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7.1 DISSECTION OF A RAT Time required: 2 periods Name: ________________ Introduction: Dissection is a technique used to study the internal organs of a dead animal. It shows students the internal structure of the animal, how the tissues look and feel, and how they are interrelated. All mammals possess similar type of organs and systems whether they are small or large in size. To understand the structure and functions of our organ systems, it is helpful to dissect smaller mammals such as rats. Since rats are herbivorous, they have 4 long, sharp incisors for gnawing and 12 molars for chewing but have no canines and premolars. The toothless space between the incisors and molars is called the diastema. Their digestive system is similar to ours except that they have longer small intestine and a large caecum to digest cellulose. They have no gall bladder. There are no differences between human and rat excretory systems. The female rats have a Vshaped uterus that can hold many fetuses where as human uterus is smaller and usually holds only one fetus at a time. Learning outcomes: After completing this dissection, students will become proficient in dissecting and become familiar with the structure and functions of various organs and systems of the rat and also note the differences between human and rat systems Group size: 2-3 per group Objectives: To dissect a rat and note the similarities and differences between rat’s digestive, excretory and reproductive systems with that of human systems. Apparatus and materials per group: A preserved male rat or female rat One female rat per class Scalpel Sharp scissors Dissecting probe Forceps A long string Plastic bag Dissecting needle Drawing pins Disinfectant solution Dissecting board A small ruler Model of human torso Disposable gloves Old newspaper Rat dissection chart and video Lab coat Observing the external features 1. 2. 3. 4. Put on gloves and a lab coat before dissecting the rat. Get the necessary dissecting equipment, dissecting board, string, drawing pins, a ruler and a newspaper. Lay the newspaper on top of the dissecting board. Obtain a preserved rat specimen for your group and note whether it is a male or a female by checking for the presence of external testes. Note: The penis in the male rat will be withdrawn inside the body 5. Observe the external ears, eyes, mouth, whiskers, feet and tail. 6. Note the body colour and type of body covering. 7. Place the rat down on its back on the dissecting board and pin the front and hind feet to the dissecting board or secure the rat firmly to the dissecting board with a string. Note: Your teacher will show you how to do this. Dorsal side Anterior side posterior side Ventral side 1 Figure1: External features of a rat http://intro.bio.umass.edu/manual/images/e/e1/ExternalLateral.jpg Oral cavity 8. Using a scalpel, cut through the angles of the jaws so that the lower jaw collapses, exposing the buccal cavity (oral cavity) Teeth Figure 2- Rat’s skull and teeth http://www.spflrc.org/user/rats/Teeth.htm 9. Open the mouth of the rat and observe the shape, size and number of incisors and molars. Note the empty space between incisors and molars called diastema. Check if the rat has canines and premolars. (Refer to figure 4) Nostrils 10. Find the openings of the nostrils in the roof of the mouth at the anterior end. The Tongue 11. Locate the tongue and note whether it is attached at the front or at the back. The Gullet and Glottis 12. Posterior in the mouth is the gullet, the opening to the oesophagus. Just ventral to the gullet is a vertical slit called the glottis. The glottis opens into the trachea, or windpipe, which branches into 2 bronchi in the lungs. START OF THE DISSECTION 4th incision 1st incision 2nd incision 3rd incision Figure 3-Guide to make incisions on the Figure 4-Internal structures of the rat ventral side of the rat http://www.biologycorner.com/bio3/anatomy/rat_head.html Cutting the skin Incision 1: 1. Grab a pinch of skin between the hind legs and make a V-shaped cut with a pair of sharp scissors. Hold the skin with forceps and continue to cut upwards along the mid-line up to the throat. (Figure 5) Incision 2: 2. Using scissors cut across both the front legs and back legs taking care not to cut the underlying muscles. 3. Using the scalpel and forceps, peel the skin off the rat and pin the skin to the dissecting board. Caution- To avoid damage to the internal organs, be sure to lift the muscles with the forceps as you cut. Point the blunt end of the scissors towards the rat .Be careful when using a scalpel 2 Cutting the muscles Incision 3: 4. Begin the vertical incision by lifting the muscle layer with forceps between the rear legs of the frog. Make a small cut with the scissors 5. Continue the incision up the midline to a point just below the front legs. Caution: Be careful that you don't cut too deeply. The muscle is thin. It is easy to damage the organs underneath Incision 4: 6. Just as you did with the skin, make a sideways incision in the muscle with the scalpel. Make the first incision between the front legs and the next incision is just above the rear legs. Caution: Again, be careful that you don't cut too deeply. 7. Separate the muscle flaps from the organs below. Pull back and hold the muscle flaps with the forceps. 8. Use the scalpel to separate the muscle from the underlying organs. 9. Pin the muscle flaps back far enough to allow easy access to the internal organs. Cutting the chest bone Incision 5: 10. Expose the chest cavity by cutting around the ribs on both sides. 11. Lift the rib cage and set it aside to expose the chest cavity. 12. Observe the lungs and the heart located in the chest cavity. Note: The chest cavity is separated from the abdominal cavity by a thin muscular membrane called the diaphragm. ORGANS IN THE CHEST CAVITY Lungs- Refer to figure 5 12. Notice the size and colour of the lungs which are at the anterior end of the chest cavity on either side of the heart. Trachea 13. Observe one trachea as it divides into two bronchi going into each lung. Heart- Refer to figure 6 14. Observe the muscular heart located between the two lungs. It is covered by a thin membrane known as the pericardium. Carefully cut through the pericardium to reveal the lower part of the heart, the ventricle, and the upper part, which is composed of two atria. The walls of the atria can be pushed in easily. The walls of the ventricle are sturdier. 15. Remove the lungs and the heart and place them aside on the news paper. Figure 5- Lungs and heart of a rat Figure 6- Heart of a rat http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg10.htm lungs http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg12.htm heart Please draw these diagrams 3 ORGANS IN THE ABDOMOINAL CAVITY Digestive system Oesophagus 16. Using forceps pick up the oesophagus as it comes through the diaphragm and note where it enters the stomach. (Refer to figure 3) Liver 17. Just above the stomach you will see the reddish brown liver which is divided into five lobes. Note: Bile produced by the liver is stored in the gall bladder and helps to emulsifies fats so that they can be digested by lipases. The rat has no gall bladder because it does not eat fatty food. Spleen 18. Note the spleen which is a flattened, reddish organ lying posterior to the stomach. Pancreas 19. A glandular organ called the pancreas is found near the stomach. Small intestine Figure 7- Rat’s digestive system Please draw a similar diagram http://kentsimmons.uwinnipeg.ca/16cm05/16labm an05/lb8pg3.htm 20. Carefully stretch the small intestines and examine the thin mesentery membrane which holds the intestine together. 21. Separate the intestines from the mesentery membrane using your fingers and pin it to one side on the dissecting board. Note: The small intestine consists of three parts: Duodenum, jejunum and ileum Large intestine 22. Now follow the small intestine all the way along until it becomes the large intestine. Note: The large intestine consists of: colon and rectum. The colon is the longest part of the large intestine, and it is further subdivided into ascending, transverse and descending colon. The last part of the large intestine is the rectum. Caecum 23. At the junction between the small and large intestines, there is a blind pouch called the caecum which is important in cellulose digestion. Note: Rats are garnivores, which means they eat seeds and grains. In the caecum are bacteria that are able to digest the cellulose in the seeds. The food sits in the caecum and essentially rots. The caecum acts like a fermentation chamber. Humans do not have a caecum because humans are nor pure herbivores. Humans have an appendix which is a vestigial organ. Rectum: 24. At the end of the large intestine there is an expanded portion called the rectum, which opens to the exterior through the anus. 25. Using a ruler Measure the length of the following organs: a) Oesophagus b) stomach d) small intestine e) large intestine and f) caecum 26. Enter your measurements in table1. 4 URINO-GENITAL SYSTEM Urino-genital system consists of excretory system and reproductive system. Figure 8- Male urino-genital system www.tutorvista.com/content/biology/biology-ii… Figure 9- Female urino-genital system please draw similar diagrams A. Excretory system Kidneys 27. Wash the abdominal cavity with water and note the colour and size of the kidneys located on either side of the backbone. Ureter and urinary bladder 28. Trace the ureters leading from the kidney to the urinary bladder which expels urine via the urethra. B. Reproductive system i) Male reproductive system Scrotum and testes 29. Examine the genital area of your rat and identify whether it is a male or a female. Note: The male rat has two external testes located in scrotal sacs and a penis located between two back legs. These organs are absent in the female rat. 30. Locate the male’s scrotum and cut longitudinally through the skin to locate the testes where sperms are produced. 31. Separate the skin from the testes and notice a sac called epididymis lying around each testis. This sac stores sperms. 32. Find the vas deferens that leads from each testis to the urethra. This duct carries sperms from the testes to the penis. 33. To either side of urinary bladder you will find two sets of glands. The smaller, round, more posterior ones are the prostate glands. The larger pair of glands are the seminal vesicles which produce semen. 34. Follow the urethra as it goes through the penis. Both sperms and urine are expelled from the urinogenital orifice at the tip of the penis. ii) Female reproductive System Uterus 35. Observe two V shaped uteri which join and open into the vagina. Note: Humans have one uterus because they usually produce one baby at a time. Ovary and oviduct 36. At the anterior end of each uterus there is a short, convoluted oviduct, which opens into a small, round ovary. 5 Pregnant female rat 37. If your rat is pregnant, cut longitudinally along two uteri and note the number of embryos. 38. On the exterior, just ventral to the tail, you will find the anus, the vaginal opening and the urethral opening. 39. Observe the reproductive organs of a male rat/female rat dissected by other groups and note the differences. Cleaning up 40. After finishing the dissection, wrap the rat and the organs in the newspaper and place it in a bag provided for disposal. Caution: Do not discard any organs in the sink. 41. Clean your dissecting board and instruments with a detergent and pack up. 42. Dispose of your gloves in the bag provided, and wash your hands thoroughly with an antiseptic solution. Results: Table 1-Enter your measurements of the digestive organs of the rat Organ measured Length (cm) Oesophagus Stomach Small intestine Large intestine Caecum Total length More than a metre 2. Label the diagrams given below: 1 2 4 3 5 6 7 8 9 A. Label the organs of the digestive system: 1. Liver 2. Stomach 3. Spleen 4. Duodenum 5. Pancreas 6. Jejunum 7. Ileum 8. caecum 9. Large intestine 10. Rectum 11. Anus 10 11 6 B. Label the male urino-genital system: 1. Kidney 2. Ureter 3. Seminal vesicle 4. Vasdeference 5. Epididymis 6. Testes 7. Scrotal sac 8. Bladder 9. Prostate gland 10. Penis 1 2 8 3 9 4 5 1 0 6 7 1 2 4 5 6 3 7 C. Label the female urino-genital system 1. Kidney 2. Ureter 3. Bladder 4. Ovary 5. Oviduct 6. Uterus 7. Urethra 8. Vagina 8 Discussion questions: 1. What covers the body of the rat? The rat’s body is covered with fur. 2. What type of teeth do rats have? Rats have incisors and molars in both upper and lower jaws. They do not have canines and premolars. The space between the incisors and molars is known as diastema. 3. Is the rat a herbivore or a carnivore? The rat is a herbivore. 4. What separates the chest cavity from the abdominal cavity? A muscular diaphragm separates the chest cavity from the abdominal cavity. 5. Describe the location and appearance of the lungs. The lungs are located on either side of the heart in the chest cavity. They are pink in colour and spongy. They are divided into 2-3 lobes and covered with a pleural membrane. 6. What is the purpose of trachea and bronchi? The trachea leads from the mouth cavity to the lungs where it divides into two bronchi. Each bronchus further divides into bronchioles which end in air sacs called alveoli. Exchange of oxygen and carbon dioxide takes place by diffusion in the alveoli. 7. Describe the location and structure of rat’s heart. The heart of a rat is located in the chest cavity between two lungs. It is a muscular organ divided into 4 chambers: the right and left atria and the right and left ventricles. 8. Describe the shape and structure of the stomach? The stomach is a J-shaped muscular organ. 9. Which is the largest organ situated in the abdominal cavity? The liver is the largest organ in the abdominal cavity. 10. Why doesn’t the rat have a gall bladder? Since the rat does not eat fatty foods, there is no need for a gall bladder. 7 11. What differences did you notice between the small intestine and the large intestine? The small intestine is narrow in diameter and is very long and coiled. The large intestine is wider in diameter and is shorter in length than small intestine. 12. Where is the caecum situated? What is its purpose? The caecum is located where small intestine joins the large intestine. It contains bacteria which help in digesting cellulose in raw plant food. 13. Why is the caecum absent in human digestive system? Since humans eat meat and cooked plant food there is no need for caecum. 14. Describe the shape and position of the kidneys? The kidneys are located on either side of the back bone. They are bean shaped and dark brown in colour. 15. What are the functions of the ureter, urinary bladder and urethra? The ureter carries urine from the kidneys to the urinary bladder where the urine is stored for a while and then is expelled by the urethra. 16. What are the functions of the following reproductive organs in the male rat: a)Penis- expels urine and sperm. b)Testes- produces the sperms c) Epidydymus-where sperms mature and are stored. 17. What are the functions of the following reproductive organs in the male rat: a) Ovaries- produce ova. b) Oviducts- carry ova from the ovary to the uterus. c) Uterus- where the fetus develops. 18. What is the difference between human uterus and rat uterus? Rats have two V-shaped uteri which join together but humans have one uterus 19. How many fetuses did you notice in the rat? 7-8 fetuses were noticed in rat’s uteri. (Answers will vary) Conclusion: Look at the model of a human torso and describe the differences between the rat and human systems in the table given below: Organs and systems Rat Human Outer covering Covered with fur Covered sparsely with hair Teeth Only incisors and molars are present Incisors, canines, pre molars and molars Has a space in between incisors and are present molars called diastema. No diastema present Digestive system Has a large caecum Has no caecum but has appendix The small intestine is very long The small intestine is shorter Reproductive system The female rat has two uteri Human female has one uterus Number of fetuses 6-8 develop at a time One fetus develops at a time Web sites: http://members.madasafish.com/~cj_whitehound/Rats_Nest/artwork/clipart/b+w_ship_rat.gif www.tutorvista.com/content/biology/biology-ii... http://www.spflrc.org/user/rats/Teeth.htm http://www.biologycorner.com/bio3/anatomy/rat_head.html http://www.biologycorner.com/bio3/anatomy/rat_head.html http://www.biolsci.monash.edu.au/undergrad/erat/erat/Shared/Main/Erat1.htm Interactive site with video http://educatus.com/main/samples/viewAll.asp?lid=801486&scid=8014860000 http://kentsimmons.uwinnipeg.ca/16cm05/16labman05/lb8pg5.htm 8