due to the growth of ·the hemispheres and the cerebellum. It is a narrow vertical cavity bounded laterally by the optic thalami and opening infront by the foramen of Monro into t.he paracoels or the ventricles of the hemispheres. (b) The optocoels are th~ cavities of the optic lobes opening in the centre into a narrow passage, the iter, which is the cayity of the mid-brain. (c) The metacoel or the fourth yentricle i8 the cavity of the hind brain. S. THE SKELETON OF THE FOWL. Owing to its larger size the fowl's skeleton is generally for study in the class. Therefore, a description of the skeleton of the fowl is given below. In fact the .differences between the skeletons of the two animals .are of a minor nature, and the following description will practically apply to either. .~iven 1. T.he Axial Skeleton. The axial skeletons consists of tho vertebral column ..and the skull. ~OH A. THE PIGEOX The Vertebral Column. There are three characteristics of the vertebral column' of a bird (i) the neck is long, being eqnal in length to· the- remaining length of the column, (iiJ the trunk region is rigid because the thoracic yertabme are fused, (iii) the shortness of the tail; the number of vertebrae itself is· short while the posterior ones fuse to form the pygostyle. The vertebrae of a bird, and for that matter the entire skeleton, is lighter as compared to the correspondingbones of other animals. (I) The cervical vertebrae are the vertebrae of theneck region. In the birds it is difficult to distinguish between the thoracic and the cervical vertebra. The usual practice is to regard the first vertebra having itt~ ribs articulating with the sternum as the first thoracic vertebra; all those anterior to it are the cervi..:als. Cotmt the 1mmber of the cervicals in a mounted specimen' and note. (a) A Typical Cervical Vertebra. (ii The vertebra is long as compared to the vert~bra of the varanus. (ii) The centrum is slender having saddleshaped surfaces, the anterior one being concave from side to side and convex from a hove downwards, while the posterior face· is convex from side to side and concave from above downwards. (iii) The neural arch is shorter than thecentrum and is notched in the middle line both anteriorly and posteriorly and the neural spine is rudimentary. • (iv) The tt:ansverse processes are fused with the cervical ribs forming a sharp back- THE PIGEON 1077 wardly directed process having a foramen at its base, through the foramen passes thevertebral artery hence called ycrtebrarterial canal. (v) 'rhe zyapophyses or are in two sets: the articular facets The pre.zygapophyses are flat articular facets on the anterior side, facing upwardsand inwards. The post-zygapophyses are the posterior articular facets projecting backwards from! tho posterior border of the neural arch, and facing downwards a.nd upwards. (b) Special Cervical Vertebrae. (i) The atlas or the first cervical vertebra isa light bony ring just like that of tl)(' varanus. Its ventral portion is thickened resombling a centr~~ and is notched above to' receive the o11Qntoid process of tho axis. Its :tnterior side bears cup-like spaces to pl'ovidt' articulation to the 0ccipital condyle of the skull. (ii) '1'ho axis is the second vertebra larger than. the atlas and smaller than the rest. It has no transverse processes and no rib", Its ccntrum is produced in front into It slender process, the odontoid process (compareits shape and position with that of the varanus). (c) The remaining cervical vertebrae are structurally similar but for variation in size and the fact that in thr binder most half a dozen cervical vertebrae the- 108 TH:E~ PIGEON hypapophyses are well developed while the last two have weIJ;leveloped vertebrae movably articulated to them. J (2) 'rhe thoracic vertebrae are five, the three .ante,rior ones are fused With the last ct'rvical vertebra, their centraJ arches and various processes are confluent. "Centra of the anterior thoracic and the posterior cervical are produced below into a compressed fenestrated plate lor the attachment of the muscle,; of the neck. The fourth .thoracic vertebra is free and the fifth fused with the first sacral vertebra. Sometimes there are <;mly four thoracic vertebrae then the first three fuse to form a common mass, wher~s, the fourth is free. All the thoraeic vertebrae carry ribs that articulate with tIle' tlternu·m. Sometimes o~';) or two cervical vertebrae also carry ribs but tht)',. do not arti.culate with thl> sternum. (3) The synsacrum is made up of next fourteen to .sixteen vertebrae fused together and supporting the immense pelvic girdle. The anterior one bearing ribs .articulating with the sternum is thoracic. 'fhen there .are about half a dozen lumbar vertebrae followed bv two .sacral vertebrae with sacral ribs. The compound synsa'Crum is so intimately fused that it becomes difficult to -distinguish the bones clearly. Last portion of the synsa.crum consists of fused anterior caudal. (4) The caudal region is short consisting of six to seven movable vertebrae of which the last consists of a vertical, laterally compressed plate, the pygoetyle. (5) The ribs and the sternum. (i) The ribs are flattened bony rods attached to the thoracic vertebrae by two processe<; THE PIGEON 109' ta) the capitulum attached to the centrum, and (b) the tuberculum attached to the transverse process of the neural arch. The ribs are united to the sternum by a sternal portion. The vertebral portion of the ribs carry backwardly directed bony proce~ses, the uncinate processes, each of which runs backward to the next rib and lies over it. (ii~ The sternum is a broad bonp covering not only the thorax but a great part of the abdomen. Ventrally -it bears a projeQting keel-like crest or carinA with which the great pectoral muscles are attached. The body of the st,ernum is boat-shaped and consists of the following important parts. (a) The manuorium is a small plate-like structure projecting vertically from the anterior end of the ventral surface of the body. ~b) The cal'ina or keel is the ventrally projecting triangular bony plate. (c) The meta-sternal processes are a pair of large processes on the sides of the anterior end of the body of the sternum. Each of them called a metasternum is direct,ed backwards and is divided into two flattened processes one of which lies just behind the last sternal rib and the other projects freely. (d) 1'he costal surfaces are depressions in the lateral borders .of the 'body with which articulate the sternal ribs. Anteriorly thecostal surface is. drawn out into a costa' process on each side. lIlO THE .PIGEON (e) The coracoid grooves are deep transverse grooves, lying about the base of the manubrium and communieating with each other through ,a hole. These provide articular facets for the coracoid bones. J3. The Skull. The bones.of the skull of a bird are fused so intimately tthat, in the adult condition, it i8 difficult to make out their outlines. Besides the orbits are large and the face is protonged into a long conical toothless beak; the occipital condyle it; one and only one ear ossicle, the columella, _,is present. (1) The cranium is rounded having usual bOiles for.ruing it.. (i) The Q~~ipj_tal region has basioccipital (base), exoccipital (sides) and supraoccipital (top), all of which enter into the foramen magnum, but the notched occipital condyle is formed by the basioccipital alone. (ii) The parietal region has well de';relopcd basisphenoid (base), the alisphenoids (sides) and the ankylosed parietals (roof) On the ventral aspeot of the basisphenoid paired membrane bones, the basi temporal are developed and beeome firmly ankylosed to it in the adult• •(iii) The frontal region. The presphenoid (base) the interorbital septum enters into and, therefore, is not, distinguishable. The orbitosphenoid (sides) are very incons- THE PIGEON III picuous bones entering into the interorbital septum but they ean be easTIy made out in a sagittal section. The fronta~l'e quite proniincnt and extensive bones. The broad posterior end of each frontal is drawn out III a post-orbital process. (2) The iympanic cavity is a cup shaped cavity at the side of the hinder end of the cranium. It is bounded by the squamosal above and the bn,sitemporal below. (i) The fenestral recess is an oval dppression within the tympanic cavity, almost, in t11e middlp, ani divided by a partition into an UPPE'l' fenestra ovalis, and a lower fenestra ,rotunda. ' (ii) The posterior opening of the eustachian canal is' a funnel-shaped opening at the ventral and anterior end of thc tympanic cavity. (3) The orbits are large spaces separated from each other by the interorbital septum which i~ formed mainly lby the mesethmoid helped by the presphenoid and orbito sphenoids. Below the interorbital septum is a ..rodlike thickening of the basisphenoidal rostrum. Dorsally it is bound by the frontal and anteriorly by the frontal and lacrymal. The lacrymals are large and ,each is produced downwards at the anterior end into a slender curved process. Below the orbit is incomplete. The intprorbital septum is fenestrated. Locate the following important fenestrae: (j) The foramen for the exit of the olfactory nerve is a median opening at the anterior ,end of the oranium which is continued 112 THE PIGEON forwards as a horizontal slit betwefn thE" inter-orbital septum and the frontal bone. (ii) The optic foramen is the large median hole in the orbitosphenoid bones opening into the posterior part of the orbit. (iii) The foramina for the exit of the third' and fourth nerves are small holes towardsthe outer side of the optic foramen. (4) The olfactory capsules are relatively smaller in size. (i) The nasals are a pair of lamellar bones which cover the sides and roof of the olfactory capsules. Posteriorly they are fused with the frontals while anteriorly they becamenotched forming the upper and lateral boundaries of the external nares. (li) The vomers are two but they unite into a single bone which is extremely small and found in continuation of the rostrum. It is often removed while cleaning the skull unless done cautiously. In the pigeon the vomers are not prrsent. (iii) The ecto-ethmoid or turbinals are poorly developed. The internasal septum is a cartilaginous continuation of interorbital septum. All these structures being entirely - cartilaginous are removed while cleaning the skull. (5) The Maxillo-palatine Apparatus. (i) The premaxilla is a large triradiate bone(really speaking formed by the fusion of THE PIGEON 113 two premaxillae) forming practically the whole of the upper beak. rtf! ascending nasal process runs backwards completing the upper boundary of the external nares, whereas, the two maxillary processes run . backwards and slightly outwards formin~ . part of the margin of the upper jaw. (ii) The maxillae a~e slender bony rods, each of which lies behind and to the inner side of the maxillary process of the premaxilla. Anteriorly each is expanded into a thin horizontal lamina, the maxillo-palatine process, with which the palatine is attached ventrally. (iii) The infra-orbital arcade is a slender bar below the orbit formed by the union of three bones-anteriorly a process of the maxilla that runs behind and joins another slender rod, the jugal, which meets the quadratojugal posteriorly. These are the components of the sub-orbital bar or the infra-orbital arcade. Posteriorly the quadratojug~l is thickened and articulates with the outer side of the distal end of the quadrate. (iv) The palatine is a slender horizontal bar anteriorly attached with the ventral aspect of the maxillo-palatine process and posteriorly it is expanded into a broad lamella the. sides of which articulate with the rostrum, whereas, its posterior end articulates with the pterygoid. 8 11-1 THE PIGEON (v) The pterygoids are stout rod-shaped bones, each of which is set obliquely articulating behmd with the inner surface of the quadrate and infront with t:le rostrum and palatines. (6, The Suspensorium. (.i) The squamosal lies above the tympanic cavity and is firmly fused with the bones of the cranium at the s:ldes of its bulging posterior part. The zYI~omatic process of the squamosal is a thin bar projecting forwards and downwards. Distally it fuses w:th the post-orbital process of the frontal. (ii) The 'quadrate is a stout triradiate bt'ne. Its dorsal arm articulate:s with the squamosal slightly above the anterior border of the tympanic cavity. Its flattened ventral arm runs ventrally along the outer side of the pterygoid and articulates with the quadratojugal. Its transversely expanded lower end provides the articular surface for the lower jaw. The third or anterior arm of the quadrate runs inwards parallel to the pterygoid. Its anterior end lies free. ~(7) The lower jaw consists of a pair of long laterally compressed rami firmly united in front. Each is thickened and expanded giving a process inwards. at the hinder end with which it articulates with the quadrate. -------~ -------- Each ramus of the mandible really consists of tive bones, OI!~ )"e'pla,_ci~~ _bone the articular and ~ i!1g bones., the angular, supra-angular, dentary and splenial, all having the same general relations as in the lizard. But these bones can be studied only in the mandibles of the young bird. 115 THE PIGEON (8) The hyoid apparatus consist.s of a short. arrowshaped body, the basihyal, bearing two pairs of cornua. The anterior cornua or ceratohyals are short. The posterior. cornua tlt thyro-hyals, lying further back, . are a long pair of joint.ed process. There are t.wo small basi-branchials, one a short rod connect.ing the two cornua with the basihyal and the other is a pointed rod projecting in betw~en the posterior cornua. (II) Tht' Appendicular Skeleton. A. The Pectoral Girdle. The pectoral girdle consists of a scapula, a coracoid ,and a clavicle. , (a) The scapula is long flattened sabre-shaped bone lying along the dorsal side of the thoracic frame·work, almost parallel to the vertebral column. Its expanded anterior end is firmly united with the coracoid by ligament. On its outer side it hears a shallow depression forming a part of the glenoid cavity. The glenoidal end of the scapula is produced into an acromial process, to which the clavicle is attached. ;I' (b) The coracoid is a stout straight bone directed ventralwards and it articulates with the coracoid groove on the anterolateral edge of the sternum. On its outer side it bears a cup-shaped depression COllipleting the glenoid cavity. The corltcoid is also produced into a process directed upwards and inwards. The clavicle is 116 THE PIGEON attached with this proeess. Thus also a.. foramen, called the foramen triosseum, is left between the thre,e bones, through which passes the tendon of the subclavius for insertion into the back of the humerus. (c) The clavicles are a pair of slender curved bones connected by their expanded upper ends with the coracoid and scapula, as mentioned above. Ventrally the two clavicles fuse forming a laterally compressed rounded plate. The bone thus, formed is. the "merry-thought" bone- or the furcula.. B. The Fore-limb or Wing. (a) The arm. / t h e humerus is an elongate bone' expanded at both the ends. Tne head is expanded and fits into the glenoid' cayity. The head is bordered by preaxial and postaxial' tuberosities. From the preaxial tuberosity extends a short deltoitd ridge. The postaxial tuberosity is larger and has a pneumatic foramen close to it on the proper dorsal face of the proximal end. Distally the trochlear end of the humerus' articulates with the bones of the fore arm., (b) The fore arm oonsists of t.wo bones the rndius: and the ulna. (i) The radius is a slender- bone articulating' with the t!'.9.C.h!.ea.!lL~!:e humerus. proximally and the oarpus distally -: - -_ - ~ ~ - - -- --...._.. THE PIGEON 117 I{ii) The ulna is stouter than the radius and also larger. A small olecranon process projects from its proximal end beyond its articular facet for the trochlear end of the humerus.' Distally it articulates with the carpus. (c) The carpus consists of only two bones in the adult (in the young birds the distal carpal row consists of three pieces which later fuse with the metacarpus). Of these the ulnare articulating with the ulna. is the larger whereas the radiale is smaller. The manus in the adult consists at a carpo:metacarpus formed by the fusion of meta.carpals with the distal ro\v of carpals. It 'consists of two rods, the proximal of which 'is the stronger of the two, and more or less straight. This represents the second meta'-carpal with the base of which is fused the first metacarpal. With the first metacarpal articulates a single pointed phalanx of the first digit. Distally the stout rod bears two prominent phalanges of the second digit. 'The third metacarpal is represented by -thl:' thinner, slightly curved rod on the postaxial side and carries a single pointed ' phalanx of the third digit. 'D. The Pelvic Girdle. The ilium, ischium and pubis, the three common {!omponents of the pelvic girdle are present in this case also though in an entirely different form. All the elements @f each side are perfectly fuse..d_ with themselves forming the os innomin~umJ both of which fuse with the lIS THE PIGEON synsacrum in between, forming a more or less, shield· shaped' structure covering the organs of the body cavity from above. As usual, the acetabulum, for the articulation of the femur, is present a.t the junction of the three bones. Each half of the girdle consists of the following parts. (i) The ilium is a rem8rkably expanded bone extending both anterior to the acetabuluill' and also posterior to it. The inner border of the ilium is fused with the synsacrum, whereas, the outer surface of its anterior' part is concave. Posteriorly it is fused with the ischium. On the outer surface, above' the acetabulum, is a projection, the antitrochanter that articulates with the great trochanter of the femur. (ii) The ischium is directed backwards parallel to the hinder part of the ilium to which it is attached posteriorly enclosing, just behind the acetabulum, an oval iliosciatic foramen. (iii) The pubis is a slender bone directed back- wards parallel to the outer and ventraD margin of the ischium, with which it is often fused. Behind the acetabuillm the pubis and ischium is separated by a slitlike opening the, obturator foramen. (iv) The acetabulum is formed by all the three bones and is perforated. It is a rimmed cup-like depression for the articulatIOn of the head of femur. D. The Hind-limb. (a) The Thi~h. THE PIGEON 119 (i) The femur is a short and powerful bone enlarged at both ends. The rounded head is proximal, on,. the outer (postaxialJ side of which is an irregular process, the great trochanter, With which bone the great trochanter articulates? ) t...tA v.."", 'l ~ to.~.. QJ~~'" ~ t'; '_ (b) Distally the femur presents infront a deeply ~~rooved surface for the patella, which is a sesamoid bone forming the knee-joint. f'2'"-P The Leg or Crus. (i) The tibio-tarsus is a long Done forming the crus. It is chiefly composed of tibia fused with the proximal row of thl? tarsus. The proximal end of the tibio-tarsus is expanded and has on its anterior face a great ridge th~ cnemial crest. Distally it articulates with the tarso-metatarsus by a pullylike articular surface. (ii) The fibula is reduced to a slender bone _ closely applied to the outer surface of the tibio-tarsus. (0) The tarsus has no free bones in the adult. The proximal row of tarsus is fused with the tibia and the distal with the meta tarsus. The ankle joint, therefore, is between the proximal and distal rows of tarsal bones, (a mesotarsal joint). (d) The tarso-metatarsus is a stout long bone proximally articulating with the tibio-tarsus and distally bearing three meta tarsals separate from each other, pach articulating with a corresponding digit. Besides another 120 THE PIGIWN incomplete metatarsal, the first meta.tarsal, is present for the hallux. It is a. small irregular nodule of bone attached to the inner and posterior surface of tarsometatarsus. Note: The tarso-metatarsus in the male often bears a pointed spinous fighting spur. \ e) The phalanges are proportion as in the directed hallux has thr.ee, the third or the fourth toe five. present in the same lizard, the backwardly two, the second toe middle toe four and CHAPTER V THE RAT A. EXTERNAL FEATURES. Examine the specimen placed before YOlt. (a) Notice that the body is covered with soft fur. How does the exoskeleton of the rat differ from that of the fish, varanus or the pigeon? Note the absence of hair from the muzzle, the external <ear, feet and tail. (b) The body can be divided into four regions, the head, the neck, the trunk and the tail. 1. The head. Notice its general shape. (a) The mouth. Did you notice any lips in the fish, varanus and bird? Does the rat have lips? What is the difference between the upper and lower lip ? (b) The nostrils. Notice their shape and position and compare them with those of the other animals studied by you. {c) The eyes are dorsolateral. Compare their size and position with those of the fish, varanus ani the bird. Are there any eye-lids or 121 12~ THE RAT eye-lashes? Notice a fold of skin at theinner corner of the eyes. This is the nictitating membrane. Hold an edge of t'c with a pair of forceps and pull it across the eye-ball. What hap pens? Id) The external ear or pinnae are at the base of the head, each pinna is large directed antero.laterally. Note the absence of hair from the external ear. (e) The vibrissae or whiskers are long hairs developed on the face at the \lide of the snout. Touch the v£brissae of a living rat (if possible) and notice what happelts~ The vibrissae are of four types according to their positions: (i) The mystacial vibrissae are those on the upper lip between the eye and the muzzle. The anterior ones are short and increase inl size posteriorly. (ii) The superciliary vibrissae are much shorter about five, usually, and located above the eye. (iii) The genal vibrissa is a single slender vibrissa. located below the eye. (iv) The submental vibrissae are numerous, slender and situated on the chin. . 2. The neck is short and thick set without prominent features. 3. The trunk is the largest part of the body. The trunk of the rat is divided into the thorax and abdomen. Supporting the trunk are two pairs 6f limbs. THE RAT 123 1. The fore·limb or arms include \ the upper arm, the fore arm, and the hand. The hand consists of the palmer region and the digits, four in numbef. The first digit or thumb is greatly reduced. What difference. yO\! find between the fore·limbs of the rat and other animals studied by you? II. The hind limbs or legs consist of the thigh, the shank, and the elongate foot composed of the planter region and the digits, five in number. Compare the hind limbs of the rat with those of the fish, the lizard and the bird. To what special use are the limbs modified in all the cases? III. The Apertures on the Trunk. (a) The teats or mammae. If your specimen is a female you will find four or five pairs of small papillae on the ventral side of the animal. These are the milk giving glands by which the female supplies its young with nourishment. (b) The anus is a small median aperture below the base of the tail. (c) The perineal pouches are a pair of depressions on the sides of the anus and slightly infront of it. The ducts of the perineal glands open into these. (d) The urino-genital openings are situated infront of the anus in both the sexes. (i) In the male the opening is at the tip of a cylindrical penis. The tip of the penis can be 124 THE B,AT retracted into a loose sheath of skin, the prepuce. (ii) In the female the opening is a longitudinal slit known as the vulva. l 4. The tail of the rat is elongate, cylindrical 'lUd tapering and often slightly longer than the body. It is devoid of hair though covered with overlapping scales. B. DISSECTION OF THE ABDOM[NAL VISCERA. Lay the rat in a tray, keeping the ventral side of the animal uppermost and nail down the feet after stretching them to their fullest extent. Feel the position of the ribs and mark the limits of the thorax and abdomen. Cut the body wall along the middle line ttpto the hinder end of the thorax. Cut along the ribs dorsally on either side .md pin the flaps. Do not open the thoracic cavity till a later stage. Note the position of the following organs :(1) The liver is a dark-red body at the anterior end How many lobes are there? Find the gall-bladder. .of the body cavity. (2) The stomach lies below the liver. .and position. Note the shape Disentangle the intestine and spread it on the table and measure its length. (3) The duodenum is the initial portion of the intestine. Compare with that of the other animals. (4) The small intestine is next portion after the duodenum. 125"- THE RAT Follow the intestine backward till where a short tube is given off. YOII come to a place- (5\ The coecum is a blind comma-shaped outgrowth at the junction of the intestine and large intestine. (Note: In the rabbit the coecum ends in a tail-like structure, the vermiform appendix; in the squirrel it is< represented by a mere protuberance and so also in the rat.) (6) The colon begins following the coecum. the first portion of the large intestine. It is" (7) The rectum is the last part of the alimentary canal and is found dilated at intervals by faecal pallets. (8) The urinary bladder is a thin-walled sac at the posterior end of the abdomen. (9) The spleen is a dark-red elongated body attached to the posterior side of the stomach. Recall the size, shape and position of the spleen in the other animals. studied by you. (10) The kidneys. 1'heyare a pair of compact bodies: having the shape of bean seeds. They lie very closely attached to the dorsal wall of the abdomen. The right kidney lies behind the li~er. Is the left in line with it? Compare the k,idney with that of the dog-fish, varanus and the pigeon. tii front of each kidney lies the adrenal body. (11) The diaphragm is the muscular partition separating the thClrax from the abdomen. (12) The peritoneum is the lining membrane of the abdominal cavity. The alimentary canal is supported by- 1~6 THE RAT a double fold of peritoneum called the mesentery reflected from the rlorsal waH of the abdomen. C. THE DIGESTIVE SYSTEM. a. The oral passage is the first division of the digestive system opening to the exterior through the mouth. rOpen the mouth cavity as fully as possible and :'lOte the following. (i) The mouth is terminal, relatively small and is guarded by movable lips, the upper of which is cleft in the middle to exp0se the incisor teeth . .(ii) The teeth are present on both the upper and lower jaws. There are a pair of molars in each jaw. The incisors are long covered and separated from the molars by a toothless region, the diastema . . l iii) The palate forms the roof of the oral passage. The anterior- part of the roof is formed by the hard palate that separates the buccal cavity from the nasal cavity. Notice the transverse ridges on its surface. Posteriorly the hard palate gives support to the soft palatp which separates th(;;Oft naso-pharynx with the buccal cavity. {iv) The tongue is an elongate, thickened straplike muscular organ occupying the floor of the buccal cavity. Its tip is free but the body and root of the tOilgue are attached by strong muscles to the hyoid bone. THE RAT (v) 127 The salivary glands are two pairs, the parotid and the submaxillary. The large parotid lies along the side of the head, in the cheek, immediately beneath the external auditory meatus; whereas the submaxillary lies in the throat region just between the angle of the mandibular bones. (vi) The pharyngeal passage is a wide funnel. shaped tube extending from the root of the tongue and the posterior end of the soft palate to the opening into the oesophagus. The passage is both respiratory and digestive in function. Posteriorly it opf'ns into the oesophagus dorsally and the larynx vent· rally. The entrance ~f the larynx, the glottis; is guarded by t~e epiglottis. What .do you think is the fUllctIOn of epiglottis? 'The internal apertures of the eustachian .tubes also open into the pharynx. What is .the function of these openings? . .~. . The Alimentary Canal. Since the abdominal viscera has already been exposed, it will be better to study the digestive organs of the ,abdomen first. Trace the oesophagus later when the thorax is dissected. Expose the stomach by turmng the ltver upwards. (i) The stomach is a sac-like expansion of the digesti ve tract lying, on thf'\ dorsal side of the abdominal cavity to the left of the median line almost transversely. The strongly rounded region of the stomach iq .the cardiac end, while the narrow cnd 128 THE RAT leading to the duodenum is the pyloric end'. that ends at the pyloric constriction. C1tt open the stomach and wash away its contents and; note. The cardia or valvular arrangement at the entrance of the oesophagus into the stomach, the sphincter ,muscles internal to the pyloric constriction and also the glandular internal lining of the stomach. <;ompare with_ the stomach of the dog-fish,the varanus and the pigeon. (ii) The duodenum is the proximal part of the intestine, which takes the form of a V-shaped. loop in th) mesentery of which is the ~!:an creas. The bile duct opens into the eXl'anded beginning of the duodenum and the pancreatic duct into the distal limb of U. (iii) The small intestine is the longest part of thealimentary canal forming coils as has been examined before. What, is the advantage of such a great length? The small intestine ends in a rounded structure the sacculus rotundus, the three way junction of the coecum and colon. (iv) The coecum is a tbin-wnlled diverticulum for the storage of the partially rligested food. (v) The colon is narrower tube having sacculated_ appearance in the beginning and widens out into a short rectum. (vi) The rectum is the last portion of the alimen-tary canal. It is sbort and opens to theexterior through the anus. ' 129 THE RAT C. The Gland,s of Digestion. (1) The salivary glands as described above. (ii) The pancreas is a diffuse fat-like gland secreting pancreatic juice. Position and structure has already been seen. (iii) The liver is very large made up of five lobes. (Compare with those of the dog-fish, the varanus and the pigeon.) It is attached to the diaphragm by a median vertical fold of peritoneum, the suspensory ligament. The gall bladder is an elongated thin-walled sac lying between the right and left central lobes. The main bile-duct arises from it and opens into the duodenum. D. THE THORACIC VISCERA. Make an incision where the ribs meet the diaphragm. Cut away the nbs taking care .not to injure the organs that tte beneath. Note the following: l. The thymus is a soft gland. like body lying on the anterior part of the thorax. 2. The heart is enclosed within the pericardium. Cut the pericardium and notice the external structure of the fottr chambered heart. 3. The lungs are soft spongy masses of tissue lying in the pleural cavity. In the rat the right lung is somewhat larger and is divided into four lobes. The left lung is composed of flo single lobe. 9 130 THE RAT 4. The diaphragm is the muscular layer separating the thorax from the abdomen. 5. The phrenic nerves are a pair of slender nerves lying bt'tween the heart and the lungs. Trace them as far as you can. Notice that the posterior end mt.>ets the diftphragm. Do not mix these 'nerves with thp pneumogastric nerves which lie a little nearer the middle of the thorax. 6. The pleurae. The lungs lie in the pleural cavity and the glistening membrane lining this cavity is known as the pleura. The right and left pleural cavities are separated by a narrow median cavity, the mediastinal cavity, in which the .heart lies. 7. The oesophagus is a muscular tube running through the thorax immediately ventral to the vertE:.hral column. It can be seen easily on lifting the left lung. E. DISSECTION OF THE OIROULATORY SYSTEM. I. The Veins. The veins are relatively superficial, it is important, therefore, to dissect the veins before the arteries. Remove the pericardium, expose the heart taking care not to injure the blood vessels. EXlmine the important veins opening into the right auricle and trace the branches opening into them. The venae cavae are three large hlood vessels return. ing.the blood from the whole of the body to the ri'~ht auncle. ). The right anterior vena cava or the right precaval returns blood from the right, side of the head and neck etc. collected through these branches : THE RAT ]31 (i) The right jugular vein that receiyes.-- (.a) The right external jugular vein from the region of the lower jaw. It runs along the side of the lower jaw at the angles of which it turns inwards running along the side of the neck. ~h) The right internal jugular vein that runs down from the brain along the side of the trachea and opens into the right external jugular vein close to its ,union with the subclavian. (ii) The subclavian, a small vein collecting blood from the fore-limb of its side and opening into the precaval. {jii) The right anterior intercostal vein, a small vessel collecting blood from the intercostal spaces and opening into the vena cava close to the auricle. ,(iv) The right internal mammary vein, another small vein running forwards on the inner surface of the ventral thoracic wall and opens into the vena cava opposite to the first rib. (v) The azygos vein, an asymmetrical vein running in the mid-dorsal line of the thorax, lying close to the vertebral column and opening into the vena cava just behind the right anterior intercostal vein. 2. The left precaval receives corresponding branches from the left side except the azygos vein. 13~ 'I'HE RAT ARTERIE.S _ •.INTERNAL CAROTID ': 'EXTERNAL CAROTID' .• COMMO« CAROTID " :---SYSTE.MIC AORTA, '.' "-PULMONAR'r v--.,~-:..,,:.--'PIJLMONAR'1 AORTA, .' " '>INTE.RCOSTAL _.···COELIAC '. -ANT-MESENTERIC" __ - .' RENAL .' GENITAL Fig. H.-The circulatory system of the rat. The left anterior vena cava has been removed. THE RAT 133 3. The posterior vena cava starts from the tail r('gion :as the caudal vein and runs forwards close to the vertebral column across the abdomen and thorax to open into the posterior end of the right auricle. During its long course .a number of other veins open into it. Beginning from the posterior end of the body the branches are: (i) The iliac veins are a pair of veins collecting blood from the limbs and opening into tho precaval in the pelvic region. Each iliac is formed by the union of an external iliac and an internal iliac. The external iliac is the direct continuation· of the femoral vein collecting blood from the outer border of the thigh. The internal iliac collects blood from the back of the thigh (ii) The ilio-Iumbar veins are two, l'etu~ning blood from the lumbar region. {iii) The genital veins (the ovarian in the females and the spermatic in the males) are a pair of veins returning blood from the gonads. In the females they are small straight vpins but in the males they are much longer and directed backwards as they descend into the scrotum along with the testes. {iv) The renal veins are a pair of small veins running from the kidneys to the vena cava. (v) The hepatic veins are two relatively larger, though short veins returning blood from the liver lobes. {vi) The phrenic veins are a pair of veins returning blood from the diaphragm just where the vena cC.va crosses it. 134: 4. THE RAT The Portal System. The portal system consists of the hepatic portal vein formed by the uniltn of small blood vessels from the intestines, pancreas, spleen and stomach etc. and opens. into the liver by several branches. The portal system 1S best seen in a fresh animal. OPen the body cavity and turn the viscera aside and carefully stretch the mesentery sttpporting the intestines and examine. Draw a figure showing aU the vei1ls. Belter draw the outline of vanozls parts and organs of the body and draw the blood vessels in relatwn to them. 5. The pulmonary veins are two main veins reiurning blood from the lungs to the heart. They open into the dorsal surface of the left auricle. II. The Arteries. Locate the origin of the systemic arch as well as tlw pulmonary arch. Trace along the systemIc arch and note the blood vessels arising from zt. 1. The systemic aorta arises from the anterior end of the left ventricle, passE's dorsal to the pulmonary aorta, and then curves round to the dorsal side of the heart and continues backwards as thE( dorsal aorta along the backbone. During its course arise the following main arteries. of the body. (A) The Anterio~ Branches. Soon after the aorta issues from the heart it giveSl off three branches the innominate, the left common carotid and the left subclavian. THE RAT 135 (i) The innominate is a very short artery soon dividing into the ri~ht subc1vian and the right carotid arteries. ,(a) The ri~ht subclavian artery runs straight to the right forf-limb and gives a small branch, the scapular to the shoulder, before entering the limb where it is known as the brachial artery. From the subclavian also arise a vertebral artery going to the vertebral column and another internal mammary supplying the inner surface of the ventral wall of the thorax. (b) The right carotid runs forwards straight along the trachea and near the angle of the jaws divides into two-the internal carotid artery going to the brain and the external carotid supplying the right side of the head and face. (ii) The left carotid artery is similar to the right carotid artery but for the fact that it arises directly from the systemic aorta. (iii) The left subclavian arises from the systemic directly and supplies the left half of the shoulder girdle and the left fore-limb. Its branches correspond to that of the right subclavian. (B) Branches oj the dorsal aorta. (i) The intercostal arteries are a series of paired arterips supplying thE' wall of the ch'_'st. 136 THE RAT (ii) The coeliac artery is the first abdominal branch of the aorta. It is a median blood vossel arising ventrally and it supplies the stomach and liver etc. through the lienogastric and hepatic branches respectively. (iii) The anterior mesenteric is another median artery arising from the aorta behind the coeliac and supplies the intestines. (iv) The renal arteries are paired arteries arising posterior to the anterior mesenteric and supplying the kidneys. (VI The genital arteries are paired arteries supplying the gonads. In the female the ovarian arteries are small and pa '38 sideways supplying the ovaries, but in the male the spermatic arterieS are long blood vessels that pass backwards along the dorsal surface and enter the. scrotal sacs through the inguinal canals and supply the testes. {vi) The posterior mesenteric is a median blqol vessel supplying the hinder part of the rectum. fvii) 1'he ilio-Iumbar arteri~es are a pair of small arteries arising from the aorta dorsally a11d supplying the body wall. ~ (viii) The common iliac arteries are two a,risin from the aorta in the pelvic region an passing into the limbs. Each iliac artery divides into an internal and an external iliac .artery. Another branch of iliac on each side, the vesical artery runs to the bladder. THE RAT 137 (ix) The caudal artery is the continuation of the dorsal aorta in the tail region. 2. The pulmonary artery arises from the anterior border of the right ventricle in the mid-ventral line, arches over to the dorsal surface of the left auricle and divides into the left and right branches supplying the two lungs. Follow the branches to the organs of their distribution. Draw a neat diagram showing all the dissected arteries faithfully. III. The Dissection of thE' Heart. Dissection of the heart should always be attempted after the dissection of the blood vessels and nerves of the neck. After cleaning the roots of blood vessels remove the heart along with a little length of each of the prominent vessel. First examine its external characters, sketch and then dissect it. (1) External Appearance. The heart appears mainly to be formed by the ventricular portion when seen from the ventral side, as .the auricles lie at its base on its dorsal aspect. (a) The auricles are two thin-walled chambers anterior to and slightly dorsal, to the ventricles. (i) The left auricle lies to the left side almost concealed by the prominent blood vessels. Into it open the pulmonary veins from the dorsal side. (ii) The right auricle, on the other hand, receives the three prominent venae cavae dorsally. l38 THE RAT (b) The ventricles are thick-walled muscular bodies separated by an oblique groove cxternally. (i) The left ventricle is the largest of the two lying behind the left auricle. Its walls are thick. The aortic arch arises directly from the left ventricle and conducts blood to all parts of the body. (ii) The right ventricle lies posterior to the right auricle. It is a small chamber pumping blood to the lUll!~s. From it arises the pulmonary arch. ,Note: It will be very helpful to dissect the heart of a goat or sheep for advantage of size. Differ-l'llces between the two are of a minor nature and can be easily made out and noted. (2) Internal Structure. (AJ The Auricles. Carefully remove the ventral wall of the auricles by Clt!ting from the sides. Try to keep the aortic and pulmonary arches as and where they are. Examine. (i) 'rhe right auricle is a thin-walld chamber mark~d internally by muscula,r ridges. Into the auricle open the three venae cavae by separate' apertures. (a) The eustachian valve lies between the apertures of the two anterior venae cavae and to some ext~n~ guarding that ofthe THE RAT posterior vena cava. It is a membranous: told and 801r remnant of the right sinuauricular valve. (b) The right auriculo-ventricular opening is a wide crescentic opening into the right auricle and is guarded by valves that will' be described along with the ventricles. (ii) The left auricle is a smaller chamber into' which open the pulmonary veins by two separate apertures. Posteriorly it opens into' the left'ventricle. (iii) The interauricular septum completely separates the cavities of the two auricles. Almost ~n the middle of the septum is an ill-defined depression, the fossa ovaHs. (In the embryonic rondJtion the two auriclescommunicate with each other through an apE'rture the foramen ovale, which is closed' after birth leaving tht' above depression.) (B) The Ventricles. With the help of a sharp blade eltt off the ventral wall' of the t'fl1trtcles, uash the ccntents and examine. The walls of the ventriclp,s are thick and muscular. The bundles of muscle fibres are so arranged as to form longitudinal ridges, the columnae carneae projectingfrom the walls of the ventricle. The interventricular septum is not entirely median but inclined to the right leaving the apex to the left ventricle. 140 a. THE RAT The Right Ventricle. ii) The tricuspid valve consists of three flaps and guards the right auriculo-ventricular aperture. (ii) 'rhe opening of the pulmonary artery is at the It'ft anterior angle. of the ventricle and is guarded by three pocket like semilunar valves. Cut open the pulmonary artery just at its beginning and examine. b. The Left Ventricle. (i) The bicuspid or mitral valve has only two flaps guarding the left auricula-ventricular aperture. (in The entrance of the systemic arch provided with three semi-lunar valves. F. THE URINO-GENITAL SYSTEM. 1. The Male. is (a) The urinary organs. You have already seen the position and shape of the kidneys. Notice that they are surrounded by fat. The notch to which the ureter is attached is called the hilus. Trace the urete" and see where it meets the bladder. Take out one of the kidneys and bisect it longitudinally, in the same way as Y01£ would sPlit a bean seed. Note the following structures. THE RAT ~i) 141' The cortex or the outer layer. (ii) The medulla or the inner layer. Observe the conical structure, the pyramid, n9ar the hilus. (iii; The pubis is the broad portion of the ureter which enters the kidney. (iv) The urinary bladder is as described above. (b) The reproductive organs. \i) The scrotal sacs are a pair of pouches in front of the anus and between: the hind legs. Cut open the scrotal sac ventrally and expose the testes lying in them. Note their sizes. (ii) The testes arE! a pair of elongated ovoid bodies attached to the hinder end of the scrotal sac by a band of tissues, the gubernaculum. fln the very young rat the testes lie in the abdomen close to the kidneys, just before maturity they descend into the scrotal sacs through the inguinal canals.) (iii) The epididymes are a pair of irregular masses of convoluted tubes lying along the inner edges of the testes. At the posterior ends of the t('stes it becomes, enlarged forming the cauda epididymis and at the anterior and it forms the caput epididymis. From the cauda epididymis emerges on each side the vas deferens. (iv) The vasa deferentia are a pair of tubes arising from the cauda epididymes. Trace their path into the abdominal cavity through the inguinal canals. In the abdominal cavity i42 THE RAT -each vas deferens loops round the ureter from the outside before it opens into the ,urethra behind the bladder. I..NUS i PERINEAL GLAND Fig. 12.-The male urino-genital organs of the rat. (v) The urethra or the urino-genital canal is the common passage for urine and spermatozoa. It leads into the penis. (vi) The uterus masculinus is a blindly ending sac, situated where the vasa deferentia and urethra join. (It is the sole vestige in the male of the Mullerian ducts.) 'vii) The penis is the organ for conveying spermatozoa into the body of the female where THE RAT 143 the ova are fertilised. Tbe urethra opens at the tip of the penis by a small slit-like opening. The loose skin covering the tip, the glans penis, is called the prepuce. (viii) The prostate gland consists of several lobes almost surrounding the walls of the uterus masculinus and opens by small ducts into the urino-genital canal. {ix) The Cowper's glands ale a pair of glandular bodies placed on the dorsal wall of the urino-genital canal, posterior to the prostates. (x) The perineal glands are a pair of elonga- ted glands lying on both sides of the penis, in the anterior walls of the perineal sacs. (xi) The rectal glands are a pair of yellowish bodies lying on both sides of the rectum. Draw a figure of the urilto-gellital organs of the rabbit and label all the parts. 2. The Female. (a) The urinary organs. ,(i) The kidneys, the ureters and urinary bladder are similar as in the male. (Ii) The urethra is a long narrow tube runfiing towards the posterior side and opening independently at the tip of the clitoris. ·(b) The reproductive organs. 144 THE RAT (i) rrhe ovaries are a pair of small yellow compact bodies about the size of a pea on either side of the kidneys. 'They are suspended in the body by a fold of peritoneum, the mesovarium. (ii) The oviducts are two, each divided into two parts having different functions. (a) The fallopian tubes are narrow convoluted tubes having their anterior ends dilated into the oviducal funnels. 'rhey convey the ova to the uteri. (b) The uteri are two' thiek-walled and dilated ends of the fallopian tubes that meet in the middle and form a common tube, the vagina. Fig. 13.-The urino-genital organs of the female rat. Your specimen may have developing embrovs in which case the uteri will be seen to be much distended. Cut open.. the uteri and examine the embroys. 145 THE RAT (c)' The vagina it> a common tl}.be formed by the fusion of the uteri from both the sides. It opens to the exterior by a slit· like opening, the vulva. Note :-The vestibule or the urino-genital canal is the common canal formed by the union of the urethra and the vagina in the squirrel and the rabbit etc. It is very short. It opens into the vulva, which is gUarded externally by a pair of thick muscular lips. But in the rat the two openings are separate. (iii) The clitoris is a small rod·like organ. IJi lies at the anterior end of the vulva. In the rat the urethra opens at its tip. Examine these organs carefltlly and draw a figure of the female genital system of the rabbit. Label aU the parts. G. DISSEOTION OF THE NECK. T he dissection is made from the ventral side of the neck. Kiep the parts moist while you work. Remove the skin carefully·and expose the following structures. 1. 2. The hyoid apparatus is deeply laid bet. ween the two rami of the mandible . . Expose the hyoid first, but take it out after the dissection of the neck is completed. The Blood Vessels of the Neck. (i) The external jugular veins. from the heart upwards. Trace them They lie to the extreme right and left. 10 146 THE RAT (ii) The yarotid arteries lie along the side of the trachea. 3. The trachea is the tube connecting the glottis with the lungs. Trace it. Note the banded structure on its ventral side. What do you think is the function of these bands? Wby do they not extend on the dorsal side also? At the anterior end of the trachea is a dilated larynx or the laryngeal chamb€lr whose walls are supported by the thyroid arytenoid and cricoid cartilages. Posteriorly the trachea enters the thorax and • divides into two bronchi going to the lungs. 4. The oesophagus is a muscular tube connecting the gullet with the stomach. Trace the oesophagus without disturbing the nerves lying besides it. 5. The thyroid is a soft vascular. gland on either side of the larynx. 6. The Nerves of the Neck. Carefully clean the carotid artery. Take special care not to damage the nerves lying beside, stretch it to one side and spread the surrounding connective tissue.· Find the vagus or pneumogastric nerve rttnning on its outer side follow it backwards along the neck and thorax and forwards to the s kuZl. The pneumogastric or vagus nerve is a stout nerve' having a ganglionic swelling neal' its origin. H runs downards from the foramen lacerum posterius until it reaches the carotid artery and then straight backwards along the neck lying outer to the earotid artery. In the t,borax it runs along the oesophagus to the stomach. THE RAT 147 (a) The anterior laryngeal nerve is a delicate nerve arising from the pneumogastric opposite the upper border of the thyroid cartilage. It crosses the carotid artery and runs dorsal to it. It bends anteriorly and innervates the mucous membrane of tIre larynx. (b) The depressor nerve is a slender nerve arisill;g from the anterior laryngeal. It runs backwards along the neck lying dorsal to the carotid artery. Trace it to the heart where it ends. (<0) The right posterior or recurrent laryngeal arises from the pneumogastric just above the heart then runs forwards alongside the trachea. Just after origin it runs dorsal to the aortIc arch which obstructs it from view. Be careful in locating it at this place. (d) ,The cervical sympafhetic nerve is a slender ne:rv.e that runs in between the pneumogastric and the depressor nerves. It swells up into a small oval anterior cervical ganglion at the level of the angle of the jaw anteriorly, and posteriorly, into another swelling, the middle cervical ganglion, lit short distance infront of the subclavian ,artery. (e) The phrenic nerve arises from the fourth cervical nerve. It runs back along the vertebral column, and passing through the thorax ends by a number of small branches in the diaphragm. Gently pull th.e ltmg and heart apart. Locate the conne· .ction of the nNve with the diaphragm alld trace it forwards to the place of its origin. 148 H. THE RAT THE BRAIN. Tn contrast with the brains of the animals studied thus far, it will be s'een in the mammals, that the cerebral hemispheres are much enlarged, as lS also the cerebellum ~ there are four lobes on the roof of the mid·brain, and the olfactory lobes are relatively small. To take out the brain first remove the skiH and then startlng from the foramen magnum cut along tf e sides of the cranium carefully and finally Sntl' away the roof. The brmn will be seen enclosed in a tough fibrous membrane, the dura mater, which should also be removed. Expose about an inch of the spinal cord by cuttmg away the dorsal half of the vertebrae, Ilft up the whole stmctztre, thereafter, alollg with half an inch of the spinal cord and keep it in a watch· glass. If the brain 1S soft and pulP'Y Jzarden it in spirit or in a mixtur.e of absolute alcohol and acetic acid iiI whIch it must rematn at least for twenty fuu1' hours. Examine the following parts. (a) The fore·brain. (i) The olfactory lobes lie at the anterior part of the brain. Each of them consists of an' olfactory tract and an olfactory bulb. The olfactory traet lies on the ventral side of the cerebral hemispheres. The olfac:tory bulbs are large ovoid structures lying just. anterior to the cerebral hemispheres. (ii) The cerebral hemispheres are large pftired structures separate along the median line by a longitudinal fissure, the median fissure. The surfa.ce· of the cerebral hemispheres is smooth in the rat. (In higheranimals it is thrown into folds and the hemi·- THE RAT 149 spheres are divided into many lobes by deep transverse and longiturlinal grooves dorsally as well as ventra,lly). In the rat they are more or less conical in shape pointed anteriorly. (iii) The thalamencephalon is narrow and has a stalk of the pineal body dorsally and ventrally it carries the optic chiasma and the illfundibulum which gives rise to the posterior lobe of the pituitary body. The anterior choroid plexus arises in the roof of the thalamencephalon. {b) The mid-brain contai.ns the optic lobes represented by four rounded eminences, the corpora quadri~emina,. on its dorsal surface. Ventrally are found the crura cerebri. (c) The hind-brain consists of the cerebellum and the medulla oblon~ata. The cerebellum is next to th~ cerebrum in largeness. J t is clearly divided into regions, being made up of median lobe, the vermis, and two lateral lobes each of which terminates laterally in the flocculus. Ventrally is the much thickened medulla oblongata. Extending transversely across the under side of the medulla and connecting the two sides of the cerebellum is a tract of fibres forming the pons varolii. In the roof of the hind brain is the posterior choroid plexus. 150 THE RAT (d) The Ventricles. Cut a sagitt'll section of the brain by a sharp knife and examine. The cavities in this case are much redllced. (i) The rhinocoels in the olfactory lobes are much reduced and almost invisible. (ii) The lateral ventricles are narroW spaces ill! the cerebral hemispheres. The lateral ventricles communicate with the third ventricle by a more or less tra,nsverse passage the foramen of Monro. The hemispheres. are connected with each other by a broad transverse commissure, the corpus callosum, lying just int!,rnal to the base of the median fissure. This will appear a'l a longitudinal fibrous band just o\':r thO' lateral ventricles in the section. (iii) The third ventricle (the ventricle of thalamencephalon) is narrow laterally and passes backwards to the equally narrow iter into which open the small cavities of the corpora quadrigemina, representing the reduced optocoel. liv) The fourth ventricle i:3 the cavity of the medulla into which opens the iter anteriorly. I. THE SKELETAL SYSTEM. For the study of the skeletal system of a mammalian type the usual practice is to study the skeleton of the rabbitr along with the skull of a dog. The sam~ procedure is followed here. (A) The Axial Skeleton. 1. The Vertebral Column. The rabbit has about 45 vertebrae. In the mammalsin general the number of vertebrae in each region is fairly THE RAT 1'51 constant. Each movable vertebra is separated by an intervertebral disc, and all the vertebrae are classified into five groups. (i) The cervical vertebrae in include the first seven. the neck region (ii) The thoracic or C;19st vertebrae are twelve or thirteen in number and' are connected to the ribs. (iii) The lumbar or loin region vertebrae in the abdominal region are six or severr in number. (iv) The sacral or pelvic region vertebrae can· SISt of three or four fused vertebrae which support the pelvis. (v) The caudal or tail vertebrae are twenty. four or more in the rat and in the rabbit they are sixteen. You have already studied the- general characters of a. vertebra before. Do you recognise thc centrum, the neural arch, the neural spine, the transverse pro· cesses, the pre-and post zygapophyses etc. ek. ? (i) The Cervical Vertebrae. (a) The first or atlas is ring-shaped. The neural spine is small and the transverse pro· cesses are broad horizontal plates The two concavities on the anterior face fit with the occipital condyles of the skull. (b) The second or axis has the centrum pro· duced.anteriorly into the odontoid process, which fits into the lower part of the ring of 152 THE RA~r the atlas. Th'l neural spine is elongated and compressed. How is an odontoid process formed? Compare the- structure of the odontoid processes of the animals studied by you. (c) The characteristic feature of a ~ervical verte- bra is that it has a canal, the vertebrarteriat' canal in addition to other common structures This cana.l is formed by the fusion of a cervical rill> with the vertebra. (ii) The Thoracic Vertebrae. Study anyone of the first six and find the following parts: 1. The centrum. 2. The neural arch. 3. The transverse processes. 4. The neural spine. 5. The zygapophyses. 6. Articular facets for the (b) the capitular. rib~:- (a) the tubercular (iii) The Lumbar Vertebrae. The lumbar vertebrae are comparatively larger. The centrum .is stout, the neural arch is surrounded by a prominent neural spine, the pre-and post zygapophyses are as usual; and the transverse processes are stout, long, flattened, directed obliquely downwards and forwards. At the anterior eni are a pair of stout metapophyses, rising well above the prezygapophyses, and below the postzygapophyses are the less prominent anapophyses. THE RAT 153 the first two or three lumbar vertebrae a ventrally directed projection the hypapophysis is present on the under side. 'These prominent processes provide for the .attachment of the muscles of the back. t{)ll (iv) The Saaral Vertebrae. The sacral vertebrae are fused togather to form the .sacrum which gives support to the pelvic arch. (v) The Caudal Vertebrae. The anterior caudal vert.ebrae resemble' the sacral, but they diminish in size towards the tip of the tail till only the centra are left. Draw figures of the vari02tS vertebrae and write notes on .each. II. The Ribs and Sternum. (a) The Ribs. Of the twelve or thirteen ribs the fifth may be taken 3,S typical. Each rib consists of a vert~bral portion and a sternal portion. The vertebral portion is dorsal and bony. It has a tuberculum~ich articub.tes with the tubercular facet of the tran:37erse process and a capitulum which articulates with the capitular facet of the centrum. Draw a figure of a typical rib and label its parts. (b) The Sternum. It is. formed of six segments, the sternebrae, the first of which is called the manubrium. The last is long and slender, the 154 THE RAT Xiphisternum and terminates in an expanded plate of cartilage, the xiphisternaL cartilage. Draw a figure of the sternum showing the manner in' which the ribs are attached to it. Label the p'arts. Fig. l4.-Diagram showing the arrangement of bones, in the skull of a mammal. (B) The Skull of the Dog. To illustrate the characters of a mammalill,n skull that of the dog is usually taken. In the present treatment emphasis has been laid on the general characters of theskull, therefore, the des('ription can be safely applbd to study the charact'jrs of the skull of rabbit with slight adjustments here and there. Before proceeding study fig. 14 that is a diagrammatic sketch showing the relation& of various bones in the formation of the skull. THE RAT 1. 155· The cranium or the brain·box. It is better to begin the study with the brain·bor which is made up of three rings placed one after another. The posterior-most ring is the occipital ring, the next the parietal ring and anterior one is the frontal ring. (a) The occipital ring is made up of (i) the basioccipital, a flat median bone forming' the base; I ii) the exoccipitals forming thesides; and, (iii) the large median supra occipital that completes the ring dorsally. (Note the difference in the shape of the' supraoccipitals of the rabbit and the dog). Between the exoccipitals lies the foramen magnum, at the sides of which are the occipital condyles. Fig. 15.-Sagittal secti':)ll of the skull of the dog. ~ (b) The parietal ring l·ies just anterior to the occipital ring and is made up of (i) a median bone, basisphenoid, just infront of the basioccipital, forming the base.; (ii) two large- THE RA'I' THE RAT 159 Internally the opening between the cranium and <olfactory chamber is closed by the ethmoid in which is the cribriform plate through the perforations of which pass the fibres of the olfactory nerve. The cavity is divided into right and left halves. by the partly cartilaginous mesethmoid the anterior end of which lies in the internasal septum. The mesethmoid rests in the y~shaped gutter on the upper margin of the vomer. The greater part of the cavity is occupied by the very 'much folded turbinal or scroll bones. (0) The optic capsules are represented by the ,orbit. The anterior wall of the orbit is formed by 'a 'Pair of lacrymals loosely wedged in between the frontals and the maxillae and each has a small notch at its outer border for the opening of the lacrymal duct. Jugal the anterior portion of the zygomatic arch forms, the ,outer boundary of tho orbit. 3. The Jaw • .(.a) The upper jaw. (i) The maxillae are two large irregular bones bearin~ soc ets or alveoli for the teeth. Ventrally the maxillq. is produced into a horizontal plate·like process which meets its fellow of the othpr side in the middle forming a bony bridge supporting the palate. These_procpsses are therefore called the palatal processes. ~ii) The palatines are a pair of horizontal plates adding posteriorly to the bony bridge supporting the palate. (Tn thl:! rabbit they are 160 THE RAT nearly vertical plates attaehed above to the ventral border of the pre-sphenoid, and: behind to the pterygoids and alisphenoids}. (iii) The pte.rygoids are a pair of narrow verti-cal plates of bone attached to the base of the skull at the junction of the basisphenoid with the a1isph~noid. , (iv) The premaxillae are a pair of bone~ formiJg the anterior part of the upper jaw. They articulate with the maxillae behind, and with each o~her in the inedian plane in front. The upper and poster}or angle of each premaxilla is produced backwards into a long nasal process which runs between the nasal and maxilla. (v) The zygomatic arch is an arch of bQ Ie extending between the maxillary and cranial region of the skull on each side. The :lnterior part of the arch is formed by the jugal bone and the posterior by the zygomatic portion or process of the squamosal. The two bone meet at an oblique suture. (In the rabbit the jugal is fused with a· process of the maxilla forming a process, the malar, that forms the zygomatic arch with the zygpmatic process (If the squamosal). (b) The lower jaw. The lower jaw consists of a single bone, the dentary on each side, and the two tegether are some times spoken of as forming. the mandible. The lower jaw articdateswith the glenoid fossa of the squamosal. Posteriorly the articular surface, however, lies between two processes, the anterior 161 THE RAT larger one, the coronary process, and the posterior smaller one the articular process. (In the rabbit, each ramus is flattened laterally and is comparatively broader. It bears an incisor tooth infront and grinding teeth behind. Between the two is a toothless space.) 4. The hyoid apparatus consists of a median body of the hyoid which lies at the base of the tongue between the rami of the mandible. Anteriorly the body bears a pair of'ar.terior cornua or ceratohyals and separate vestigial stylo.hyals ; and posteriorly a pair of long backwardly directed posterior cornua or thyrohyals are present. 5. The Teeth. The teeth in mammals are of different forms, heterodont. Those infront of the jaws are called the incisors followed on each side by the canines, and then by the cheek teeth made up of the premolars and molars. (i) The incisors (the upper one of which are always borne on the premaxillae) have a sharp chisel edge and are used for biting off the food as in the rabbit, but in the dog they are used for prehension. o (In the case of the rabbit the wear on the teeth being excessive the pulp cavity remains open and the teeth continue to grow through out life. Such teeth are said to have persistent pulp). I (ii) The canines are the characteristic teeth of the dog. like mammals. They are larger, 11 162 THE JRAT commonly concial and pointed, used for seizing the prey. (In the rabbit they are absent). (iii) The cheek teeth usually have broader crowns and the root has more than one fangs. They are made up .of paemolars and molars modified for grinding in some mammals (e.g. rabbit), but in the dog the last pair of premolars in, the upper and the first molars in the lower jaws are pointedly ridged and sharp edged, for shearing the flesh, and are called the carnassial teeth. (iv) The dental formu]a. , The number of teeth of each typO' varies in different mammals and to represent this a kind of equation called a dental formula has been evolved ba,sed upon what is regarded as the typical number in the upper and lower jaws on each side~ The following is the typical dental formula possessed by the pig .. i. Dental formula of the dog. " " " " 6. Rabbit Man ~-, c- i.~. i. t, c. f, pm. 1, ll.l. ~ = 44 h pm. t, m. i = 42 = 28 c. §, pm. i. f, c. f, pm. i, m. ~ -L m. -~ = 32 Foramina of the Skull. Locate the following foramina on the surface of the skull andsketch. THE RAT (i) 'The optic foramen is a large median hole in the orbitosphenoids for the optic nerves. ~ii) The foramen lacerum anterius is a vertical slit below the optic foramen a bit to ihe posterior side between the basisphenoid and the alisphenoid. It is for the exit of the third, fourth and sixth nerves and the maxillary and ophthalmic bran,ehes of the fifth cranial nerve. ,( iii) The internal orbital foramen is a small aperture a quarter of an inch infront of the optic foramen (exit for the nasal branch of the ophthalmic of the fifth cranial nerve). .(iv) The infra-orbital foramen is a prominent aperture on the zygomatic process of the maxilla (exit for the infra-orbital branch of the maxillary of the fifth cranial nerve). .(v) The anterior palatine foramen is a slit-like aperture anterior to the palate just internal to the incisor and separated from its fellow of the other side by the palatal process of the premaxillae (exit for naso-palatine branch of the maxillary of the fifth). (vi) The posterior palatine foramen is in the bony palate at the junction of its maxIllary and palatine processes (exit for the anterior palatine branch of the maxillary of the fifth). -(';vii) The foramen lacerum medium is between the alisphenoid and the periotic just infront of the tympanic bulla le~it of the mandibular branch for tlw fifth)_ IG4 THE RAT (viii) The stylomastoid foramen is about the mid:lle of the posterior border of the t.ympftnic bone, between it and the mastoid process of the periotic (exit for the seventh .cranial nerve). (ix) The foramen lacerum posterius is between the occipital condyle and the tympanic bulla (exit for the glossopharyngeal, pneumogastric and the spinal accessory nerves and the jugular vein). (x) The condylar foramina are holes in the exoccipitals just infront, of the condyles (exit for the hypoglossal nerve). (xi) The lacrymal foramen is in the r.uter border of the lacrymal bone t exit for lacrymal duct). (xii) The pituitary foramen is a median hole in the centre of the basi-sphenoid. (xiii) Th'1 carotid foramen is in the ventral surface of the tympanic bone close to the occipital condyle (passa€!.e for the internal carotid artery. Among other apertures of the skull are the anterior narial openings, the openings of the eustachian tubes the external auditory apertures and the foramen magnum which the students are familiar with. (B) The Appendicular Skeleton. 1. The Pectoral Girdle. The pectoral girdle is composed of THE RAT :3.. 165 The scapula which is a triangular bony plate, having at its apex the glenoid cavity into which fits the head of the humerus. The coracoid is fused with it giving a small coracoid process or spur. lb. The suprascapula is reduced to just a strip of cartilage. On the outer surface of the scapula is a ridge, the spine, the free vt'ntral edge of which has two projections, the acromion process and the metacromion process. The acromion is a free downwardly directed process, whereas, the mentacromion is a long process projecting backwards. c. The clavicle is a slender slightly curved bone running from the sternum to the scapula. Draw a figure of the pectoral girdle and write down the differences between the girdle of the varanus, the pigeon .and the rabbit. 2. The Fore-Limb. (a) The arm consists of only one humerus. bone, the Note the head of the humerus that fits into the glt'noid cavity and also the trochlea with a small hole called the supra-trochlear foramen. Note the deltoid ridge. (b) The forearm consists of two separate bones immovably articulated. Both are almost equal in size. 166 THE RAT (i) The radius is an elongated bone, slightly curvtd forwards, proximally it articulateswith the trochlea of the humerus and distally with the bones of th{, wrist. (ii) The ulna is the post axial oone of the forcarm. It has an en18>rgcd proximal end on the anterior surface of which is the sigmoid notch for articulation with the trochlea. Behind the elbow-joint, the bone forms the large laterally compr('ss{ld olecranon process. Distally the ulna narrows considerably and articulates with the car_D.us. (c) The wriSt or carpus. The carpus is made up of eight bones. I n the proximal row are three, the radiale and intermedium articulating with the radius and the ulnare articulating with the ulna. The centrale is a small central bone wedged between the two rows, thongh it appears to belong to the distal row. The distal row consists of four bones. (d) The hand. The hand consit;ts of five metacarpal bones followed by five digits with typical numbE'l"' of phalanges. Count the number of phalangeS in each digit and note. Draw figures oj the arm and forearm with the wrist, palm and dIgztS. Compare them with the (orresponding bones in other animals. THE RAT 3. 167 The Pelvic Girdle. It is made up of two halves. Anteriorly they are firmly attached to the sacrum and posteriorly they unite with each other by means of a strip of cartilage at the symphyeis. Each half-the innominate bone-of the girdle is made up of: a. The ilium, a blade-like pOl:'tion running parallel to the vertebral column. b. The acetabulum, a' concavity into which the head of the thigh bone fit!!. c. The ischium, the posterior and dorsal portion of the innominate bone is the portion opposite to the ilium. d. The pubis, the smallest bone of the three is a slightly transverse bone near the acetabulum. The pubis, symphasis and ischium enclose a cavity known as the oblurator foramen. Draw a figure oj halJ the pelvic girdle showing all the parts and compare it with that oj the dog- fish, varantts and pigeon. 4. The Hind-Limb. I. A. The I thi~h. I (i) The femur is the stoutE'st bone in th{' rabbit. At its Janterior end it bears a rounded head which :t:its into the acetabulum, and below the head there are three projections: the ~reat trochanter, which is anterior most, the lesser tr~chanter, which is opposite 16') THE RAT this, and the third trochanter, which is just below the head. The diE>tdl end hears two large condyles separated by a notch, the intercondylar notclil, into which fits the patella' or knee.cap. The condyles articulate with the tibia. (ii) The patella or knee cap iB a large bone connected by ligaments with the tibia and slides in the groove on the front of the lower end of the femur. Feel your patella, move yoltr leg and see how it slides. II. The leg is made up of two bones of unequal Rizes. (i) The tibia is a stout straight bone. Itfl enlarged proximal end bears two oval surfaces on its upper end for the -articulation with the condyles of the femur. Below the head is the prominent cnemial crest, a ridge ruoning down the anterior surface of the bone. DistalJy it articulates with the tarsal bones. (ti) The fibula is a slender bone fused distally with the tibia. III. The tarsus consists of six bones arranged in two rows with a central bone between them. a. The proximal row of the tarsals consists of two bones, the astragalus and the calcaneum. 'The astragalus lies on the inner or preaxial side and articulates with the dil>tal end of the tibia. The calca.DllUm is the largest tarsal hone, along the ou·ter side of astraga· Ius, and is produced into a large calcaneal process or heel. THE RAT 169 b. The central bone or centrale (naviculare) is placed infront of the astragalus. c. The distal row of tarsals consists of three bones that support the foot. IV. The foot has four toes only, each consisting of a long metatarsal bone, followed by three phalanges, the last of which bears a claw. The first digit or hallux is rudimentary and represented by a backward1y directed processe at the proximal -end of the second (apparent first) metatarsal, and articuJating with the central tarsal bone. Draw figures and write notes on each important bone. CHAPTEr. VI HISTOLOGY, Having studied the gross anatomy of the organs it becomes necessary to study the minute structure of the-'organs and the tissues of which they are composed. Such a study is called histology or might equally well be called miscroscopical anatomy Do you know what is a tissues? The word is often used somewhat loosely. In general it is applied to any aggregate of more or less similar cells, together with any in1iercellllktr secretion produced by them. The study of histology is (lone either by "whole mounts" i.e. taking bits of tissues and mounting them as such, or by cutting sections and mounting them. The whole mounts where only very small animals, or small pieces of organs of larger animals are concerned are studied in one of the following ways: (A) Saline mounts. (i) The simplest form of fre8h cover-slip preparation is to take fresh tissne (or small animal)r keep It in salt solution, put a cover-slip and examine. Salt solution is used as it keeps the tissues in fresh condition for a long time. (ii) There are certain dyes that can stain the tissue:; in their living state. Neutral red 170 HISTOLOGY 171 i:; one of the commonest of such stains. It· is dissolved in salt solution. Take thematerial, say connpctive tissue, spread it on a slide and cover it with neutral red. Put a cover slip and examino. Methylene blue is Itlso used for staining fresh and even living tissues. It is prepared in clilute salt solution. (B) Glycerine mounts. Material fresh or fixed is. teased stained, mounted in glycerine and examined. (C) Permanent mounts. PreparatIon of permanent slides takes a sufficiently long time and needs great car~. The following is the method of making permanent, preparations: (i) Fixation. Every fresh tissue, for such a study" has to be killed in such a way that the protoplasmic structures may remain, more or less, in the same condition in which they exist in life. For this purpose the tissue is' treated with such a reagent which kills it, penetrates inside a,nd "fixes" its various contents. Such a reagent is call'"ed a fixative and the process is called fixation. INxatlOn, therefore, is a process of conversion of the more or less soluble and diffusible substances constituting protoplasm into insoluble and non-diffusible substances, in such a way as to preserve the structures present in' the living state. (a) The fixatives. For the present purposes the best fixatives and hardening reagents are; 1. Absolute alcohol. 2. Acetic acid glacial or its 5 per cent solution which is specially used for rendering nuclei more distinct. _ft 72 HISTOLOGY 3. Formalin ordinary 40 per cent which is very helpful for gelatinous tissues. (b) Procedure. Take the tissue on a slide or in a watch glass and cover it with the fIxative to be used. Leave for 10 to 15 minutes. If the tissues are thick the time ma v be increased upto half an hour. . Note: Formalin prrserved material, if given, need not be treated by the above reagents as it is already fixed. (ii) Washing. After taking out the tissue f£om the fixative wash it in water changing it twice or thrice, till the fixative is thoroughly washed out. If the fixative is not washed out completely the stain will not act properly. Important. The material is directly transferred from water to stain, in case aqueous stains are to be used. Sometimes the stains are alcoholic. In that case the tissue has to be stained while it is being dehydrated. (iii) Dehydration is a process with the help of which water is removed from the tissues. To make a permanent; preparation it is always necessary to remove water entirely from the tissues. This is done by means of yarions _grades of alcohols. In fact the real dehydrating agent is .absolute alcohol, but if the material is put in the absolute .alcohol directly from water the tissue gets distorted. It is, therefore, treated with various lower :zrades -of alcohols beginning usually from 50 per cent alcohol. This is how it is done: Transfer the material from water, whether it is after staining in aqueous stains or directly after washing the fixative, to 50 per cent alcohol and keep in it for at least .'5 minutes. 'Then transfer it to 70 per cent alcohol for.'5 minutes; from this to 90 per HISTOLOGY cent alcohol for 10 minutes. From 90 percent thematerial should be taken to absolute alcohol for 10-, minutes. It is always better to change the absolute alcohol twice. So far as possible dehydration should be done in small corked tubes. (iv) Clearing. After removing moisture the material has to be cleared. This process is called clearing which is donl with the help of certain reagents called clearing agents or clearatives. The best clearatives for the present purposes are: 1. Xylol. 2. Cedar-wood oil. 3. Oil of cloves. 1Iethod: Add any of these to the absolute alcohol containing the material. If the quantity of absolute alcohol is enough drain off a little leaving only a small amount in the tube and then add the clearing agent. Atter a couple of minutes drain off the mixture and add only the clearative. Leave the material in the clearative tilI'it becomes transparent. (v) Mounting: Now the material has to be transferred to some such 'medium that may penetrate the tissue removing the clearing agent and at the same time solidify. Thee are many such mounting media, as they arc called, but the most suited for t,he present purposes is canada balsam. 1-Iethod: Transfer the 'clear' material from the staining tube to a dry slide by means of a soft brush. Keep. 174 HISTOLOGY the material in the middle of the slide. Cover it with a ,drop of canada balsam. Put the cover slip in such a 'Way that no air may get inside. This can be done thus: Keep the coverslip perpendicularly on t.he slide .beside the drop of balsam, support the free edge of the .coverslip on a needle and lower it gradually till it covers the matedal (vi I Staining: To study the £iner structure of cells in details It is better to colour the tissues.. The colouring c!Jgents are of two types, those that stain the nucleus and those that stain the cytoplasm only. ]ror the present purpose the most popular 'nuclear' stains are carmine .and haematoxylin, though they also give a general colouration to the cytoplasm. Among the useful cytopJasmic stains are eosin and light green etc. Often the material is stained in one stain only ana .mounted; but sometimes, it has to be counter.;tained with a cytoplasmic stain differing widely in colour from the nuclear stain to be used first. For eXi~mple if haematosylin (blue) is used for the nucleus, eosin (red) may be .used for the cytoplasm. Such a staining is called double staining. With respect to the solubility of the eolouring materials .the stains may be aqueous ie. dissolyed in water or ,alcoholic ie. dissolved in alcohols. (a) Aqueous StaiRS. Tf the aqueous stains have to be uspd tran~fer the material into the stain directly after washing away th~ :fixative. Keep it for 10 to 15 minutes depending upon the size of the materiaL Wash the stain in tap water and examine. If the stain is more than needed differen-tiate the stain in acid wa.ter. Then dehydrate" HI~TOLOGY 175 Gb) Alcoholic Stains. If the stains to be used are alcoholic such as borax carmine, which is dissolved in 70% alcohol then pass the material from 50% stage to the stain giving it 10 to 15 minutes. If overstained differentiate it in acid alcohol .and _upgrade. Double staining; In case the material has to be .stained with two stains say haematoxylin and eosin proceed as follows: After washing off the fixative pass on the material in haematoxylin. Keep it for 5 to 10 minute. Wash it in tap water so the material becomes blue. Upgrade upto 70% stage. Pass it in eosin for i to 1 mts. Wash in 90% quickly and pass on to the absolute. Do not leave in the absolute alcohol for long otherwise eosin will be washed off. Clear the slide quickly and mount. (vii) Differentiation: The superfluous stain has to .be washed out if the material gets overstained. For this purpose water with 1 % mtric or hydrochloric acid is used for aqueous stains and 70% alcohol with same percentage of acids as above is used at 70% stage for ,alcoholic stains. To get good result it is always better to overstain the material and differentiate. Differentiation does not only wash away superfluous etains but jm proves the tone of the colour. PREPARATION OF, TISSUES. (a) I. Epithelial Tissue. The Squamous Epithelium. Gently scrape off the outer covering oj the skin oj a preserved jrog alld examme it under a microscope. When 176 HISTOLOGY a piece showing ,the characterist~c structure of squam~l(s epithelium 1S obtamed dehydrate tt a)7er thoroughly washtng the preservative in water. Stain the tissue in borax carmine at 70% stage. Dlfferntiate in acid alcohol, upgrade, clear and mount 1n canada balsam. Peritoneum is other suitable material for the preparation of this type of epithdium. (i) Note the cells are hexagonal with distinct large nuclei. (ii) Intercellular space is very much reduced. II. Columnar Epithelium. Break up a shred of epitheliu:I1t from a minttie piece of the mucous membrane oj small intestine of a joramlin pseserved frog, and examine for columnar epithelium. Dehydrate upto 70% alcohol, stain it in borax cramine di iferentiate, upgrade and mOftnt in balsam. 1.{ake another similar preparation and try double staining in haematoxylin and eosin. (i) The Cells are elongated rod-like lying side by side. (ii) Nuclei are towards their inner Or deeper ends. III. Ciliated Epithelium. (i) Gently scrap gff the m'lICOUS membrane of thepharyugeal region of the buecal cavity of a recently chloroformed frog mount it in salt solution and examine. The lashing movements of the cilia can be seen. 177 HISTOLOGY (ii) After thus isolating the cells fix them on the slide in a week solution of formalin, for at least 10 minutes; wash the fixative well and up. grade to 90 % alcohol. Now stain in borax carmine for 5 mts. only, differentiate, up. grade and mount. The whole process should be completed on a slide. (iii) Ciliated epithelium is also prepared from a trachea. Take a bit of trachea and treat it with 1 % osmic acid for some hours. There· after leave it in waler for at least a day or two. The cells can easily be separated by gently tapping the coverslip. Dehydrate and mount. (i) The cells are usually columnar. (ii) At their free ends they bear extre. mely fine tufts of cilia. IV. Glandular Epithelium. (a) Simple Glands. Examine a stained prepared section of the large intestine following. 1)/ the rabbit and note the (i) The glands are surface. simple depressions of the (ii) The glandular epithelial cells lining the pits or depressil!ns are short columnar. granular cells, many of which are swollen to form goblet cells. 12 178 HISTOLOGY (b) Compound glands. Examine a pre pared slide of the stomach of the frog alta note the characteristics of a gastric gland. (i) The cavity is not simple pit like depression but subdivided or branched in a tubular manner. Each branch is of tolerably uni. form diameter. . (ii) Notice the neck of the gland and also the mouth. The liver and the pancreas are examples of rftCemose glands. The description of these i~: given iIlt the next chapter. (B) Muscular Tissue. (a) Striped Muscles. Take a shred of muscle from a recently killed frog or rat ()n a dry slide and carefully tease the tissue along the length of the bundles so as to separate single fibres. Stretch them out and cover them with 4% acetic acid for 1tearly 15 minutes. Wash with water thoroughly and stain ill haematoxylin. Counter-stain in eosin at 90 % stage, dehydrate, clear and mount. Examine and note : (i) The transverse striations. (ii) The spindle-shaped lauclei scattered here and there. (iii) The sarcolemma-delicate elastic sheath. , 179 HISTOLOGY (b) Unstriped Muscles. Vnstnped muscle fibres can be mounted exactly in the -same way as the striped. The material should be taken fr01It the ttrinary bladder of a frog or jrom the In Itscular wall of the mtestines. Examine and note : (i) The hands of muscular fibres. (ii) A number of fusiform, nucleated cells forming the hands muscle (iii) The cells are not striated. ~C) The Connective Tissues. (1) The Blood. There are many methods of studying the blood. Of these only two given below, will be quite helpful for the :present purposes. (a) Obtain a drop oj frog's blood on a dry slide altd mix it with a drop oj salt solution {tpon the slide. Examine with a high power. (i) Notice t,he shape of the coloured torpuscles both when seen fiat and sideways. (ii) Notice the colourless corpuscles, smaller than the red corpuscles and pall;' in· colour. Do not confuse them with the rounded liberated nuclei of crushed red corpuscles. ~iii) Also try to make out blood pIa.telcts. the spindle-shaped 180 HISTOLOGY ~b) Make a thin film of blood on a dry slide and dry it quickly in air. Fix the film by immersing for aboztt 30 mts. in a tube containing 90 parts of absolute alcohol and 10 pa~ts. of formal, or by simply pottring absolttte at~oltol on the sh~e and keeping tt for 20 to 30 mts. Stam w haemutoxyltrt first and upgrade. Counter-stain· in eosin after 70% alcohol. stage. Qttickly ttpgrade and moullt in canada balsam. Important. To make a film take two dry slides. Keep a drop of blood on a slide. Keep tlte ground edge of the other slide just in umtact with the drop and then keeping tt' almost perpendicular quickly draw the shde evenly over the surface oj the other slide. This will leave a thin film of blood on the slide. If the film 1S not thin or even try it again. Make preparations of the blood of the bird, the varanUB and the rat and examine similarly. Note the salient rlointB of differences between the blood of these animals. In case a preparation of human blood has to be made prick the finger above the nail or the tip wront. Be careful to sterilize the needle in flame before pricking. Proceed further as usual. (2) Areolar Tissue. (a) Take a bit of the subcutaneous tissue and spread it out into a large thin film with needles on a dry slide. Keep the centre moi~t by occasionaUy breathing on it, but allow the edges to dry on the slide. Put a droj) ·of salt solution on a glass cover-slip and now iuvert this over the film. Examine: with a high power and sketch. Note (i) Wavy bundles of whit,e fibres. (ii) Straight, slender yellow elastic fibres .. (iii) The connective tissue eorpuscles-. HISTOLOGY lSI (b) To make a permanent prepar.ation make another film -of the subcutaneous tissue as directed above and fix it by pouring 10% formal and keeping in it for about 20 mts. Wash in water and stain in haematoxylin. Counter-stain in eosin. ilehydrata and mount in balsam. Examine and sketch. (3) Cartilage. Cut two or three thilt sections of a fresh cartilage of a joint or of epi-or xiphi·sternwlt of the frog. Mount ilt salt solution and examine. Make some more sections and treat them with 1% acetic acid for 10 mts. wash well with water and stain with haematoxylin. llfolmt in glycerine. Perm:ment slides of this can be made by the usual methods bitt as the sections are relatively thicker it 1S no use making a permanent slide. Only mount in glycerine, examine .and sketch. Note (i) The intercellular matrix. (ii) The cartilage cells embedded in the matrix in definite cell spaces. Dividing cells are also seen. (4) Bone. For the study of bone examine prepared slides showing ;the structure of ta.) simple bone (b) compound bone. (a) Simple bone. (i) The outer osteoblast cells. (ii) The bony lamellae surrounding the marrow cavity. (iii) The lacunae 01' cell spaces. (iv) The canaliculi or the fine branching canals connecting the lacunae. 182 HISTOLOGY (v) The inner osteoblast layer. (vi) The marrow cavity. ·tr1.b) Compound bone. ~ - -:..r::~~ 1\ ,._J,.,. tt (i) The Haversian~systems form the greater part of the bone, and are readily recognised by the concentric arrangement of the lamellae' round the central callaIs, the Haversian canals. (ii) The lacunae or cell spaces cover the lamelllj,e as usual. (iii) The canaliculi. (iv) The marrow or the m'ldullary cavitT is as 'usual in the centre. (v) The circumferential lamellae are a series of concentric lamellae parallel to the surface of the bone, and forming its most superfecial layer. (vi) The prepledullary lamellae are a series of concentric l'amellae lining the central medul· lary'cavity of the bone. Also examine a longitudinal section of a long bone. (D) Nervous Tissue. (a) Nerve Fibres. Take a piece of a big nerve, say the vagzts, stretclt it fltlly witlt two needles and keep it in 1 % osmic aCid for 24 hOltrs. After washmg in water separate a small piece of nerve into HISTOLOGY 183 its fibres keeping in a dilttte solution of glycerine. Mount in glycerine and examine under low and high powers and see if yOlt can locate. (i) The medullary sheath. (ii) The nodes of Ranvier, (iii) The nuclei of neurolemma etc. (b) Nerve Cell. Put a small fragment of spinal ganglion of the frog into 1 % osmic acid for a few hours, place it in water containing a fragment of thymol for two days or more. Tease in dilute glycerine and examine the strllctttre of a nerve cell or neuron. (i) Eac'j neuron consists of a cell-body from which arises a system of branching processes. (ii; Tho dendrons and their subsequent branches, the dendrites, are small branches. (iii) The axon is a long branch of each neuron that passes into a nerve fibre. CHAPTER VII HISTOLOGY (CONTINUED). For the study of the histology of various organs prepared slides will be provided. Stndy them in comparative manner and note down the salient features of differences between the histology of organs of the different classes of animals. 1. Skin. Examine a vertical section of mammalian skin,-skin of the rat, rabbit or man and note the following feattlres ; (A) The epidermis, the outer layer made up of (i) The outer stratum corneum in which the cells are arranged in stratified layers. (ii) The innermost malpil~hian layer, a layer of columnar cell'l. D_o you know the function of this layer ? (B) The dermis or corium is the deeper layer made up of a net-work of connective tissue having both elastic and non-elastice fibres. Some fibres lie horizontally while others are perpendicular to the surface. This layer is richly supplied with blood vessels, therefore, numerous blood vessels cut in longitudinal or transverse sections are seen. Cutaneous glands, seb!tceous glands and roots of hairs, derived from the epidermis, become embedded 184 HISTOLOGY' 185 in the corium. Fat cells in the lower layers are numerous. Skin muscles are few. Where do you find them ? (Jompare the skin of mammals with that of the frog. Also examine the skin of fishes, reptiles and birds and note the differences. (a) Skin of Fishes is scaly. (i) Epidermi::o is highly glandular, not differentiated into corneal and Malpighian layers. (ii) Dermis or corium is a typical mesh-work of connedive tissue more stratified in its deeper parts and bearing the embedded scales which it gives rise. How do the .dermal scales of fishes differ from those of the reptiles? (b) Skin of Birds. Epidermis reduced to a delicate thinness, the corneal Jayer Qf epidermis is thickened only in those parts not covered by feathers like the shank etc. Feathers, beak, leg-scales are only epidermal. Dermis also reduced much. 2. Structure of Tooth. Examine a longitudinal section of tooth of mammal. The tooth consists of three parts externally. (a) The crown is the outer enamel capped portion. (b) The root is the end portion buried in a bony ,socket or alv~olus. 186 HISTOLOGY (c) The neck is a constricted region between the root and the crown. Internally each consists of : (i) 'The pulp cavity is a longitudinal centrltl canal, opening at the end of the root. (ii) The dentine is the bone-like substance that forms the large mass of the tooth. It sur- rounds the pulp cavity. At no place on the tooth does the dentine reach the surface since the crown or neck are covered with enamel while the root, is surrounded by a heavy cement. 3. The Stomach. Examine a transverse section of the stomach of the rat or rabbit and note the following parts. (i) The serous coat derived from the peritoneum. (ii) The muscular coat consisting of two layers of muscle fibres. (a) The longitudinal or outer layer in which the bundles run longitudinally. (In the frog this layer is replaced by connective tissue layer called subserosa). (b) The circular or inner layer which consists of muscle fibres disposed of circularly. (iii) Th~' submucosa consisting of areolar connective tissue and having a number of branches of blood vessels. HISTOLOGY 187 (iv) The mucosa in which is embedded the glandular tissue--the gastric glands mainly. muscularis mucosae is a thin double layer, composed of circular muscles (inner) aJld longitudinal muscles (outer), in between the mucous coat and the submucosa. (v) The Examine the gastric glands under high power and sketch. A gastric gland is a tubular gland consisting of one to five secreting tubules t.hat form the body of the gland. The tubules opE'n into a narrow neck which opens into the lumen of the gut through the mouth. 4. The intestine. Examine a T. S. of the intestine of the rat. Also exa· mi 11 C T. S. through duodenum of tl e rat and ilium of the cat. Note the following parts: 1. The outer serous coat of peritoneum. 2. The next muscular coat of longitudinal and circular muscle fibres. 3. The submucosa consisting of areolar tissue. 4. The mucosa having epithelium. simple layer of glandular In the region of the duodenum and near it the following other structures are also met with. (i) The crypts of Lieberkuhn arc simple tubular glands lined throughout by columnar epi. thelium with scattered goblet cells, and lying between adjacent villi. 3188 HISTOLOGY ~ii) The Brunner's gland are small tubulo-mcemost> glands situated in the submucosa. They open into the lumen of the gut t>ither between the crypts of Lieberkuhn or into them. I (iii) The Peyer's patches. The mucous membrane betwr.en the glands is mainly composed of reticular tissue, which contains nodules of lymphoid tissue, here and there. When they occur singly the nodules constitute so-called solitary glands of intestine, but when aggregated together they ·form the patches of Peyt'r. The Peyer's patches are chiefly formed in the ilium. 5. The liver. Study sections throltgh the live:r of the rat, pig and other mammals, note the following fecrtures aud sketch. The liver is a solid glandular organ made up of hepatic lobules separated from one an~ther by connective tissue. In some animals such as the pig this separation is complete, but in other animals such as the rat or the rabbit it is incomplete. (iJ The hepatic cells. Each lobule consists of a mass of polyhedral cells pierced <:lvery where with a network of blood vessels. Cells are granular in appearance a nd contain a spherical nuclei. (ii) The interlobular bran<:hes of blood vessels are found between the lobules ie. they are at the periphery of lobules. (iii) The intralobular veins are centre of the lobules. found in the HISTOLOGY 189· (iv) Tho hepatic ducts. The smallest ducts commence between the hepatic cells in the form of intercellular bile-channels, which lie b{'tween the adjacent sides of the cells and receive the secretions of the hepatic cells._ At the periphery of the lobules the intercellular channels pass into interlobular bile-ducts, which formally form the biIeduct proper. The bile-ducts are lined bycolumnar epithelium, outside which is a basement-membrane. In larger ducts some fibrous and plain muscular tissue may alsobe noted. Knowing this can you distinguish a section ofa bile duct with that of a blood' vessel ? (v) The Glisson's capsule is a capsule of loose connective tissue enclosing the branches of the portal vein, the hepatic artery and the bile-duct and also the lymphatic capsules. In transverse section such capsules appear as circular or oblong spaces enclosing smallm circles representing the various types of vessels. 6. The Pancreas. The pancreas is a racemose gland with longer and tubular alveloli. The connective tissue between the alveoli is somewhat looser. Examine a T. S. of the pancreas of the rabbit or man and note the following characteristics. (i) The pancreatic cells lining the alveoli are polyhedral in shape with granular cytoplasm. (ii) The acini (singular acinu8~ of the pancreas· are reduced and often occupied by spindle- 190 HISTOLOGY shaped central cells, hollow. instead of being ,(iii) The islets of Langerhans are clusters of lightly staining cells. The area of these clusters in section is considerably greater than that of a single acinus. The islets are ductless glands of the pancreas that secrete insulin . .7. The Kidney. A. Cllt a ktdney of the goat or sheep (or even the rat .if big szzed kidney is not '!vailab.e) lengthwise into two Aalves and examine. To the naked eye it appears to be formed of two Jlortions. (i) The cortex or the outer region which has a contorted arrangement of the uriniferous tubules • .(ii) The medulla or the inner region in which the tubules have a straight course . •(a) The pyramid is a cone-shaped l:Itructure the base of which rests against the cortical layer while the apex, the papilla projects into the cavity of the pelvis. ~b) The pelvis is the dilated commencement of the ureter. B. Study the sectwns of a mammalian kidney under low and high following: powers and 110te the 1. The cortex shows a number of rounded structures scattered here and there. These are HISTOLOGY 191 different-sized sections of the Malpighian bodies passing through their various aspects. Along with these longitudinal as well as transverse sections of convoluted uriniferous tubules are also seen. {i) The Malpighian body is the dilated commencement of the uriniterous tubule hollowed out at the tip; and containing the glomerulus in it. \(a) The Bowman's capsule is the mme given to . the hollowed out commencement of the uriniferous tubule. ,(b) The glomerulus is a lobulated knot of blood vessels lodged into the Bowman's capsule. Ui) The neck is the narrower portion of the tubule immediately below the capsule. (The neck is very prominent in the frog and not so in mammals.) (iii) The body of the tubule is the remaining portion. The tubule leaves the capsule by the neck and is convoluted in the cortex, then it becomes nearly straight and passes down into the medulla towarqs the pelvis. II. The medulla presents section through a -number of uriniferous tubules lying parallel to each other. III. The blood vessels are seen cut transversely as 'well as longitudinally all over the surface of the kidney. 8. The Testis Examine a T. S. of the testis of the rat or the rab'Jit a.mder low power and note the following structures: 192 HISTOLOGY (i) The tunica albuginea is the outermost' fibrous capsule. (ii) The crypts are the circular oval, and sometimes elongate surfaces representing' the transverse sections through the seminiferous ~ubules. 'The glandular substance of the testes is wholly made up of convoluted tubules, the seminiferous tubules. In transverse sections only their cut ends are visible. , Examine a crypt fInder high power. (a) The germinal epithelial cells form the outer margin of the crypts. (b) The spermatogonia, the speunatocytes· and the spermatids are found in the successive internal layers ; of these the spermatids. have very prominent nuclei occupying almost entire cytoplasm. (c) The spermatozoa are found in groups or bundles in the lumen of the crypt; and they\ can be easily recognised by their tails and. elongated heads mainly occupied by elon-· gate nuclei. (d) The Sertoli cells are elongate cells found here and there among the germinal epithelial cells of the ou1jermost layer of the crypt., They conduct nourishment to the interior. (iii) 'The interstitial cells are spindle-shaped' cells embedded in the connective tissuefilling up the spaces between the adjacent tissues. 193 HISTOLOGY Compare the structure of the transverse section of the testis of a. illammal with that of the frog and the pigeon. 9. The Ovary Study the transverse section of the ovary of tlte rat or the rabbit both under low and high powers. (i) The germinal epithelium is the outermost layer of the ovary. (ii) The stroma is the fibrous tissue forming the main bulk of the ovary. Scattered all over the stroma can be seen sections of blood vessels (iii) The ova of different sizes in the various stages of development can be found through. out the stroma. The smallest of these, the • oogonia, are found near the outer surface, the larger ones, the primary oocytes, the secondary oocytes, the primary follicles and the Graafian follicles etc. are placed more deeply in the stroma, although as the Graafian follicles incren.se in size they extend towards the surface. (1) The primary follicle. In the early stages the primary oocytes are not surrounded by the epithelium of follicle cells. In the smallest follicle the epithelium is formed of a singl!.'- layer of cells. In still larger follicles the epithelial cells are in two layers and are more or less columnar in shape. In still further stages each of the two layers becomes several cells deep and in between them fluid begins to collect at one place. This liquid is the liquor folliculi. 13 194 HISTOLOGY (2) The Graafian follicle. Examine a Graafian follicle under high power. (a) The theca externa and the theca interna are two layers forming the wall proper of the Graafian follicle. These layers are derived from the stroma. {b) The membrana granulosa is the outer layer of follicle cells often formed of several strata of cells. -( c) The cumulus oophorus or discus proHgerus is the mass of follicle cells which surround the ovum. {d) The liquor folliculi is 3. liquid filled' space in the follicle cells bound by the membrana granulosa externally with the discus proligerus on one side. ()RAPTER VIII THE DEVELOPMENT OF THE CHICK. 'rhe requirements for the practical work on embryo~ 'ogy for the standard for which this book is meant are iimited. As such the following account consists of mere 'explanatory note:> to explain the characteristic features of certain important stages in the development of the chick .so tha.t the student may find it easy to study them. 1. The Egg. The egg of the fowl is large-yolked i.e. telolecithal. In other word the cytoplasm of the egg ill charged with yolk (deutoplasm), which serves as a source of nourishment for the Fig. 17.-The egg ofa hen with young organism until half of the shell removed. it can feed for its'Jlf. ,:\he yolk is, as a rule, aggregated chiefly towa,rds one pole of the cell, the vegetative pole, whilst the egg nucleus lies near the other pole, the animal pole. The egg of the fowl is surrounded by tertiary membrane (the shell) deposited around th'3 egg during its passage down the ,oviduct. SH~(,,~ .... ~~ 1otE.r.leRArfE~ 'VITE:lLtNE MtMeR~N£ Remove half of the shell of a hen's egg carefully and ,examine the internal structures. 195 EMBRYOLOGY 196 (i) The blastodisc or ~~erminat disc is a smaH area of cytoplasm forming a cap at theanimal pole. This is completely surrounded by yolk. In the centre of the blastodiso lies the germinal vesicle (egg nucleus). (ii) The yolk consists of a central mass of yellow yolk. A careful examination reveals that the yellow yolk is not of uniform composition. It consists of a central mass of whitish yolk around which are arranged alternating concentric layers of yellow and white yolk (not to be confused with the" white of an egg "). (iii) The vitelline membrane is the transparent membrane investing the oocyte. The "white of egg" gets deposited around the egg during its passage down the oviduct. Because oHhe rotating movements of the egg the "white of egg" gets deposited in a spiral fashion which is most clearly shown by twisted cords of albumen, chalazae, at each end of the egg. The "white of egg'" is secreted by the glandular walls of r,he upper parts of tue oviducts, whereas, the lower part secretes watery albumen and the material forming the outer tough shell membranes. The calcareous shell is secreted by still lower portion of the oviducts. 2. Fertilisation. The ovum is fertilized in the upper part of tho oviduct before the deposition of'the albnmen around it. Therefore, the hen's egg, when laid, is a zygote, a double structure composed of both malo anci female nucleus and protoplasm. EMBRYOLOGY 197 3. Cleavage. This is the process by which the 'Zygote divides into many cells or blastomeres which become smaller and smaller as cleavage progresses The process of cleavage is said to end with the completion of the embryonic stage termed blastula, though the division continues throughout life. As the egg is heavily yolked, cleavage is ('onfined to the blastodisc only. Such cleavage is of the meroblastic type. Cleavage begins soon .after fertilisation and continues till the egg passes down. Examine prepared slides showing various- important phases vf the. development of the chick and sketch. (i) The blastoderm is the name given to the blastodisc after cleavage has started. After a couple of divisIOns the cells of the blastoderm become completely delimited. A vertical section of the blastoderm reveals a. row of more or less rectanguhr cells just above the yolk cells. Fig. 18 -Median section of chick blastoderm showing segmentation cavity (ii) The blastocoel is the subgerminal cavity formed below the central cells of the blastodisc. The marginal cells remain as a layer directly living on the yolk. The embryo is now in the blastula stage. (iii) The extra-embry,onic endoderm. Further horizontal cleavages make the blastoderm 198 EMBRYOLOGY I several cell-layers thick. The blastoderm becomes larger spreading over the yolk which is engulfed and digested by the marginal cells as they encroach upon it. The marginal cells are several layers deep, except, at the extreme periphery. This layer lies directly on the top ot the yolk and is known as the yolky endoderm which later on forms the extra-embryonic endoderm. Examine a slide presenting the surface view of the blastoderm. (iv) 4t this stage the blastoderm shows two distinct areas. A centrally situated relatively clear area, the area pellucida, lyhg just. over the subgerminaJ cavity; and a darker zone surrounding it, the area opaca. Fig. 19-Yertical section of blastoderm showing formation of the endoderm 4. Gastrulation. In gastrulation a re-arrangement of the cells of the blastula takes place. In the chick gastrulation is separable into three distinct phases, namely, the laying down of th~ embryonic endodt'rm, mesoderm formation and the formation of the embryonic axial structures. (i) Endoderm Formation. Examine a medwn longitudinal sc'ction of the blastoderm showzng formation of the endoderm. EM:BRYOLOGY 199 As gastrulation begins the blastoderm expands and the cells of the area pellucida become re-arranged to form a single layer of cubical cells in the form of a definite epithelium_ The cells from this layer migrate onwards at the posterior end of the blastoderm and form a complete floor of the subgerminal cavity. This layer is the embryonic endoderm Thus a two layered embryo is formed, ~he outer layer the ectoderm and the inner endoderm. (ii) The Primitive Streak. (a) Examine a whole mount of blastoderm showing the prtmitive streak. --./' '·:lios~. Fig. 20-Surface view of the blastoderm showing the primitive streak and the forlJlation of the notochordal process The primitive streak is a linear groQve flanked on either side by ridge-like thickenings extending from the inner margin of the area opaca to approximately the centre of the 200 EMBRYOLOGY blastoderm. The primitive streak marks the longitudinal axis of the future embryo. The end adjacent to the area opaca is posterior whereas the other is anterior. At the anterior end the streak has a distinct IjIwelling, the primitive knot &r Hensen's node. (b) Examine a median lon~i'tudinal section of the blastoderm showing thl~ e'J.rly appearance of the primitive streak. In its early stage the primitive streak is just a thickening of the blastoderm. In section therefore it appears as a thickened area. of ectoderm. cells. (c) Examine a median longitudinal section of the blastoderm showing the appearance of the primitive groove. Fig. 21-1'ransverse section of the embryo showing the migration of mesoderm. The primitive groove is a narrow groove developing later on the surface of the primitive streak In section this appears as a shallow depression. (iii) Mesoderm Formation. Examine a median longitudinal section through primitive streak of at last 15 hour embryo. EMBRYOLOGY 201 As the primitive groove becomes fully formed there is, at this iime, a complete fusion of ectoderm and endoderm in the region of the primitive streak. From either side of the streak bud off certain cells and migrate side ways. TheRe are, the primitive mesoderm cells. They congregate, form a mass on either side and are further drawn into definite mesoderm sheets. (,iv) The Notochordal Process. Examine a slide showing the formaliOJ1 of the notochordal process. The notochordal process. The cells of the primitive knot migrate in a forward direction so as to form a column of cells connected behind with the primitive knot and lying in the middle line. (v) Neural Tube Formation. (a) Examine a slide showing the formation the neural plate. of The neural plate is formed in the early development stages by the convergence of the cells of the area pellucid a in the anteroposterior direction to finally occupy the longitudinal axis of the body just above the notochordal process. The neural plate extends from the anterior eni of the embryo to the posterior end of the primitive streak. As it becomes longer the primitive streak diminishes in size and ultimately disappears (this happens at about 40 hours incubation). (b) Examine preparations to show the neural folds both in surface view and in sections. 202 EMBRYOLOGY Soon after the formation of the head fold the lateral margins of the medullary (neural) plate are elevated to iOlLm the neural folds between which appears a deep groove the neural groove. These folds as they increase. in size bend inwards, toward each other, and meet in the midline Dorming a tube, the neural tube. :Fig. 22-Surface view of lO-somite embryo. (c) Examine more preparations to study the for- mahan oj the nemal ttebe and its primary dimsions. The anterior end of the neural tube become:;· enlarged and rapidly differentiates i~to three regions separated by constrictions. (1) EMBRYOLOGY 203' The anterior divison is the forebrain that expands laterally into the optic vesicles; (2) the middle one is the midbrain and (3) the hindbrain plainly marked by constrictions into a number of neuromeres. (vi) ~ead Folds. (/,t) Examine an entire mount showing the forma- tion of the head fold. Fig. 23-Lollgitudinal section of a young embryo showing the head fold. The head fold appears as a wide transverse crescentic fold across tho area pellucida just infront of thc head process, where the blastoderm consists of only two genu layers ectoderm and endoderm. The head process is the forward growth of the primitive ~tren,k which appears ill front of the primitive knot after about 18 hour's incubation and is the forerunner of notochord. The appearancc of the head fold marks the first step in the separation of the embryo from the blastoderm. (b) Examine a longitttdillal section formatiolt of the head fold. illustrating the :204 EMBRYOLOGY The formation of the hell,d fold raises the curved anterior end of t,he medullary plate and as the fold moves b!wk under the head a pocket lined with endoderm is formed beneath the medullary plate. This is the primordium of the foregut. (vii) Paraxial mesoderm. Examine entire mounts as well as sections showing somitcs. ID'ig. 24-Surface view of the embryo showing head fold and formation of two somites" 205 EMBRYOLOGY Ali the early head fold stage the lateral wings of mesoderm spread from the sides of the notochord and laterally across the area pellucida. The thickened inner margin of eactl wing, the paraxial mesoderm, forms a distinct band of cells bordering the noto. chord on either side, whereas, the outlying mesodermal sheet forms the lateral plate. The paraxial mesoderm becomes segmented by transverse grooves leading to the for.· mation of somites. Fig. 25-Transverse section of the lO·somite embryo showing the formation of splanchnocoel. Fate of somites. A somite when first formed is a block of cells. Each somite givcs rise to three structures. (i) The sclerotome which later develops into vertebrae and connective tissue. (ii) The dermatome which is transformed into the dermal layer of the skin. (iii) The myotome muscles. develops into voluntary (viii) Lateral plate. Examine a transverse section of the embryo showing the formation of the amniocardiac vesicles or pericardial cavity. 20() EMBRYOLOGY The lateral plate of mesoderm splits into a somatic layer above and a splanchnic layer below enclosing a cavity, the amniocardiac vesicle, in between. This occurs at about six-somite ·stage. The somatic layer of mesoderm and the adjacent ectoderm, with which it Boon forms a close contact, constitute the somatopleure th'J splanchnic layer with the endoderm form the splanchnopleure. The space between these two layers is common to the cavities of the pericardium and the extra-embryonic coelom. The simultaneous formation of small spaces in different parts of the lateral plates and thcir subsequent connection with one another and with the amniocardiac cavity results in the formation of a s;ngle large cavity, the coelom, bounded above by the somatopleure and below by the splanchnopleure. Later this single cavity may be divided into embryonic coelom and extraembryonic coelom. (ix) The Gut , Formation. Fig. 26-Transverse section through a seven-sorr ite embryo showing the formation of fore-gut. Examine a transverse section of all embryo of six to -seven somites. EMBRYOLOGY 207 Fr\Jm the above it is apparent that two inwardly .directed folds arise from the right and left sheets of splanchnopleure and advance towards each other. These lateral folds finally meet and fuse below the embryo so at first side walls, and later, the ventral wall of the fore gut are established. The fore gut continually increases in length as the fusion of right and left sheets of splanchonpleure progresses. (x) The Heart Formation. o Fig. 27-Transverse section of the embryo showing the formation of the heart. Examine transverse f'ection of the 7 -somite and lO-somite embryos to study the formation of cardiac tube. J n the region of the hind-bra.in, as tlie amniocardiac vesicles approach each other in the midline below the pharynx there appears a large blood vessel, the rudiment of endocardium, on each side. Both these later fuse to form a single tube, the cardiac tube, below the pharynx. The inner wall of the amnio-cardiac vesicles which lie against the endocardium form the muscular wall of the heart and are known as the myocardium. (xi) The Amniochorionic Folds. 208 EMBRYOLOGY Examine preparations shJwing the head folds' of the amnion. The amnion and chorion arise as folds of the ectoderm of the blastoderm on either side of the midline. Each of these folds is called the head foldl of the amnion. Each begins to grow rapidly over the head of the embryo to form a complete arch. Finally they fuse fmd leave two layers-the outer having ectoderm outermost and mesoderm innermost is the chorion, whereas, the inner layer having its ectoderm next to the embryo and its mesoderm outside is the amnion. (xii) The Allantois. Examine a sli~e of a 28-somite stage of the embryo the study of allantois. fO? Soon after the hind gut ha~ been formed by the tail fold of the embryo the ventral wall of the hind gut, jURt infront of the anal plate, grows out t,o form a sort of bud which projects out into the coelom. This is the allantois. It grows in size rapidly and becomes a pearshaped vesicle lying between the amnion and chQrion. Later the mesodermal layer of the allantois fuses with that of the chorion forming, the allanto-j~horion. Further examine sections of the various stages of embryo passing through the heart, the kidneys, lhe lungs and the limbs etc. CHAPTER IX MAIN GROUPS OF VERTEBRATES The classification of the animals is merely a convenient filing device in which each organism is place::l in a particular position in accordance with its resemblances and differences (phylogenetic relationships) with others. Accordingly the vertebrates are divided in five classes that will be des ribed in the following pages. CI.. ASS PISOES , Class pisces is divided into five sl1b-classes. Sub-Class l. " " " Marsipobranchii (Cyclostomata.} 2. Elasmobranchii. 3. Ganoidei. 4. Teleostei.. 5. Dipnoi (Lung Fishes.) Sub-class Marsipobranchii.- Vermiform animals with smooth scaleless, slimy skin ; suctorial mouth devoid of jaws; piston-like tongue with horny epidermal teeth; vertebral column unsegmented; notochord carti- 209 14 210 CLASSIFICATION laginous and p~rsistent ; single median nasal organ; intestine straight; segmentally arranged mesonephros ; one or two semi-circular canals in the ear; without paired appendages, generative ducts and conus a.rteriosus ; caudal fin practically diphycercal. ~mily Petromyzontidae.-Lampreys with seven pairs of external gill apertures leading into a subpharyngeal tube. Branchial basket present; mouth surrounded by circular lip and is provided with yellow horny teeth; nasal organ in the middle of the dorsal side of the head and the nasal duct ends blindly. Eyes present. Two dorsal fins. Intestine with spiral valve; gall. bladder absent. Found in the frest-waters and coasts of the temperate regions of both hemispheres. ~¥mple Petromyzon.o", L()..V"'-\,A,\ . vffamily Myxinidae.-Hagfishes with six p'lirs of branchial sacs opening by a pair of external apertures and leading directly into the pharynx; branchial basket; mouth without lips, but sourrounded by barbels; nasal aperture at the anterior end of the body and the nasal duct opens into the pharynx; eyes very imperfeot and hidden under the skin; dorsal fins and intestinal spiral valve absent but a gall.bladder is present; <:set'ies of segmentally arranged mucous-glands present on either side of the body. Attach themselves to living fishes and gradually bore their way into the coelom, devouring the flesh as they do so. Marine, found in temperate regions of both hemispheres. ~ample Myxine. Sub·class Elasmobranchii.-Sharks, rays and, chimaeras. Fishes with a cartilaginous el'ldoskeleton, placoid scales cover the body; and abdominal pelvic fins provided CLASSIFIOATIO N 2.J.l with claspers in the ma.le. The externa.l opening of the doaca serves as a common outlet for the rectum as weH as the rena.l and reproductive ducts. Operculum is absent except in the holocephali and the gills are la.minar. There is a conus arteriosus, an optic chiasma and a spiral valve in,bhe intestine. The oro-nasal groove, the pericardioperitonial canal and the large sinuses in the venous system are the characteristic features of the sub-class. The eggs are large and provided with a horny case. There is no air-bladder. With very few exceptions exclusively marine. Order 1. " " " " Pleuropterygii (extinct) 2. Acanthodi " 3. Ichthyotomi " 4. Holocephali 5. Selachii Suo-order 1. 2. Notidani. Squali. 3. Raji. -Drder Holocephali.-Unique; with four gill. clefts covered over by an operculum; adult with naked skin; spiracle absent; skull autostylic; eyes large and high up on the head; mouth small, ventral and bounded by lip-like folcls; three pairs of teeth in the form of tritural plates ; nostrils confluent with the mou'th; anterior dorsal fin has a strong spine on its front border; tail prolonged in chimaera into a long filament; extra claspers on the head and is front of the normal abdominal pore; 212 CLASSIFICATION notochord unsegmented and with numerous calcified rings; urino-genital part of the cloaca, is separate from the rectum. Found on the coasts of Europe, Japan, Australia, West coast of North America and the Oape of Good Hope. ~ample Chimaera. OnJer Selachii. Sub-order Squali.-Shark..!h.. with five gill apertures laterally 'placed, spiracle present or absent, never large ~ with two dorsal fins and with or without anal fins . ./""Family Scyllidae.-Stegostoma-,!:_iger-shark. With /alternating briUiant dark-brown and white bands )ver the body and fins. Attains a length of fifteen feet. Inhabits Indian Seas.. / . Family Carcharildae.-Zygaena-Ham mer-headed shark. Head transversely expanded with ey;~s at the extremity of the head lobes; mouth ventral and crescentic ; spiracl~ absent; viviparous. Temperate and Tropical Seas. F amity Cestraciontidae-Bull-head sharks " " Lamnidae-Mackerel sharks RlmlOdontidae-Whale-sharks Rhinidae-Ray-like sharks Pristiophortdae-Shark " " Scymnidae Spinacidae ~~ CLASSIFICATION 213 Sub-order RajL-Skates and rays_ Body flattened dorso-ventrally; pectoral fins enormously expanded anteroposteriorly; gill openings ventral, five in number; spiracles rpresent and dorsally placed behind the eyes; .caudal region usually slender; dorsal fins, if present, on .the tail; without anal fin. Family Pristidae. ample Pristis.-Saw-fish. Snout produced into a process of cramum and provided with lateral saw-like teeth; body disc-like and gradually passing into the comparatively thick tail. Tropical and sub-tropical. Family Rhi1tobatidae. Example Rhinobatus.-Guitar-fish. Tail long strong and without serrated caudal spine; rayed portion of pt ctoral fin not continued to the snout; electric organ .absent, viviparous. Tropical and sub-tropical. ~ Family Torpedinidae. v-Example Torpedo.--With an electric organ between the pectoral fins and the head and occupying the whole Bpace between the the dorsal and ventral integument, and consisting I)f vertically arranged columns supported by walls of connective tissue and divided by horizontal septa of the sam..: material into a number of compartments filled with gelatinous tissue through the middle of which runs a horizontal plate composed of a fine granular nucleated substance and of numerous nerve endings. Shocks from these can disable a man. Mediterranean, Atlantic and Indian Oceans. Family Rajidae. Example Raja.-Skates. Disc broad and rhombic; pectoral fins not extended to the front end of the snout; 214 CLASSIFICATION caudal fin rudimentary; males with patches of claw-like retractile spines on the upper side of the pectoral fin. Seas of both hemispheres. Family Trigol1idae. Example Trygon.-Sting-rays. Tail long and slender and with long serra"ted spine. Temperate and tropical seas. / Family M yliobatidae. Example Myliobatis.-Eagle-rays. Large pect()ri\! fins which are not present at the side of the head b~, reappear at the extremity of the snout as a pair of detach-cd fins; tail long thin and whip-like; vi.viparous. Temperate and tropical seas. ----- - _ . Sub-order Notidani-\Vith six or seven branchial apertures and a small spiracle; with one dorsal fin without spine. Vertebral column imperfectly segmented. Example Heptanchus. Chlamydoselachus, Hexanchus I 'and Sub-class Ganoidei.-Fishes with scales embedded' in the dermis and frequently covered by a layer of ganoin. Fins frtquently possess a single or double row of fulcra on their anterior edge; with a conus arteriosus, optic chiasma, free pectinate gills, operculum, abdominal pelvic fins and spiral valve in the intestine. The oviducts aI]ll urinary ducts unite and open by common aperture behind the anus. Skull hyostylic and tail heterocercaI. Order Chol1drostei. Example Acipenser sturgeons.-With five rows of keeled plates in the skin; snout elongated; mouth small without teeth; four barbels in front of mouth; fulcra on CLASSIFICATION 215 the dorsal lobe of the caudal fin ; notochord persistent and unsegmented. Large fishes in the seas and rivers of northern hemisphere. ~ Order Lepidostei. Example Lepidosteus Garpike.-Body covered with thick, rhombic scales arranged in oblique rows and connected with each other by peg and socket articulation; tail heterocercal; snout much elongated; vertebrae opisthocoelous ; paired fins non-lobate, and all fins bear paired fulcra; air-bladder with cellular walls; heart with a long valvular conus. Fresh-waters of North America, cuba and china. Order Amioidei. thin~ A m p l e Amia.-Scales cycloidal, over lapping and without ganoin ; fins without fulcra; tail practically homocercal ; vertebrae solid and amphicoelous; spiracle absent; air-bladder bilobed, cellular, lung-like and opens by a duct into the pharynx; heart with small valvular conus; bulbus arteriosus well developed. Amia approaches more closely to the teleostei than to I1UY other ganoid fish. Fresh waters of United States of America. O~der Crossopterygii. . Example Polypterus.-Vertebral column 0ssified ; tail diphycrrcal ; paned fins with scales basal lobe of which is rounded, tribl1sal and rhipidostichous ; two jugular plates between the rami of the lower jaw; body covered with rhombic ganoid scales; dorsal fins numerous; pelvic fins present. \Rivers of North and Equatorial West Africa. ~b-class Teleostei. Fishes with a bony endoskeleton, amphicoelous vertebrae, pectinate gills, four gill- 216 CLASSIFICATION slits, and a branchial operculum; without spiracle, conus arteriosus, optic chiasma' and intestinal spiral valve ; airbladder often present; scales thin, overlapping and in the form of bony plates; testes not connected with the kidney and with separate ducts and openings; caudal fin homocercal. This order includes the vast majority of living fishes, fresh-water as well as marine. The sub-class is divided into thirteen orders. In the following account only a few specialised and interesting teleosts are described. Order 5 ymbrallchii. Example Amphipnous.-Bo'tly eel-like ; scales minute or absent; paired fins absent; unp!Jired fins reduced; anus far from the head; air-bladder aho,ent; gill-openings confluent in a single slit; with a lung-like branchial sac on each side. Bengal. Order H eteromi ,Example Fierasfer. Without pelvic fins; vent at the throat eel-like, small shore-fishes of tropical seas often living as lodgers in the cavities of holothurians, star-fishes and bivalve molluscs; often commensal with the pearl oyster. Order Hemibrallchii. / Example 1. Syngnathus.-Pipe-fishes. Snout produced ; mouth small and at, the tip of the snout. Skin with bony plates arranged in ringe; gill-openings very sl.:lall and near the upper pOl:!terior angle of the gill cover ; one soft dorsal fin ; pelvics absent; males with an egg-pouch pla~d on the vdntral side of the abdomen, usually formed of two folds of skin ; eggs retained here till sometime after hatching.; tail prehensile and with a caudal fin. CLASSIFICATION 217 'v.(1fxample 2 Hippocampus.-Sea-norse. Tail prehensile ; without caudal fin, rest as in syngnathus . ./ Order Acanthopterygii ~ Example 1. Ditrema.-Fins supported by rigid spines; pelvic fins far forward. Viviparous fish; eggs develop in a saccular enlargement ot" the oviduct. Temperate North Pacific. ~mple 2. Pseudorhombus.-Flat-fishes. Cranium twisted anteriorly to allow the two orbits to be on the same side. In the young the two sides of the body are alike and the eyes normal; body' compressed laterally lower side of the body colourless; dorsal and anal fins long and without spines. ~ple 3. Echeneis.-W~th a suctorial, transversely laminated oval disc on the upper surface of the head (modified dorsal fin) ; pelvics thoracic. Found in all seas; attach themselves to floating objects and other fishes. Example 4. Dactylopterus.-Flying gurnards. With pectorals enormously extended in an antero 'porsterior direction; take short gliding leaps in the air with the help of pectoral fins. Order Ostariophysi. Example Clarias.-Cat-fish. Skin naked ; barbels present; with an accessory 'branchial apparatus in the form of It highly vas ular arborescent organ lodged in the dorsal half of the branchial chamber; are able to cross land in searoh of other waters. Muddy and marshy waters .of Africa and l'3outh Asia. Sub-Class Dipnoi.-Lung-fishes. Specialised fishes with gills, lungs, external and internal nares, reduced 218 CLASSIFICATION scales; skull cartilaginous and covered with few largesoutes ; jaws sh?rt ; teeth in the form of orushing plates; notoohord persistent and unsegment('d ; an operculum covering the gill apedures; paired fins with unsessmented cartilaginous axes; caudal fin diphycercal ; lungs are very efficient respiratory organs ; vascular system and heart well developed to deal with pulmonary respiration. Slwamps of Tropical Africa. Example Protopterus. OLASS AMPHIBIA. Class Amphibia is divided into the following four orders.Order " " " 1. UrodeIa. 2. Anura. 3. Gymnophiona. 4. Stegocephalia (extinct). Order Urodela.-Scaleless amphibia with a well developed tail" hich assists in the progression of the animal; eyes small, sometimes functionless and without lids, except in salamandridae ; tympanic membrane and tympanic cavity not developed; vertebral column long and vertebrae opisthocoelous; usually oviparous and mo~tly aquatic. Division Perennibrar.chiata.-Larval gills persist throughout life in the form of three plume-like appendages on either side; gill-slits present. 2H)" CLASSIFICATION Family Proteidae. ~xample 1. Necturus.-vVater Newt. With twogill-openings; eye-lids absent; fore and hind limbs present; fingers and toes four; trunk elongated and is separated by a slight constriction from the depressed head' and passes insensibly into the compressed tail which is· bordered by continuous median fin-rays. North America. I , ~mple 2. Proteu5..-With three fingers and two toes; eyes hidden beneath the skin; tail provided with a caudal fin unsupported by fin-rays. Rest as in Necturus. Inhabits the Carniola subterranean waters. Family Sirenidae. ~mple Siren.-Mud-eeL Body eel-like; gill-0penings three; premaxiIIaries and mandibles tooth:ess but with horny beaks; eyelids and hind limbs absent; fore limbs weak and with four fingers. South East United States of America. o-JUo ~ C;\A-~ Lo..u Division Derotremata -Larval gills di!lappear but gillopenings are retained by the adult. Family Amphiumidae. Example 1. Amphiuma.-Body eel-like; gill-openings two :rairs; branchial arcl1es four; maxillary boneS' present, both jaws bear teeth; two pairs of very small and functionless limbs wit.h two or three digits. Swamps and muddy waters of North America. Ex~mple 2. teeth III Ambl~stoma.-With a series of palatal a nearly straIght, transverse line; toes five; 220 CLASSIFICATION shows phenomenon of paedogenesis. uma. North and Central America. Rest as in Amphi- ~le 3. '21mblystoma and Axolotl Larva.-In the two.genera. Triton, the larvae have the power of developing sexual organs and reproducing (Paedogenesis, Neotony). The axolotl larva of Amblystoma has a tail fin ; three pairs of external gills and four pairs of gillopenings. It breeds several times a year and is founri in a state of nature in various parts of Mexico and the United States of America. The causes of this phenomenon appear to be the abundance of food and other favourable conditions of life. Division Myctodora.-Adult devoid openings. of bo~h gill- Family Salamandn:dae. Example 1. Triton cristatus.-The crested Newt. Aquatic newt with eyelids and laterally compressed and serrated tail; fingers four and toes five; larvae attain sexual maturity if prevented from metamorphosing. Great Britain and Europe. ~Example 2 Salamandra-Lafid Salamanders-t- tail sub-cylindrical and retained throug out life. ~o ~ ..u.DXtl-a Order Anura. Scaleless amphibia with a large depressed head and a wide mouth; eyelids 'Yell developed; nictitating membrane pr8s~nt ; tympanic membranes large and some times concealed beneath the skin, vertebral column remarkably short; ribs and tail absent; nasal apertures far forward and on the extremity of the snout; hind-limbs much lamer than fore-limbs; posterior part of the tongue is completely free and can be protruded as a prehensile organ. CLASSIFICATION ~ple 221 Family Bufonidae. Bufo.-Toad&.. Teeth absent ; tympanum usually distinct; fingers free; toes more or less webbed, tips dilated into small discs; skin covered with ugly warts. /" Family Hylidae. VExarp.ple Hyla.-Tree-frogs. Upper jaw with teeth; terminal pha1anges claw-shappcj and swollen at the base and carry adhesive cushions with which these animals climb trees ; tympanic membranes large. Excepting H. Arborea and two other species found in North India and South China, the rest are exclusivply American or Australian. Family Ranidae. ~Ple Rana Pipens.-The skin on the back of the female becomes soft and spongy during the breeding season and the eggs are placed on it by the males. Each egg singly into a little pouch of the skin covered by a gelatinous film_ Embryos do not pass through any larval stage, but are hatched in the adult form. -D-- ~ In addition to the above families the order contains three more families viz. Engystomatzdae, Pelopattdae and Dtscoglossidae Order Gymnophiona. Vermiform amphibia without limbs or limb-girdles, tail very small or absent, with numerous small scales pmbedded in the skin arranged in transverse rows; eyes small, functionless and covered by skin; tympanum and tympanic cavity absent. Burrow in the surface soil in damp places near streams. Central and South America, ~uatJrial Africa, India and the Malay Archipelago. , -222 CLASSIFICATION Example Ichthyophis gl~ \ Order Stegocephalia. Extinct; salamander-like or lizard. like ; dorsal surface of the skull completely covered by dermal bones; with bony dermal plates on other parts of the body; parietal foramen present. Earliest known pentadactyle animals. Examples Branchiosaurus, Dolichosoma and Laby.rinthodon. CLASS REPTILIA. Class Reptilia is divided into l,he following super,orders. Super-order 1. Anapsida. Order 1. " 2. Super-order II. " " " Chelonia Synapsida Order 1. " Cotylosauria (extinct.) Pelycosauria (extinct) 2. Dicynodontia 3. Theriodontia 4. Sauropterygia 5. Ichthyopteryg;ia Buper-order III. Order 1. ,. " " " Diapsida Thecodontia (extinct) CLASSIFICATION Order 2. 3. " Rhynchocephalia Squamata Sub-order 1. " " " " " " 223 Lacertilia 2. Ophidia 3. Pythonomorpha (extinct) 4. Saurischia (extinct) 5. Ornithischia 6. Crocodilia .7 Pterodactyla textinct) Sub-order Lacertilia. Lizards. Reptiles with horny ,epidermal scales, movable quadrate bones, transverse anal ,opening, a cloacal bladder and two sacral vertebrae. The two rami are firmly connocted at the symphysis; eyes with movable eyelids and nictitating membFane; tympanic eavity present. The sub-order contains twenty-one familirs. Only a few sp('cialised and interesting forms are described below. Family Agamidae. Example Draco.-Flying Dragon. Ribs much prolonged supporting a lateral wing-like dermal expansion by means of which the animal IS able to glide among trees. Osteoderms absent. fjound III East Indies, Family Anguidae. Example Ophisaurus.-Body elongated and snakelike limbs absent but limb-girdles present; osteodermal 224 CLASSIFICATION plates present on head and' body; eyes with movabfe lids. TerrestriaL South East Europe, North Africa, South West Asia, America and Mexico. Family Chamaele ontidae. Example Chamaeleon.~Tongue protractile to It length equal to that of the body, club.shaped and viscose; digits arranged in two bundles of tw() and three; eyes la.rge and covered with a thick granular lid pierced by a small central opening for the pupil ; tympanum arid tympanic cavity absent; parietals single, forming a knoblike crest; tail prehensile, coiling ventral wards. Thesfr animals undergo remarkable changes of colour. Africa •. Madagascar, India and Ceylon. Family Helodermatidae. Example Heloderma.-Gila monster. The only poisonous hzard. Body elongated and covered with ugly tul)crcles; limbs short, well developed and powerful; tail thick with reserve fat; teeth fang.like and grooved with labial poision-glands; osteoderms present. Found in Mexico. Order corocodilia.-Tail long and with a dentated dorsal crest, paired in front" but single behind; fore-limbs with five digits, hind· limbs with four webbed digits; nostrils at the tip of the long snout, ean be closed; false palate present; back and belly with transverse rows of bony plates which are overlapped by epidermal plates; the plates of the back are pitted, sculptured or keeled; abdominal ribs and nictitating membrane present; t.~eth lodged in . sockets ; pubis excluded from acetabulnm; long prepubis presl:'nt; quadrate immovable. The two living families of the order are Gavialidae and Crocodilidae. 225 CLASSIFICATION Family Gavialidae. Example gavialis.-Snout long and slender; nasals widely separate from nasal aperture. India and Burma. Family Crocodil'idae. Example 1. Crocodilus.-Head long; teeth unequal; fourth mandibular teeth fitting into a notch in the upper jaw; without bony nasal septum and ventral armour; AfrictL, South Asia; North Australia and tropical America. Example 2. Alligator.-Head short and broad; teeth very unequal; fourth mandibular tooth fits into a pit in the upper jaw; with a bony nasal septum and with ventral scutes. China and North America. Example 3. Caiman.-Without bony nasal septum; dorsal and ventral armour articulated together. Rest as in Alligator Central and South America. Order Squamata. Sub-order Ophidia.-The snakes are elongated reptiles without limbs and limb girdles with the exception of a rudiment of a pelvic girdle in some, e.g. Pythons). Horny epidermal scales without oste(}derms. The mouth is very extensible and even the upper jaw can move relative to the cranium-quadrate bones ate movable. The middle ear apparatus is lacking. The tongue is sensory. Some snakes have poison fangs. The vertebrae are procoelous and bear extra articulating facets. The lower border of both temporal vacuities has vanished. The anal opening is transverse. Without eyelids urinary bladder and sacrum. The rami of the m,andibles are united at the symphysis by an elastic ligament. 15 226 CLAS SIFICATIO N About 400 genera and 1,800 species are known. Very few fossil forms ~re known. Family Boidae. Example Boa.-The pythons-usually large snakes in which the maxillary and mandibular teeth gradually decrease in size. The ventral seales are transversely enlarged. The nostrils are placed between two or three nasals and these are separated from those of the other side by small scales. Eyes are functional and free, the pupil is vertical. Vestiges of hind limbs and pelvis present. The tail is short and prehensile. Prefer wooded districts, climbing trees; crush their prey 'in the coils of the body. World wide in distribution except New Zealand. Family Vipendae. Example Russell's Viper.-Nine genera and about fourty ~pecies of the vipers are known. Of these Russell's Viper is one of the scourges of India, Burma and Ceylon. The upper surface of the body is covered with ..;mall, imbricating, usually keeled scales. The general colour is pale brown above with three longitudinal series of black, light-edged rings, often encircling reddish spots. The under surface is yellowish white uniform, or with small crescentic black spots. Usual length is five feet. Family Crotalinae. Exomple Crotalus.-The Rattle-Snakes. These have a deep cavity or pit between the eye and the nose, lodged into the hollowed out maxillary bone hence also lalled "Pit-Vipers." Crotalu3 has got a rattle at the end of the tail. The rattle is composed of 8: nu,mber of horny bells which fit into each other. The terminal bell is in reality the horny covering of the tail. Now and then some of the bells break off, probably when worn o~t~ and a new CLAS SIFICATIO N 227 set gradually replaces them. As the snake moves the rattle produces strange rattling noise, a shrill sound like that of a rattling alarm-clock. The upper surface»£ the head is covered with small soales. The supra-ocular shields are broader often transversely striated. The largest of Rattle-snaks attains a size of 8 feet. Family Hydrophinae Example Enhydrina.-Sea Snakes. The characteristic feature of the sea-snakes is that the tail is strongly (!ompressed. All the scales are small, often the ventrals .are not enlarged. Eyes are small with round pupils. All the sea-snakes are very poisonous and live in the sea with the exception of one species of Disteria which is found jn a fresp. water lake in the Phi lipines. They live on fish. In the Enhydrina the maxillaries carry two or more small grooved teeth besides the poison fangs. The back is olive 0': dark-grey wi,h black transverse bands. The under surface is white. This speceies is found in the Indian Ocean. Family Colttbrinae. Ex!)-mple Naja.-Naja belongs to sub-family elapinae which comprise about 150 species. The tail is cylindrical. The pair of large grooved poison-fangs are separated by an interspace having from one to three small, fa5ntly grooved ,teeth near the posterior end of the maxillaries. The scales are smooth and ·without pits, and are arranged jnto fitteen to twenty-five oblique rows on the trunk, although more occur in the region of the neck. The head is slightly distinct from the neck. Each nostril lies between two nasals and the internasal. The pupil is round. The neck region can be expanded int.o a hood as the ribs of the region spread and move headwards. S(.'veral 'species of naja are known. The cobra is N. tripudians_ A black and white spectacled mark on the dorsal side of 228· CLASSIFICA'rION the hood is a characteristic feature of its colouration. The cobra is widely distributed and ascends to 8000 feet above. sea level in the Himahyas. ]~arge specimens may attain six-feet length. The 'cobra prefers places which afford it a convenient hole to retire into, for instance deserted hills of termites and ruins etc. It makes itself at home in inhabited house especially in villages. Order Chelonia.-Turtles and Tortoises. Body encased between the carapace and the plastron; single nasal open~ ing ; quadrate immovable; single temporal arcade; usually with an epidermal exoskeleton of horny platf's; thorl.l cia vertebrae and ribs firmly fused with the carapace; pectoral and pelvic girdles within the ribs ; limbs terminated by cla1vS or paddle like; tpeth absent but the jaw.; have a horny investment. Sub-order' Athecae.-Vertebrae and ribs not fused: with carapace; skin leathery, without horny plates; parietal bones without descending processes; head net retractile; limbs paddle-like. Indian, Atlantic and Pacific Oceans. Example Dermochl!lys. Sub-order Thecophora. Thoracic vertebrae and' ribs immova bly united with a series of neural and costal plates; parietals prolonged downwards meeting tbe pterygoid or separated from them by an epipterygoid. Example Chelone, TriollYX: etc. Order Rhynchocephalia. Lizard-like; verte'nae amphicoelous; quadrate immovable; upper and l(Jwer temporal arcades present; acrodont teeth; premax'llae resembling incisors of rodents; mandibular symphysis ligamentous; skin with horny scales; vertebrae contain persistent remains of notochord; ribs single-headed and CLASSIFICATION 229 with uncinate process. At present these animals are restricted to some small islets in the Bay of Plenty. Example Sphenodon. CLASS AVES. Class Aves is divided into two ord{'rs. Order I. " 2. Archaeornithes. Neornithes. Order Archaeornithes. Example Archaeopteryx.-Oldcst bird known from the lithographic slates of Solenhofen (Eavuia) showing avian as well as reptilian features. Teeth embedded in sockets in both jaws; manus with three clawed digits; a long tapering tail composed of about twenty separate vertebrat'l present; thoracic ribs without uncinate processes; .sternum not well developed; vertebrae amphicoelous; metacarpals free; pelvis and hind limbs avian. Order Neornithes.-All living birds; metacarpals fused with one another'; thoracic ribs with uncinate processes; tail much reduced; pygostyle present. Sub-order Ratitae.-Flightless birds; sternum without keel; coracoid and scapula nearly parallel; palate dromaeognathus; upper articular head of quadrate undivided; barbs of feathers disconnected, the barbules without hooks; syrinx absent; wings reduced in size; pygostyle, uncinate process, and pterylae absent. 230 CLASSIFICATIO N Family Struthionidae. Example Struthio.~Ostrich. With naked head and neck; neck long; legs powerful ; pes with only two digits. viz. 110S. 3 and 4; feathers without aftershaft. Inhabit. the plams and deserts of-Africa and Arabia. Family Apterygidae. Example Apteryx.-Kiwi. Small; four-toed; neck short; beak long and weak having the nostrils near theextremity; feathers without aftershaft and are long, hair-like ansi hang down loosely; wings vestigial, hidden and functiona) ; eyes remarkably small; legs short, powBrful and scaly; nocturnal. New Zealand. Family Rheidae. Example Rhea.-With partially feathered head and neck, with three-toed feet, and with ischiadic symphsis. Feathers without aftershaft. South America. Family Casuaridae. Example Casuarius.-With high, compressed beak; helmet-shaped bony knots on the head; with short p.eck, and thrpe-to('d f('ct. Aftershaft as large as main shaft. Australian region. :FamiliE's Dinomitlzidae (Moas) and Aepyornithidae are extinct. Sub-order Carinatae.-Inclm1cs grC'at majority 01 birds, palatines and ptcrygoids articulate with the basisphenoidal rostrum; quadrate triradiate; basipterygoid processes present or a.bsent; sternum with keel; coracoid and scapula nearly at right angle; barbules carry hooks. 231 CLASSIFICATION CLASS MAMMALIA. Class mammalia is divided into the following sub. classes : Sub-class 1. " " Proto theria (Monotrema ta) 2. Metatheria (Massupia lia) 3. (Placentalia ). Eutheria Sub-class Prototheria.-Mammary glands without teats; pectoral girdles with large coracoid, T-shaped interclavicle and precoracoids; vertebrae without epiphyses; ribs with single capitulum ; marsupial bones present; true teeth absent in the adult; ear without a pinna; male with a horny spur on the hind leg; right auriculo-ventricular valve incomplete; testes abdominal; urpters open into a urinogenital sinu.s which communicates with the alimentary canal; cloaca present; brain simple and without corpus callosum ; eggs large and meroblastic and laid at an early stage; body temperature relatively low. But. these animals are undoubtedly mammals because of the presence of hairs, warm blood, non-nucleated blood-corpuscles and a left aortic'arch. Family Echidnid{1e. Example Echidna.-Spiny-anteater. Skin covered with spines and 'hairs ; snout elongated; tongue long and protractile ; tail very short ; with an anterior abdominal vein, five cldwed digits on each limbs and a temporary pouch in which the single egg is placed. Burrows in sand and earth. Australia. Tasmania and New Guinea. Family Ornithorhynchidae. Example Ornithorhynchus.-Duck-billed platypus. Body covered \, ith a dense soft fur; flnout broad, elonga- 232 CLASSIFICATION ted and covered with a leathery skin produced into a fold at the base of the snout; molar teeth horny; f('et webbed; aquatic in habit and burrows in the banks of streams ; lays two eggs in a nest. Australia and Tasmania. Sub-class Metatheria.-Marsuplals. Mammary glands have teats and enclosed in a marsupial pouch in which the young are carried ; coracoid reduced to a process of scapula; epipubic bones present; va~ina double; scrotum in front of penis ; anus and the opening of mino-genital sinus enclosed by a common sphincter muscle; posterior part of the palate with vacuties; tympanic bulla, formed by alisphenoid; corpus callosum and fossa ovaHs absent; only the last premolar is replaced; glans penis bifurcated; an allantoic placenta, is absent. Metatheria are ccnfint'd to the Australian region (with the exception of New Zealand) and to South America. Order Didactyla.-Marsupials with all toes separate; usually with more than i incisors (polyprotodont dentition) ; marsupium opens ventrally or posteriorly. Examples Didelphys (Oppossums of America), Dasyurus, Thylacinus etc. Order Syndactyla.-Marsupials with the second and third toes of the hind foot united by a common sheath. Pouch opens forwards. Examples Perameles (Bandicoots of Australia), Phascolyms (Womoat), Macropus (Kangaroos) etc. Sub-class Eutheria.-The "higher mammal,\". Frino-genital organs mostly open independently of the rectum, vagina generally single, marsupium and epipubic bones absEnt; mammae and mamma,ry glands present; tympanic forms a bulla embryo with allantoic plact'nta; carpus callosum well developed ; testes generally pass OLASSIFIOATION :into a scrotal sac which hind the penis. i~ 233 placed at the sides of or be- Order Edentata.-Teeth absent from the anterior !part of the mouth; with scratching or curved claws on the extremities; with numerous rootless grinders. Family Manidae. Example Manis.-Pangolins. Edentlflous, cove.red with large overlapping epidermal scales which can be erected and with hairs between the scales; tongue long and extensile, limbs short and carry five digit8; walk .on the dorsal surfaces of the claws of their fore-feet and on the soles of the pes. Terrestrial and burrowing; can Toll themselves into a ball. Other examples of the order are Tatusia (Armadilloes) Myrmecophaga (Anteaters), Bradypus (Sloths) etc. Order Sirenia.-Sea-cows. Short necked; thick skinned; naked or with very sparce hairy covering; ·external nares anteriorly directed; pinna absent; two pectoral mammae ; fore-limbs paddle-like and very mobile; hind limbs absent; tail horizontally expanded into a caudal fin. Large herbivorous· animals adapted to life in estuarine waters or shallow seas of South America, Africa, Oriental region and Australia. Example Manatus and Halicore. Order Cetacea.-Wbales and Dolphins. Body fishlike and streamlined, naked, but with a thick layer of ,sub-cutaneous blubber; nasal apertures on the top of the head; caudal fin horizontally expanded; fatty dorsal fin present; external ear minute and devoid of a pinna; .eyes very small and at the angle of the mouth. Live in 2~4 CLASSIFIC.A. TION 'schools' (herds) and swim with grl~at strength and speed; camivorous and predaceous; found in all seas. Sub-order Mystacoceti.-Whalebone whales. Without teeth. Example BalaEma. Sub-order Odontoceti.-Toothed whales. With nUJ)1erous small homodont and monophyodont teeth. 'Example Platanista Gangetica. Order Hyracoidea.-The biblical Conies. Small rabbit-like creatures with close set fur, emaIl ears and a split snout; tail reduced. Most of these live on the ground except Dendrobyrax which lives in tre(~s. Inhabit Africa. Example Procavia. Order Proboscidea.--The elephants.. Large animals with enormous head; with a long, mobile proboscis serving aA a prehensile organ; nostrils at the end of the proboscis; limbs massi,-e; bemiplantigrade; inci&ors of great size and form tusks; canines and premolars absent; molars lophodont. Love dark and shady forests; herbivorous. "fith only two species confined to Africa and Indi \ (induding Ceylon, Burma, Malaya, Sumatra' and Cochin China). Examples Elepbas indicus and Elepbas africa 'IUS. Ortrer Artiodactyla.--The Even-toed Ungulates. Herbivorous; usually of large size; digitigrade; the axis of the limbs passes between digit,s nos. 3 and 4, these digits bein:; symmetrict.l an~l having their metapodia. CLASSIFICATION 235 closely applied together or united into a cannon bonc; digits bear hooves ; cheek teeth either bunodont or solenodont ; alisphenoidal canal absent,; femur without t,hird trochanter; dorso·lumbar vertebrae thirteen; stomach simple. Distributed aU -over the world except Australia and New Zealand. Examples Sus (Pigs), Hippopotamus Cervus (Deer), Ovis (sheel?) etc. Oamelus Order Perissodactyla.-The Odd-toed Ungulates. Axis of the limbs passes through the middle of the digit no. 3 and the toes of the hind foot are never more than three; premolars and molars alike; first milk molar replaced; alisphenoid canal present ; nasals expanded posteriorly'; dorso-fumbar vertebrae more than twenty two; femur with a third trochanter; tympanic small: orbits generally open behind; fibula does not articulate with calcaneum. With only three living gen~ra. Examples Equus Horses, ·(Malaya, Borneo, Sumatra and America), - Rhinoceros (India, Java, Sumatra and Africa). Order Carnivora Sub-order Fissipedia.-Terrestrial carnivora with . large projecting, canines; three incisors; cutting premolars; m.olars tuberculate; carnassial teeth present; clilVicles absent or reduced; scaphoid and lunar fused in the carpus; lini'bs unguiculate and provided with strong claws; orbits not closed behind; sagittal crest formed at the union of parietals; tympanic forms a bulla ; zygomatic arch is strong and stands well out from the cranium to allow for the powerful jaw muscles; gut short. Example Felis, (Cat), Canis, (Dog) etc. '236 CLASSIFICATION Sub-order Pinnipedia.-Aquatic carnivora with short "pentadactyle limbs; digits united by It membrane; nails .small or absent; incisors conical; molars and premolars alike; cranium rounded and capacious; tail short. Aquatic, mainly marine, but some ascend rivers; or inhabit inland seas and lakes. Examples Of aria (eared seals), Trichechus (walrus), Phoca (Seals) etc. Order Rodentia.-Small; herbivorous; plantigrarle ; unguiculate, incisors large, sharp, rootless and chiselshaped; canines absent; with wide diastema betwcen incisors and check-teeth; articular surface of the lower jaw longitudinally elongated; jaw muscles much enlarged; clavicles absent; brain little convoluted; caecum large; body covered with fine fur which may be modified llto spines nails usually claw-like; the habit of gnawing food is universal. Examples Sciurus (Squirrel), Castor (Beaver), Mus (Rats), Hystrix (Porcupine), Lepus (Hares and Rabbits) ·etc. , Order Insectivora -Very primitive ; small; covered with f1?-r or spines; plantigrade or semiplantigrade ; g6nerally pentadactyle, unguiculate feet; digits with claws; ,extremity of muzzle projects beyond the end of the man,dible ; incisors, canines and premolars not differentiatlJd ; molars rooted and tuberculated; skull elongated and depressed ; orbit not enclosed. by bone ; palate with vacuities; jugal weak or absent; testes never descend into scrotum; cerebral convolutions abse n t. \Videly dIStributed ovc'r the earth's surface; ab3ent from Australian ,regio-n and South Ameri~a. Examples Talpa (Mole), Tupaia (Trec-shrews), Erinaceus t Hedgehogs I, Sorllx (Shrews) etc. CLASSIFICATION 237 Order Chiroptera.-Nocturnal; flying mammals ; forcl·limbs modified as wings; second to fifth digits greatly elongated to support a broad web of skin exten:iing back to the hind-limb; wing comprises of the antebrachium, the wing membrane and rthe inter-femoral; tail, if present, is included in the patagium; sternum with a keel; ulna vestigial; pollex small; knee directed bltck wards; cerebral hemispheres smooth; teeth heterodont. and diphyodont; penis pflndent ; testes abdonimal. Sub-order Megachiroptera.--Frugivorous ; large animals; snout elongated; foliaceous appendages absent; second digit of manus with a claw; molars devoid of cusps. Tropical and sub-tropical regions of Eastern Hemisphere. Example Pteropus (Flying fox) Sub-order Microchiroptera.-Insectivorous; small animals; snout short; foliaceous appendages present; second digit of manus without a claw ; molars wi th sharp cusps. Tropical and temperate regions of both hemispheres. Example Vespertilio and other ordinary bats. Order Dermaptera.-"Flying lemurs". With hairy, muscular parachuting patagium extending from neck to the wrist and down the sides of the body to the ankle and thence to the tail; brain primitive; lower incisors comb-like; molars multicuspidate. Examples Cynocephalus and Galeopterue. Order Primates, Sub-order Lemuroidea.-SmaIl, nocturnal animals with a long nonprehensile tail; opposable pollex and 238 CLASSIFICATION hallux; low brain case; uncovered cerebellum. Orbit not_completely Sh'lt off from tempo-ral fossa; jugal posteriorly extended; nasals large; humerus with entepicondylar foramen; femur with third trochanter; lower incisors procumbent; lower canines incisiform; second toe and finger with a sharp claw; tympanic ring pnclosed inside the bulla, uterus bicornuate. Madagascar, Africa and the Oriental region. Examples Lemur, LOl"ls etc. Sub-order Torsioidea.-With large eyes and ears, long thin tail tufted at the end, very long tarsus, fingers and toes ending in flatt~ned discs; inner upper mcisor large and in contact with its fellow; molars with nuwerous pointed cusps; orbit largely separated from temporal fossa; digits two and three of the pes with claws; calcaneum and navicular elonga"cd. Indo-Malaya Archipelago and Phillipines. Example Tarsius spectrum. Sub-order Anthropoidea.-Cranial cavity spacious ancJ rounded; orbits look forward and shut off from temporal fossa; dentition complete, with bunodont premolars and molars; two thoracic mammae; humerus without entepicondylar foramen; radius and ulna separate; thumbs opposable; femur without third trochanter: hallux opposa hIe·; cerebrum covers cerebellum; uterus without cornua. Group Platyrrhina.-New-World Monkeys. With broad nasal septum; three premolars; tympanic ring forms a bony canal to the external auditory meatus; tail -may be prehensible. CLASSIFICATION Family Hapalidae. Example Hapale. Family Cebidae. Examples Cebus, Ateles (Spider Monkeys), Mycetes .(Howlers) etc. Group Catarrhina.-Old-World Monkeys and Apes. 'With narrow nasal septum; two premolars; bony external -auditory meatus, ischial callosities, and cheek-pouches present; tail not prehensile. Family Cercopithecidae. Examples Macacus ; Papio (Baboons), Cercopithecus ·etc. Family Simiidae. Examples Hylobates (Gibbon), Gorilla, Simia (Urang), Pan (chimpanzaee) etc. Family Hominidae. Example Pithecamhropus, .Homo (Man). Sinanthropus, and APPENDIX PROTOCHORDATES The group chordata is divided into two sub-phyla (i~ sub-phylum acrania and (ii) sub-phylum craniata. Thecraniata are more highly organised chordates including the vertebrates; whereas the acrania or protochordata are simple or aberrant cnordates devoid of skull (hence acrania). The proto chordates fall into three classeshemichordata (acorn worms), urochordata (tumnicates or sea-quirts) and cephalochordata (lancelets). I. Class: Hemichordata.-Worm-like marin" animals differing from the chordates in many respects. Thebody is not segmented.metamerically but is divisible into three main regiol1s-pre-orallobe (proboscis), collar and trunk. The anus is triminal therefore thl're is no postanal tail. The notochord is restricted to the proboscis, region and develops rather late in the embryo. The hemichordates are grouped into two orders. Order: Enteropneusta.-Elongated wormlike burrowing animals with numerous gill-slits; ciliated glandular epidermis. Example Balanoglossus. Order: Pterobranchia.-Sedentary, oolonial animals ; the pre-oral lobe bears ciliated tentacles; gill-slits. two in number or absent. 240 :?41 APPENDIX Type: BALANOGLOSSUS. A. External characters. Take out a specimen of balanoglossZts in a dish and examine its characteristic features and sketch. (i) The proboscis is more or less oval muscular structure which helps in digging holes. The mouth lies at its base, on the ventral side, covered by the anterior edge of the collar. The mouth remains constantly open. (ii) The coilar is band shaped muscuiar structure externally very smooth. ,It .helps. in burrowing. , .. (iii) The trunk is the last portion of the, body differentiated into the anterior brancbio-~enital region having gill-slits, the hepatic re~ion •. the. ab<,lominal', regbn and the caudal regions. ~n the hepatic region,am ~Elen a. number of finger-like outgrowths of the sid~s. . ~hey. are actually the projections of the alimentary canal called hepatic diverticula. 'Che. anus is sit'uatedf at the terminal end of the trunk. .1' B. Structure of body wall. Examine a transverse section·oJ the bod.y wall carefully and sketch. .i' ( • (i) The ciliated epithelium forms the outermost layer. Seattered here 'and' there are seen unicellulaI' gland cells. ',ii) The circular muscle f~bres form the next layer underlying the ciliated epithelium. 16 24:.: APPENDIX (iii) The longitudinal muscle fibres form innermost layer. C. t e The internal anatomy. 1. Examine transverse section thr01tgh the posterio region of the proboscis of the animal, 110te the following alt/ sketch. (i) The body wall as described above with section of strong proboscis musculature. (ii) The proboscis cavity in the middle. (iii) The proboscis gland in section. (iv) The notochord in section. 2. Examine a transverse section through the branchiogenit"l region of balanoglossfts and note the following structures. (i) The body wall as usual. (ii The gill-slits opening behind into the gillpouches. (iii) The gill-pouches are two:, one on either side opening into the pharynx: through the gillpores. (iv) The enteron is divided into two regions la) an upper respiratory and \b) the lower digestive region which is glandular. I{V) The gonads are seen closely associated with the gill-pouches in sections of mq,ture specimens. APPENDIX 243 Also ,examine a longitudinal section passing through the anterior part of the body and transverse sections thro16gh the collar and through the posterior part of the trtmk, if available, and note important characters. II. Class: Urochordata.-Remarkable ciliary-feeding marine animals-the sea-squirts and their allies. Except members of one group the most are sedentary. The chordate characters are lost in the adult. Hut their free-swimming larvae possess important chordate characters. The notochord is found in larval stages only restricted to the tail (hence urochordata). A we11 developed endostyle is present. At metamorphosis the .larvae settle on the sea-floor to which they adhere by means of glandular papillae on the head. Rapid growth in size takes place and the animal undergoes retrogressive metamorphosis with the result that (i) the larval nervous system is lost; (ii) the body becomes surrounded by a gelatinous or horny eoat the tunic or test; (iii) the number of stigmata increases and the pharynx becomes enlarged; the stigmata open into an ectoderm lined cavity the atrium; and (iv) the tail is cast off and with the tail the notochord is also lost. Order: Larvacea.-1!'ree-swimming pelagic forms retaining larval characters in the adult stage. Two gill-slits only. Atrium absent. The test is thin and periodica11y cast off. Example: Oikopleura. Order: Ascidiacea.-Sedentary animals usually fixed to the sub-stratum such as the sea-squirts. Often colonial. Reproduction by sexual methods. Metamorphosis complete. Example: Ascidia, Clavelina, Botryllus, Pyrosoma. 244 APPENDIX Order: Thaliacea.-Large free-swimming forms with transparent gelatinous test. MuscleH of the body wall arranged in distinct bands, encircling the body wholly or partially. l'wo distinct phase!! in the life history which differ from one another structurally (Polymorphism); besides one reproduces sexually and the other asexually. Example: Salpa, Doliolum. Type: A. ASCIDIAN. External Characters. Take out an ascidian, keep it in a dish before you and note down tlte outstanding features. (i) The tunic or test is the thick outermost C:1Ve- lop secreted by the epidermal cells of the body wall it covers the body entirely. (ii) The peduncle or the base fixes the animal to the sub-stratum. (iii) There are two openings at the distal end (a) the oral pore and (b) the atrial pore. The atrial aperture is dorsal the britnchial (oral) aperture is terminal and indicates the morphological anterior end of the animal. Each opening is surrounded by a fringe of the tunic . .B. The Body-wall. Study a transverse section of the body wall and note the following ·structures. (i) The tunic forms the outermost layer. It consists of test cells embedded in the hyaline matrix of tunicene. APPENDIX " 245 The mantle is the next layer. It consists of (a) outer epidermal layer made up of a single celled layer and (b) the muscle fibres and blood vessels. (ii) C. The pharyn~eal wall. Carefully separate the pharyngeal wall from the mantle, and make a permanent balsam preparation (see page 171 for method). Examine and sketch. Note the following structures. (i) The sti~mata. (iiI The primary ~ill-bars. (iii) The secondary ~iIl-bars. (iv) The synapticulae. D. The Tailed Larva. The embryo hatches as a tailed larva which leads a free-swimming existence for a short time. This stage is the highest in its chordate organisation when the larva -swims actually through the sea, by vibrating its long tail. The larva soon develops adhesive papillae. The heart and pericardium develop as a diverticulum from the stomach. After leading free life for some time the larva attaches itself to the sub-stratum and the retrogressive metamorphosis sets in. Examine' a preparation oj the tailed larva oj ascidialt and note tlte jollowing structures. (i) The notochord embedded in the tail. 24 Ii APPENDIX (ii, The brain with the nerve chord, the' eye and the otocyst etc. (iii) The mouth, the pharynx with stigmata, the endostyle and the stomach. (iv) The atrium just beginning to develop around the last parlj of the alimentary tract. (v) The adhesive papillae. n T. Class: Cephalochordata. -The Lancelets. Marine, sand inhabiting animals, fish-like in appearance, possessing chief chordate characters.. The notochord is persistent throughout life and extends the whole length of the body even to the tip of the snout (hence ctlphalochordata). Pharynx is large and sac-like with numerous gill-slits. There is a prominent endostyle and a definitely ciliated tract answering the ciliary feeding habit. The gill-slits open into the atrium. There is a definite coelom. Excretion carried through definite nephridia. Metamerism is well marked. Example: Branchiostoma (Amphioxus). Type: A. AMPHIOXUS. External Features. Examine a specimen of amphioxHS. Fish-like elongated body pointed at both the endsand bears fins. (i) The dorsal fin ext.ends as a fold of skin along the whole length dorsally. \. APPENDIX 247 )i) The "ventral fin is on &he ventral side ex- tending about a third of the length of the body from the posterior edge. At the tip of the tail it is continuous with the dorsal fin. (iii) The metapleural folds are two down. growths from the dorso-Iateral body wall extending from the atriopore to the oral hood• .(iv) The oral hood. At the anterior end the dorsal and lateral parts of the body project forward to form a hood, the oral hood. Posteriorly the oral hood is continuous with the metapleural folds. From the edge of the hood arise about twenty stiff tentaclelike processes, the oral cirri. The under side of the oral hood bears a complicated series of a ciliated grooves ood ridges which collectively from the wheel organ. (v) The atriopore is situated on the ventral side just where the ventral fin ends and the metapleural folds begin. B. The pharyngeal wall. Take out a small piece of the pharyngeal wall of amphioxus and make a permanent preparation (see page .J, 71 for method). Examine and sketch. Note (i) The primary rods forked at their ventral ends. (ii) The secondary or tongue bars. (iii) The synapticula, cross connections of the primary 8S well as secondary bars. 248 C. APPENDIX Internal Anatomy. Examine transverse sections of the amphioxus passing through the pharyngeal region. Note the following ~tructures s"k~tch. and (i) The dorsal fin on the dorsal side supported by fin-ray. (ii) The metapleural foMs ventral side. are two on tJw (Iii) The body wall consists, of (a) The epidermis, the outer layer wJnsisting of a single layer of cubical epithehal cells ~ which bea.r a thin 'but a distinct cuticle. (b) The fibrous connective tissue forma a. continuous tough layer below the, epidermis. ; ~ (0) The segmentally arranged muscle blocks, the myotomes' 'or myomeres form the greater part of the body wall .. tiv} The neural tube is situated dorsally abovethe notochord. (v) The notochord appeo.rs, more or 1008 circular in section, below the neural tube. (vi) The pharynx occupies the wh9Ie of the coelom which is extremely reduced in this region. The pharvngeal wails show thE} gill-slits and gill-blll'a etc. in sections. APPENDIX 219 (v) The gonads lie in the ventral part of the lateral body wall in the pharyngeal reg.ion. A section through the pharyngeal reglOn, therefore, will show the gonad toward its ventral margin. 2. Examine transverse sections through the region of the intestine behind the atrio pore, ani through other sitch regions, study und sketch in the manner described above.