Download External Anatomy

Cats!
External
Anatomy
Cats!
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External Anatomy
A.
The body is divisible into head, neck, trunk, and tail. The body is covered with
fine, closely set hairs, forming the characteristic mammalian fur.
B.
As is generally true of the smaller species in the cat family, the head is somewhat
larger in proportion to the trunk than in other typical quadrupeds. The larger size of the
head is due to greater development of the enclosed brain—especially the occipital lobes,
which process vision. Concomitantly, the eyes are somewhat larger in proportion to the
head than in most other quadrupeds—another characteristic generally true of the
smaller cat species.
C.
The head may be divided into an anterior facial region (from the orbits
forward) and an enlarged posterior cranial region. The mouth has well developed
lips; the upper lip is broadly cleft in its center, as it is in the gnathomorphs such as
rabbits (thus the term “harelip”). This cleft exposes the small row of incisor teeth.
Lateral to the incisors are the prominent canines (=fangs), in both upper and lower
dental arches. The external nares are large and elongated, and overhung by the
mobile nose. Around the nose and mouth are a number of long stout hairs, called
vibrissae (=whiskers), which have a rich nerve supply and serve as important tactile
organs for measuring lateral space.
D.
The eyes have upper and lower lids and nictating membranes, which can
be grasped with tweezers and drawn over the eyeballs. The ears have a long, flexible
external fold called a pinna, which springs from the rim of the external auditory
meatus—a passage which descends deeply into the skull, allowing sound to reach the
tympanum (=eardrum). Pinna and meatus together constitute the external ear.
E.
The trunk is divisible into an anterior thorax, supported by the ribs, and a
posterior abdomen. On the ventral surface of the trunk occur four or five pairs of
teats (=nipples), which are the openings of the mammary glands in female
specimens. As in the majority of mammals, there is no cloaca; the anal and
urogenital openings are separate. The region which includes both openings is called
the perineum.
F.
In female specimens, the urogenital opening is situated immediately anterior to
the anus. It is enclosed by folds of skin called the labia majora (=greater lips); the
labia and opening together constitute the vulva. In male specimens, the urogenital
structures consist of the penis and the scrotum (or scrotal sac); the scrotum is formed
by a double pouch of the body wall, inside which the testes are located. In cats, the
scrotum is conspicuous as a pair of rounded eminences just anterior to the anus. Just
anterior to the scrotum is a fold of skin, the prepuce (=foreskin). The opening in the
center of the prepuce is not in fact the urogenital opening, but simply the depression left
by the withdrawal of the penis into the prepuce. In male cats, the penis is generally so
far withdrawn that it is not visible externally.
 In your observation section, identify the gender of your cat and
explain WHY.
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G.
The trunk bears two pairs of limbs, composed of the same parts as in other
vertebrates—though with their own proportions typical of the cat family. The upper
sections (upper arm and thigh) normally lay close aside the trunk, and in life their
movements are partially concealed by the skin and fur which cover the trunk. The
elbows and knees are usually on a level with the trunk’s ventral surface; thus the lower
sections of the limbs (forearm and shank) are the prominent extensions. The lower
limbs end in a short carpal assembly in the forefeet, and in exaggerated tarsals in the
hind feet. All four limbs terminate in clawed digits—five in the front and four in the
back (the first hind toe is absent). The claws of cats and their relatives are retractile—
their tips can be withdrawn well behind the distal margin of the toes.
H.
The limbs have evolved dramatically from their position in the cat’s primitive
ancestors. Instead of extending laterally from the body (as in a lizard), they project
ventrally and are elongated, so that the trunk is carried high above the ground. Thus,
this evolution has involved a ventral rotation of 90 degrees in each limb. In addition,
the hind limb has become rotated 90 degrees forward, so that the original dorsal surface
of the limb is now anterior, the knee joint faces anteriorly and bends caudad, and the
toes point forward. On the other hand, the forelimb has become rotated 90 degrees
posteriorly, so that the original dorsal surface now faces posteriorly and the original
preaxial surfaces face laterally; the elbow faces caudad and bends anteriorly. [In these
respects, the limbs are oriented as they are in a human who is “walking” on all four
limbs, but using only fingers and toes to touch the ground.]
As a result of the caudal rotation of the upper arm, the toes would be expected to
point posteriorly. But an additional torsion has evolved to bring them forward. This
torsion (called pronation) involves the crossing of the two forearm bones: the distal
end of the radius turns over the distal end of the ulna, becoming medial to it. [This
prone position of the forearm is analogous to a human letting the arms hang by the side,
keeping the elbows close to the trunk and pointed behind, and then rotating the hands
so that the backs of the hands face forward. If sitting, resting the palms on the lap or a
table is prone. In prone position, the crossing of the two forearm bones can be felt by
palpation. Rotating the hands back to a palms-forward position returns the two bones
to the primitive, parallel position (called supine). Thus, a human can change the
forearm position through a continuous range from supine to prone, whereas in most
four-footed mammals the forearm position is fixed in the prone position.
J.
The posture of the feet varies widely among mammals. Humans walk on the
whole sole of the foot, from tarsals to digits, in the primitive style called plantigrade.
Cats and dogs—and most quadrupedal hunters—walk on their digits, with the
metacarpals, carpals, metatarsals, and tarsals all elevated, in a style called
digitigrade. Horses, cattle, and most other grazers walk on their toenails, which have
broadened into hoofs, in a style called unguligrade.
 With your cat on it’s dorsal surface, sketch an external view of the cat and
label the following structures: head, vibrissae, pinna, neck, thorax, abdomen,
teats, perineum, upper arm, forearm, thigh, shank, and tail
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Skinning the specimen
I strongly recommend reading this entire skinning procedure before you
start.
K. Begin skinning the animal, and while doing so, become familiar with all of
the structures and regions named in Sections L-R below.
1. Use scissors to make incisions, not a scalpel.
2. To avoid cutting any structures below the skin:
(a) with your free hand, pinch the skin ahead of the scissors and pull up on it;
(b) keep the scissors at the shallowest possible angle and keep pulling the
skin away from the body as you snip it.
3. To separate the cutaneous membrane from the underlying muscle groups,
refer to the pattern on page 4 and follow these steps:
(a) Snip the skin and make a circular cut around each wrist and ankle.
(b) Start at the midpoint of the chin; cut along one mandibular margin (edge of
the jaw bone); proceed around the side of the head just behind the ear; repeat
this pattern along the other mandibular margin. The skin should still be
attached at the base of the posterior skull.
(c) Snip the skin at the base of the tail; cut downward around the back of one
leg; around the anterior margin of the genitals; then upward around the
back of the other leg to meet the starting point.
(d) Very carefully cut a midsagittal line along the dorsal surface, starting from
the cut at the base of the tail [see (c) above] and continuing forward until you
reach the midpoint of the cut across the back of the skull [see (b) above];
(e) Now cut “I-serifs” (aka “flap cuts”; aka “book cuts”) as follows:
•ANTERIORLY: start at the midsagittal cut at the level of the shoulder
blades; proceed laterally and continue along the dorsolateral side of the
forelimb until you reach the “wrist” cut [see (a) above];
•POSTERIORLY: start where the midsagittal meets the cut around the
base of the tail; proceed laterally along the dorsolateral side of the hind
limb until you reach the “ankle” cut [see (a) above].
4. Starting at the middorsal cut, steadily lift the skin away from the underlying
torso. Notice as you pull, the subcutaneous fascia stretches into an opaque, white
foamy appearance. Use light, smooth strokes with the back of a scalpel blade
point to shave through this foamy hypodermis. This will allow you to separate
cutaneous tissue from underlying muscles and other structures safely. As you go,
remove all the fatty tissue you find. You may also snip through the small
cutaneous blood vessels where found. [See M below.]
5. When you have finished, only the fur on the head, paws, and genitals should remain.
The rest of the skin should be one piece which is still attached at the base of the skull so
that it can be used to wrap around the cat at the end of each session.
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6. Carefully rinse the cat using a squirt bottle to remove stray hairs.
7. Now proceed to carefully and completely clean away the superficial fascia and fat
from the surface of the muscles. Be sure to completely remove the larger masses
of fat from the axillary and inguinal regions. This process is tedious, but must be
done very thoroughly.
8. At the end of the session, spray the exposed tissue with preservative fluid and then
burrito-wrap the specimen in its skin and double-bag it. Make sure the bag is
properly labeled.
9. Wash, rinse, and dry your scissors and scalpel well, and return them to the drawer.
Discard your gloves at the end of this session.
Superficial tissues
L.
The skin is connected to the underlying muscles by a loose web-like material, the
superficial fascia (or hypodermis, or subcutaneous connective tissue).
 EXACTLY WHAT TISSUE TYPE IS THIS? (answer in dissection journal)
The hypodermis is impregnated with fat, in some places heavily.
 WHAT TISSUE TYPE IS THIS FAT? (answer in dissection journal)
Below the hypodermis, adhering directly to the surface of the muscles, is a much
thinner, but firmer and tougher connective tissue called the deep fascia.
 WHAT TISSUE TYPE IS THE DEEP FASCIA? (answer in dissection journal)
M.
Cutaneous circulation. Passing from among the torso muscles and into the
skin—at regular intervals reflecting the ancient body segments—will be seen slender
strands composed of artery, vein, and sensory nerve. These may be severed. Other
blood vessels, not segmentally arranged, will also be seen passing onto the under surface
of the skin—usually directed from anterior and posterior regions toward the middle of
the trunk. If the latex injection went well, arteries will appear bright red and veins
will be blue. If the injection timing was poor, veins may appear dark reddish brown in
color. All blood vessels passing to the skin may be severed.
N.
Dermal (or integumental) muscles. These are thin muscle sheets which
have acquired insertion on the skin, and which act to move either the skin itself or
dermal structures such as hairs, bristles, pinnae, scales, etc. Dermal musculature is
particularly well developed in mammals, where they accomplish movements such as
shivering, fur/hair erection, movement of vibrissae, facial expressions, etc. Dermal
muscles are extensions of trunk musculature, especially of the pectorals, the brachials
(e.g., trapezius), and the superficial neck muscles.
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O.
Platysma. Extensions of the neck muscles form the platysma—a collective
name for the dermal muscle of the head—with numerous subdivisions acting to move
the lips, cheeks, eyelids, ears, etc. The finely divided parts of the platysma bear separate
names, and we may try to see some of these later, but accurate study of the platysma is
extremely difficult in preserved specimens.
P.
Cutaneous maximus. As the skin is loosened from the torso, you will note
a thin layer of muscle fibers on the under surface of the skin, appearing as a fine
striping. Toward the chest and shoulder region this assumes the form of a thin sheet.
This is the generalized dermal muscle, the cutaneous maximus. It covers the entire
lateral surface of the thorax and abdomen, although it is more prominent anteriorly. As
you continue to loosen the skin forward and ventrally, you will find the cutaneous
maximus takes its origins from the latissimus dorsi (just posterior to the shoulder);
from the axillary muscles (the ventral fold of the armpit); from various points on the
pectorals (ventral chest muscles); and from the midventral line of fascia called the
linea alba (“white line”). All these points of origin of dermal muscle strands may be
carefully cut through, and the cutaneous maximus removed along with the skin to
which it generally adheres.
Q.
In female specimens, the mammary glands will be noted as a thin, irregular
layer of tissue spread out on the under surface of the skin in the ventral region, from
chest to abdomen.
R.
It will now be seen that the specimen’s exposed surfaces mostly consist of skeletal
muscle tissue (pinkish tan masses with faintly visible parallel fibers); whose surfaces are
covered with a thin, almost transparent sheet of deep fascia (dense irregular CT). In
some places, the deep fascia consists of much thicker, denser, and tougher white sheets.
The posterior half of the back is covered by such a sheet in several layers, called the
lumbodorsal fascia; and in the median ventral line is a thin strip of it called the
linea alba.
S.
The angle between the base of the thigh and the abdominal wall is called the
inguinal region; the angle between the upper arm and chest is called the axillary
region (or simply axilla).
 Be sure to describe cutaneous circulation, dermal muscles, cutaneous
maximus, deep fascia, fat tissue, and mammary glands (if female) in your
reflection.