Cats
Ventral Cavity
Guide
Cat Guts p. 1
The Ventral Cavity
A.
The trunk is divided into the thoracic region (anterior) and the abdominal
(posterior) region. Each of these regions contains cavities, which ontologically are portions of
the coelom (intramesodermal space). The thoracic region has four subdivisions: two lateral
pleural cavities; the mediastinum; and within the mediatinum, the pericardial cavity.
The abdominal region contains the abdominopelvic cavity: the abdominal (or
peritoneal) cavity, which is enclosed laterally and ventrally by an abdomial wall of soft
tissues (no bones), contains most of the digestive organs; the pelvic cavity, which which is
the region within the pelvic girdle, contains the organs of urination and reproduction. These
two portions of the abdominaopelvic cavity are essentially continuous with each other, and
their functions overlap.
Thoracic Cavities
Using scissors, carefully make the shallowest possible cut along the midventral line of the
chest about 0.5 cm off the center line, cutting the costal cartilages from the edge of the
sternum, from end to end. Then, from the anterior end of this cut, carefully make shallow
lateral cuts (chest wall only) dorsally and caudally to a point under the original location of the
belly of the latissimus dorsi. From the posterior end of the sternal cut, make lateral cuts
following the anterior edges of the last true ribs around to where the dorsal surface lies on the
work surface. You may break the ribs near the spine in order to pin back the chest walls, to
make study of the thoracic contents easier.
B.
After the chest is opened, the revealed cavities are the left and right pleural cavities—
sometimes called the pleural sacs. The pleural cavities contain the soft, spongy lungs.
Anteriorly, the pleural cavities narrow toward the junction where the trachea, major blood
vessels, and esophagus pass to the throat. The lateral walls of the pleural cavities are the
interior surface of the chest wall; and the posterior wall of the pleural cavities is formed by a
dome-shaped partition of thin muscle, the diaphragm.
Cat Guts p. 2
C.
GENERAL PLAN OF THE PLEURA: Each pleural cavity is lined by a smooth, moist
membrane, the pleura, which is defined in two parts: the parietal pleura lines the inside
surface of the thoracic walls, covers the anterior face of the diaphragm, and forms the doublelayered mediastinal septum (ventrally) and the walls of the mediastinum (dorsally). The
visceral pleura is that part of the pleura which lines the surface of the lung, to which it is
indistinguishably fused.
In the median region, under the original location of the sternum, lies the heart. You
may notice (if it hasn’t been damaged too much by the opening of the chest) a membranous
partition that reaches from the heart toward where the sternum was. This dense irregular
connective tissue (D.I.C.) is called the mediastinal septum (“middle divider”). It actually
consists of the medial portions of the left and right parietal pleura sacs in contact with each
other. Heading dorsally, the two membranes separate and pass around the heart, which is
further enclosed in its own membranous sac, the parietal pericardium. The “space”
between the two membranes of the mediastinal septum is called the mediastinum. Dorsal
to the heart, the mediastinum also contains the esophagus, the branchings of the trachea
into the bronchii (which enter each lobe of lung), and the major ascending and descending
blood vessels of the chest (the dorsal aorta and the pre- and postcaval veins).
D.
Examine the left lung. It is a soft, spongy organ divided into three lobes: a smaller
anterior and larger middle and posterior lobes.
Use a scalpel to make a ½”-deep cut into the left posterior lobe; note that macroscopically
the organ appears solid, but is really composed of innumerable minute air sacs called alveoli.
E.
Note that the right lung is larger than the left lung. It too is divided into anterior,
middle, and posterior lobes; but notice that the posterior lobe may also be divided into two
lobules, a medial and a lateral. The medial lobule projects into a pocket formed by a
special, dorsally directed fold of the mediastinal septum, called the caval fold. The caval fold
has the function of supporting the large postcaval vein, which can be found in the fold’s
dorsal margin, extending from the diaphragm to the heart.
F.
Examine the heart and pericardial sac. The pericardial sac, or parietal pericardium,
is a sac of thin tissue (areolar/D.I.C.) enclosing the heart but not attached to it except at the
anterior end, where the great vessels enter and leave the heart. The heart is freely movable
within the pericardial sac. The narrow “space” between the heart and its pericardial sac is
called the pericardial cavity, and is another portion of the coelom. Cut and fold back the
pericardial sac to expose the heart. The surface of the heart is invested by a thin membrane,
the visceral pericardium, indistinguishably fused to the heart wall. The visceral
pericardium is continuous with the parietal pericardium at the anterior end, where the great
vessels enter and leave the heart.
G.
Look carefully in the mediastinum, in the median line, ventral and anterior to the heart,
and extending forward. There will be found a mass of glandular tissue, the thymus, which is
larger in younger specimens, but may be almost absent in some older specimens. As you
probe for the thymus, be sure not to injure the large blood vessels occurring in this region.
Cat Guts p. 3
H.
Now lift the left lung and heart and gently but firmly press them over to the right,
looking into the anterio-dorsal region of the left pleural cavity. You should be able to see that
each lobe of the lung is anchored to the mediastinal wall by a narrow radix, or root,
consisting of a 3-tube package of artery, vein, and bronchus enclosed in a thickened bit of
pleura called the pulmonary ligament.
Carefully cut through the root of the left posterior lobe and remove the lobe.
Now you can see the radix in cross-section.
I.
Note that the lungs are attached along most of their length to the dorsal thoracic wall by
the pulmonary ligament (D.R.C.), a thickened fold of the pleura. Note that dorsal to the
root of the lung, the pleura continues onto the dorsal and lateral surfaces of the pleural cavity;
that is, the visceral pleura is continuous with the parietal pleura. Further, observe that
certain other structures can be seen lying against the median dorsal wall of the chest, at the
back of the mediastinum. The most conspicuous of these structures is the dorsal aorta, a
very large vessel that arches away from the top of the heart to the left, then turns and
descends toward the diaphragm. Ventral and adjacent to the dorsal aorta is the esophagus,
which should be carefully traced to where it penetrates the diaphragm.
J.
The diaphragm is a curved sheet forming the posterior wall of the thoracic cavity and
completely separating it from the abdominal cavity. The center of the anterior face of the
diaphragm is seen to consist of connective tissue forming a circular tendon, the central
tendon. The rest of the diaphragm is muscular, taking its origin from the ribs, sternum, and
vertebrae, and inserting on the central tendon.
 Sketch a ventral view of the thoracic cavity. Be sure to include the following:
Lungs
Thymus
Common carotid
Diaphragm
Submaxillary gland
Left subclavian artery
Heart
External jugular vein
Great saphenous vein
Trachea
Pulmonary trunk
Esophagus
Aorta
Peritoneal and Pelvic Cavities
As you work your way through the abdominal (or peritoneal) cavity, keep in mind that these are
inexpensive specimens. The timing of the arterial/venous/anesthetic injections was bound to be
inexact among specimens of varying ages, body masses, and metabolic rates. As a result, in tripleinjected specimens, capillary color may be all yellow or all blue or blended green. [—in doubleinjected specimens: all red or all blue or blended pink.] Just realize that if a particular tissue is dense
with color, but you can’t discern individual vessels with the naked eye, this indicates a capillary bed.
Of course, the abdominal digestive tract is richly vascularized in all species.....
K.
GENERAL PLAN OF THE PERITONEUM: The peritoneal cavity is lined by a membrane, the
peritoneum. As in all coelomate animals, that portion of the membrane on the inside of the
body wall is called the parietal peritoneum. In both dorsal and ventral regions, the
peritoneum is deflected from the body wall and passes over the surfaces of the visceral organs,
forming a covering layer for them. This layer is called the visceral peritoneum (a.k.a.,
serosa). In passing to and from the body wall and the viscera, the peritoneum forms doublelayered membranes, generally called the mesenteries. The mesenteries act as “tethers”
which help stabilize the convoluted digestive organs by interconnecting them with each other
and with the abdominal walls. The ancient ventral mesentery persists in modern
Cats Guts p. 4
mammals only in the regions of the liver and urinary bladder. The dorsal mesentery,
however, is still very extensive; most of its various portions have their own names, which are
identified in the following sections.
L.
GENERAL PLAN OF THE PERITONEAL CAVITY: The anterior wall of the peritoneal cavity is
formed by the concavely arched diaphragm which, when intact, completely separates the
peritoneal and pleural cavities. Just posterior to the diaphragm, and shaped to fit its concave
surface, is the large, lobed liver, generally grayish brown in preserved specimens. Posterior
to the liver, the peritoneal cavity appears filled by the coils of the intestine. The intestine
will be at least partially covered ventrally by a thin double membrane impregnated with
streaks of fat, called the greater omentum. Forming part of the complicated stomachrelated peritoneum called the mesogaster, the greater omentum helps anchor the small
intestine and the transverse colon to the visceral peritoneum of the stomach, and to the
parietal peritoneum of the dorsal wall of the abdominal cavity. (The organization of the
mesogaster and of the greater and lesser omenta are described more thoroughly below, in
sections P and Q.)
M. By raising the liver and looking dorsally, find the stomach, with the spleen attached
to its left border by a mesentery called the gastrosplenic ligament. The spleen is the largest
lymphoid organ of the mammalian body.
LIVER
N.
The liver presents a convex anterior surface, fitting against the posterior surface of the
diaphragm; and a concave posterior surface, fitting over the stomach and the most anteriorventral part of the intestine. The liver is divided into right and left lobes, each of which is
further divided into median and lateral lobes. The left lateral and right median are usually
distinctly larger than the other two lobes; and the left lateral is further cleft into two lobules.
To understand these divisions, lift portions of the liver and study the roots of attachment.
When you do, you should note there is one further, small lobe, called the caudate lobe,
Cat Guts p. 6
situated inside a mesentery called the lesser omentum between the median lobes and the
stomach. Nestled between the two right lobes, find the yellowish green gall bladder.
STOMACH
O.
To examine the stomach, raise the liver and press it craniad. (The exposure of the
stomach may be facilitated by slitting the diaphragm on its left side.) The stomach is an
elongated, irregularly shaped, fairly muscular organ. Find where the esophagus emerges
from the diaphragm and enters the anterior surface of the stomach. This junction is called the
cardia, and the region of the stomach adjacent to it is called the cardiac region of the
stomach; the sac-like bulge of the stomach to the left of the cardia is called the fundus; and
the muscular, transverse section of the stomach is called the body. At its right end, the
pylorus of the stomach narrows conically to a constriction called the pyloric sphincter,
which regulates the passage of food from the stomach into the intestine. [Just beyond the
pylorus, you’ll see that the intestine makes an abrupt caudal-and-leftward U-turn.] The shorter,
laterally concave anterior surface of the stomach is called the lesser curvature; the larger
convex posterior surface is called the greater curvature.
P.
RELATIONS OF THE STOMACH TO THE PERITONEUM. Raise the fundus and note the
mesogaster (peritoneum related to the stomach) extending from the dorsal cavity wall to
the stomach. Only a small portion of the mesogaster actually passes directly to the stomach;
the greater part “descends” posteriorly to form the greater omentum, which envelopes and
supports the intestine [as discussed above in section L]. Then it returns anteriorly, passing
onto and around the pancreas and duodenum and finally onto the stomach wall along the
greater curvature. To the left of the stomach, the spleen is enclosed in the ventral wall of this
sheet; the portion of the omentum directly between spleen and stomach is called the
gastrosplenic ligament. From the greater curvature, it passes over the stomach surfaces,
forming the visceral peritoneum (or serosa) of the stomach.
Q.
At the lesser curvature of the stomach, the peritoneum becomes mesentery again,
passing to the middle of the posterior face of the liver as a strong ligament called the lesser
omentum (a.k.a., gastrohepatic ligament). A portion of it, called the hepatoduodenal
ligament, then continues rightward from there to the first section of intestine.
R.
IN THE HEPATODUODENAL LIGAMENT.....run the bile ducts. To see them distinctly,
carefully dissect away the ventral face of the ligament. The cystic duct can be traced from
the gall bladder; and the hepatic ducts can be traced from the lobes of the liver—
especially large ones from the right lateral lobe. The cystic duct and the hepatic ducts unite to
form the common bile duct, which should be traced to the duodenum. Also in this
ligament, to the right and dorsal to the cystic and common bile ducts, lies the large hepatic
portal vein. Try not to injure this vessel, but trace it to its branch into the right lateral lobe
of the liver.
S.
HEPATIC LIGAMENTS. As we have seen, the lesser omentum extends to the middle of the
posterior face of the liver, where it becomes the serosa of the liver. Here its two walls part
and follow the surface of the liver, fused with it as visceral peritoneum, around to the anterior
face, where they again unite to form ligaments. The falciform ligament (now torn by
opening of the ventral abdominal wall, and so probably not easily recognizable) normally
extends from between the two median lobes of the liver to the median ventral line of the
abdominal wall (where it becomes parietal peritoneum again); in vivo, it is a thin sheet with a
Cat Guts p. 7
concave posterior border; anteriorly and dorsally, it is continuous with the coronary
ligament, a stout ligament which attaches the liver to the central tendon of the diaphragm.
The coronary ligament is circular in form, and its ring of attachment to the liver bounds a
small area of the liver’s anterior face that is actually free of serosa.
INTESTINE (formerly called the “small intestine”)
T.
Now trace the intestine, starting from the pylorus. As you proceed through sections U,
V, W, carefully but thoroughly detach such mesentery as necessary to unravel the intestine
and spread it out in the work surface. BUT! leave the colon (“large intestine”) in its
original position until later.
U.
The first portion of the intestine, the duodenum, begins by abruptly curving caudad,
and then turning left. Its caudad curve is bound to the liver by the hepatoduodenal ligament,
and the transverse portion is supported by a dorsal portion of the greater omentum, the
mesoduodenum, and is also attached to the right kidney by the duodenorenal
ligament.
V.
Located in the mesoduodenum is the pancreas,which is more easily seen by gently
spreading the mesentery. The pancreas is a loosely organized, usually tan or pinkish gland
which extends leftward into the dorsal wall of the greater omentum posterior to the greater
curvature of the stomach. There are two pancreatic ducts. The principle one joins the
common bile duct at the point where the latter enters the duodenum (you may need to pick
away the substance of the pancreas to see this duct); the slightly swollen common chamber
where bile and pancreatic ducts unite is called the ampulla of Vater. The secondary, or
accessory pancreatic duct, enters the duodenum slightly posteriorly to the ampulla, but is
usually hard to isolate.
W. From the duodenum trace the coils of the remainder of the intestine. These are
supported by the major portions of the greater omentum (in this region, usually called simply
the dorsal mesentery, or sometimes the mesentery proper). The second, longest
Cat Guts p. 8
portion of the intestine, called the jejunum, begins where the duodenum turns posterior and
begins markedly coiling; and the last portion, where the coils begin to relax, is called the
ileum; but there is no definite boundary between the two portions.
Note the coils of the mesentery that accompany the intestine, and the frequent fusions that
occur between these coils. Follow the ileum to where it enlarges into the colon. (This
usually occurs in the right posterior region of the abdomen—but the location can vary in
individual specimens, especially in pregnant females.) The junction of the intestine with the
colon (see below) is marked by a baggy projection called the caecum. [By the time evolution
reached the primates, the caecum became reduced to a vermiform appendix, such as you were born
with.] Again, it may be necessary to partially tear the mesenteric fusions, but keep the organ
structures as intact as possible.
COLON (formerly called the “large intestine”)
X.
The colon passes anteriorly as the ascending colon; then turns left and extends
across the abdominal cavity as the transverse colon; then turns abruptly and proceeds
straight posteriorly as the descending colon. The portion of dorsal mesentery supporting
the colon is called the mesocolon (no surprise). At the left corner, where the transverse
turns to descending, the mesocolon is secondarily fused to the mesogaster (see sections L and
P above).
Y.
The terminal portion of the descending colon is the rectum. Both the rectum and the
urinary duct (urethra) pass to the exterior through the ring formed by the pelvic girdle and
the spinal column. (We will follow these to the anus and to the penis or vagina when we
dissect the urogenital system of the cat.)
Now cut through the exact junction of the small and large intestine and note an elevation, the
ileocolic valve, projecting into the ileum. Also make short incisions at various points along
the small and large intestines to see how the lining of the intestine varies from one region to
another.
KIDNEYS
Z. On the dorsal wall of the peritoneal cavity, at about the level of the posterior ends of the
liver lobes, and outside [behind] the parietal peritoneum, are the kidneys, which can be seen
by gently lifting the coils of the intestine.
BLADDER
AA. The urinary bladder is a sac occupying the posterior end of the peritoneal cavity,
immediately internal to the ventral body wall, and ventral to the colon. When intact, a
mesentery called the median ligament of the bladder (median umbilical fold in human
anatomy) extends from the ventral surface of the bladder to the median ventral line, and
extends forward for some distance. Near the exit of the bladder from the peritoneal cavity,
each side has a small ligament called the lateral ligament of the bladder.
LYMPHATICS
BB.
The small round bodies that you may notice in the mesenteries, often buried in
fat, are lymph glands, parts of the lymphatic system. Small portions of lymphatic tissue,
called lymph nodules, are also abundant in the walls of the intestine. Aggregations of the
Cat Guts p. 9
nodules are called Peyer’s patches. They appear as thickened oval spots on the surface of
the intestine, of a slightly different color than the surrounding intestinal wall.
REPRODUCTIVE STRUCTURES
CC. In female specimens:
The cat’s uterus is quite different from a human. The uterus in a cat is Y-shaped and is called a
bipartite uterus. The base od the Y is the body of the uterus and the upper two branches are
the uterine horns (where multiple fetuses may be located if your cat is pregnant). In the
pelvic cavity, locate the small, oval ovaries posterior and lateral to the kidneys and the small
uterine tubes that have tiny fimbriae curved over the ovaries. Note the thin mesentery that
attaches these structures to the body wall.
DD. In male specimens:
Because a male cat has a retractable penis, you may need to check for the external urethral
orifice first to find the penis and the sheath-like prepuce covering it. To observe the glans
penis, make an incision in the prepuce. Identify the scrotum or scrotal sac covering the
paired testes, which may not be very obvious if you have a young male. Carefully, make a
lateral incision in one side of the scrotum and remove the loose fascia and inner fibrous
connective tissue to expose one testis. Note the Epididymis on the medial and posterior
surfaces of the testis, and inspect its tiny, coiled tubules (you may want to use a hand lends for
this). Identify the ductus (vas) deferens that begins at the tail of the epididymis and travels
toward the body in the spermatic cord. Observe the spermatic cord and cut away the
connective tissue to identify the ductus (vas) deferens, testicular artery, testicular
vein, and autonomic nerves within it. Follow the ductus (vas) deferens through the
inguinal canal into the pelvic cavity. Trace the path of the ductus (vas) deferens in the
abdominopelvic cavity as it arches around the ureter, and continues posterior to the bladder
to join the small prostate gland at the urethra. Inside the pelvic cavity, the testicular blood
vessels and autonomic nerves travel near the ureters, taking a different route from the
ductus (vas) deferens.
Cat Guts p. 10
 Make sure to observe the dissection of a cat belonging to the opposite gender
at some point during the class period.
 Sketch a ventral view of the abdominal cavity. Be sure to include the following:
Liver
Mesentery
Esophagus
Intestine
Kidney
Ureter
the appropriate reproductive structures
Stomach
Ascending colon
Urinary Bladder
Spleen
Transverse colon
Gall Bladder
Descending colon
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BLOOD VESSELS
Double injected cats are usually used to identify blood vessels. Arteries are injected with red
latex, and veins are injected with blue latex. Blood vessels differ slightly in location from cat to
cat. It is important to understand that these slight differences in location are normal and also
occur in humans. Observe the fascia that protects and secures blood vessels. Carefully
remove the fascia with blunt instruments to separate blood vessels from other
structures.
ARTERIES
A.
If you have not done so already, cut open the pericardial sac surrounding the heart with
scissors. Expose the heart and identify the pulmonary trunk exiting from the right
ventricle. Locate its branches, the right pulmonary artery and the left pulmonary
artery, and follow them to the lungs.
Cat Guts p. 11
B.
Identify the ascending aorta as it exits the left ventricle. Identify the aortic arch. In
cats, there are only two branches off the aortic arch, the brachiocephalic artery (first branch)
and the left subclavian artery. Identify these branches.
C.
The brachiocephalic artery divides into the right subclavian artery, the common
carotid, and the left common carotid. Locate the right subclavian artery as it turns laterally
and travels toward the upper extremity. Locate the right and left common carotid arteries as
the travel along the trachea. At the level of the larynx, the common carotid arteries divide to
form the external and internal carotid arteries.
D.
The first major branch off each subclavian artery is the vertebral artery. Follow the
right and left subclavian arteries to the first rib. As each subclavian crosses the first rib, it
becomes the axillary artery. Follow the axillary artery into the arm, where it becomes the
brachial artery. Distal to the elbow, the brachial artery divides to form the radial and ulnar
arteries.
E.
Lift up the heart and follow the aortic arch as it descends and forms the thoracic aorta.
Follow the thoracic aorta and observe where it passes through the diaphragm with the
esophagus and inferior vena cava, and becomes the abdominal aorta. The abdominal aorta
is retroperitoneal. You must move aside the visceral organs and remove the parietal
peritoneum lining the dorsal body wall to observe the aorta.
F.
Locate the celiac trunk, the first branch off the abdominal aorta. The celiac trunk
branches into the hepatic artery, the left gastric artery, and the splenic artery. Posterior to
the celiac trunk is the superior mesenteric artery, whose branches can be observed
traveling through the mesentery of the small intestine. Follow the abdominal aorta to the level
of the kidneys and observe the paired renal arteries branching off and traveling to the
kidneys.
G.
The gonadal arteries, testicular arteries in males and ovarian arteries in females, are
the next major branches off the abdominal aorta. Follow these arteries to the gonads (testes
in males and ovaries in females).
H.
The inferior mesenteric artery branches off the abdominal aorter posterior to the
gonadal arteries. Branches of the inferior mesenteric artery travel through the mesentery of
the large intestine. Iliolumbar arteries are large branches off the abdominal aorta posterior to
the inferior mesenteric arteries.
I.
The abdominal aorta ends when it divides into right and left external iliac arteries,
and the internal iliac artery. There is no common iliac artery in the cat. Follow one external
iliac artery into a thigh, where it becomes, the femoral artery. The femoral artery travels
down the thigh and becomes the popliteal artery in the popliteal area.
VEINS
Blood leaving tissues travels through veins back to the heart. Remember that some veins are
superficial (close to the surface), whereas others are deep. Many of the deep veins are adjacent
to arteries with the same name
Cat Guts p. 12
J.
Observe the large superficial vein traveling along the medial surface of the leg
ascending into the thigh. This is the great saphenous vein, and it joins the femoral vein, a
deep vein, traveling through the thigh adjacent to the femoral artery. The femoral vein
becomes the external iliac vein in the groin region. The internal iliac vein joins the external
iliac vein to form the common iliac vein. The right and left common iliac veins unite to form
the inferior vena cava (postcava in cat).
K.
The hepatic portal vein probably does not contain blue latex and may appear brown
from the presence of coagulated blood. The hepatic portal vein receives blood from the
digestive organs and carries this blood to the liver. The hepatic portal vein is formed from the
gastrosplenic vein and the superior mesenteric vein.
Follow the inferior vena cava through the diaphragm, into the thoracic cavity, and into the
right atrium.
L.
Locate the radial and ulnar veins in the forearm. These veins are adjacent to their
corresponding arteries. The radial and ulnar veins merge to form the brachial vein. The
brachial vein becomes the axillary vein that is adjacent to the axillary artery in the axillary
regions.
M.
In the shoulder area, the axillary vein becomes the subclavian vein. Each subclavian
vein unites to form either the right or left brachiocephalic vein. The brachiocephalic veins
merge to form the superior vena cava (precava). Follow the superior vena cava until it enters
the right atrium.
N.
Blood draining from the face and skull enters the external jugular vein. The internal
jugular vein drains the brain. Identify the large external jugular vein traveling along the lateral
surface of the neck until it joins with the subclavian vein to form the brachiocephalic vein.