Animal Biology 74
Body cavity Dissection
anteriorAbdomen dissectionA. Abdominal cavity— On the ventral surface Figure3anterior to the umbilical cord locate the
sternum (breast-bone) and the rib cage.
Locate the lower edge of the ribs. With your finger trace the edge of the rib cage (your make initialfingertips will trace in an arc across the belly slit here
and around to the sides). With your razor thoraxblade make an initial slit in the abdominal 122cavity just below the rib cage and above the edge of rib cage3umbilical cord [figure 3(1)]. You need to cut umbilical chordabdomendeep enough to enter the abdominal cavity, 44
but not so deep you cut into the liver, a large 55brown organ right under your incision. Once
you have entered the cavity, switch to
scissors and cut the abdominal cavity open by following the same arc you traced with your
finger [figure 3 (2)]. Now go back to the
posteriorcenter again and make two incisions around
the umbilical cord and down to the legs [figure 3 (3) and (4)]. Now make two incisions
[figure 3(5)] over the top of the legs and toward the back (dorsal surface).
; The umbilical cord should still be attached by the
umbilical vein to the liver. You will need to cut this
vein in order to completely open the abdominal
junction ofclavicles andsternum
cavity (see figure 4) umbilical vein
B. Thoracic Cavity— To keep the
diaphragm intact, locate it again (above
the liver) in the abdominal cavity. Intestine
Position your scissors on the sternum
(breast bone) a short distance above the
diaphragm (figure 5). Puncture the
thoracic wall [figure 5(1)] with the point
of the scissors (you may want to use a
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razor blade to make the first cut.) The thoracic wall is thicker than the abdominal wall and you will be pushing the scissors through cartilage, so a fair amount of pressure will be needed to enter the cavity. You are posterior to the lungs and heart so you will not damage these organs. Once you have entered the cavity cut an arc along the ribs (similar to the one made in the abdominal cavity) both to the left and the right side[figure 5(1)]. Now look for a hard bump between the arms where the collar bones (clavicles) and breast bone (sternum) meet. At the
location where you entered the thoracic cavity make a second cut up the sternum to this bump [figure 5(2)]. You need to be careful to cut only the body wall, including the cartilage of the ribs and sternum. The lungs and heart will be just below where you are cutting and are attached by some of the membranous lining of the body (called the pleural membranes over the
lungs and the pericardium around the heart). It is okay to cut the lining, but not the organs. Stop at the sternal bump and then cut laterally toward and under the arms to the back [figure 5(3)]. Spread the rib cage (you need to break the ribs) and expose the heart and lungs (figure 8).
; Some of the specimens may contain excess preservative fluid or coagulated blood. You may need to wash out the exposed cavities. Hold the pig under a moderate flow of water and rinse gently. You may find it necessary to move the intestine, or other organs slightly to remove all of the excess coagulated blood. Use paper towels to soak up excess water.
; Many of the organs in the abdominal and thoracic cavity should now be observable. It will, however, be necessary to move some organs from their natural positions in order to see those “hidden”.
I. Abdominal Cavity — Identify (figures 6 and 7) the following structures:
1. Liver— This dark brown organ dominates the abdominal body cavity. It should be
observable at the location where you made the first incision into this cavity. There are 5
lobes. Four are easily seen, while the fifth can only be seen by moving the intestinal
coils to the left.
Animal Biology 76 .
Figure 62. Diaphragm— This is a dome-shaped
muscular wall that separates the thoracic gall bladderumbilical veincavity from the abdominal cavity. Use liver
the liver to orient your search for the
diaphragm. The diaphragm is just above
the liver and is a sheet of muscle bile ductcompletely separating the body cavities. umbilicalstomachThe diaphragm arches over the top of cord
3. Gall bladder— Lift the right central lobe
spleenof the liver. The gall bladder, a
collapsed balloon-shaped reservoir, is intestineembedded in the liver’s under surface.
umbilical4. Stomach— This is a large muscular arterypouch on the left side of the pig. It is
partly under the liver. The esophagus
enters the stomach from the thoracic cavity. It can be observed by lifting the liver and finding the top of the stomach. The esophagus is easier to find in the thoracic cavity.
At its lower end the stomach forms a short “U” or “L” and ends in the muscular pyloric
sphincter. The pyloric sphincter will feel hard and forms a bump.
5. Small intestine — There are
three regions to the small
intestine. The upper portion is a
short region called the
duodenum The duodenum in
Latin means “12 finger”, = 12
inches today. This is about the
length of the human duodenum.
It is continuous with the pyloric
sphincter. Small tubes called
ducts open into the duodenum
from both the gall bladder (bile
duct) and the pancreas
(pancreatic duct). Trace the bile
duct from the gall bladder down
to the duodenum. You may look
for the pancreatic duct later.
The remaining small
intestine is composed of two
functional regions; however,
externally they are not
distinguishable. Notice how the small intestine is coiled and held in place by a thin membranous
material. This membrane is called the mesentery. Lift a coil of the small intestine and stretch
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the area out. Note the narrow vessels (blood and lymphatic) and lymph nodes interlaced in the mesentery. You will not be able to distinguish the blood vessels from the lymphatic vessels, but the lymph nodes will appear as small light brown oval-shaped structures. Stretch out a coil of the small intestine and cut the mesentery with your scissors. Continue to stretch out coils and cut the mesentery until you have completely unraveled the small intestine. Stop when you reach the large intestine (colon).
6. Large intestine— At the end of the small intestine (ileum) there is a short blind sac
called the caecum. In humans the caecum has an appendix. The large intestine (colon)
is shorter, darker, and thicker than the small intestine. The rectum is the uncoiled
posterior portion of the colon. The rectum descends along the back and disappears
under the urinary bladder. The human colon is shorter than that of the pig, and is not
7. Spleen— This is the reddish-brown leaf-shaped organ seen on the left side near the
stomach. The spleen is attached by a membrane to the stomach and you have probably
been pushing it out of the way as you traced the stomach and small intestines. 8. Pancreas— Lift the stomach and spleen (and the small intestine if it is still coiled at the
duodenum.) The pancreas is a lobular light-colored structure that has a beady
appearance. Its main part lies in the loop of the duodenum, but it extends to the left
toward the spleen. The pancreatic duct enters the duodenum from the pancreas, but it is
small and difficult to find, so do not spend time looking for it.
9. Kidney— These large bean-shaped organs are embedded in the dorsal body wall, one on
each side. Notice they are covered by a thin membrane that not only wraps them but
covers all of the dorsal body wall. This is the peritoneum. The mesentery of the small
intestine is also a part of the peritoneum. You cut through the ventral portion of the
peritoneum when you entered the abdominal body cavity. To remove the peritoneum
from the pig’s left kidney, use your forceps to break and pick up the peritoneum on the
lateral side of the kidney and pull it slowly from the kidney and the ureter (a white wavy
tube) that descends from the kidney. Remove the peritoneum carefully from the ureter
as you trace it to the point where it enters the urinary bladder. Most of the female and
some of the male reproductive structures are located at the base and a little
anterior to the urinary bladder. Be careful not to pull or remove any tubes or
oval-shaped structures as you remove the peritoneum. We will look at the
reproductive system on the demonstration pigs.
10. Urinary bladder — In the fetal pig, the urinary bladder is an elongated sac that is
attached to the inner surface of the ventral abdominal wall and the umbilical cord. The
two vessels, one on each side of the urinary bladder, are the umbilical arteries.
II. Thoracic Cavity: Use figures 8 and 9 to help you located the thoracic structures.
Find the following organs and glands:
1. Thymus gland — This granular tissue partially covers the heart and extends up into the
neck. The gland is enlarged in the fetus and in young animals. It is greatly reduced in
size as an animal matures. You will need to remove this tissue from the heart, but be
careful not to remove the atria (upper chambers; atrium singular) of the heart at the
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same time. This will not be a problem if the pericardium is intact, however if it is cut, be careful. The atria are usually a darker color and not granular.
2. Pericardium— You will need to remove this sac from the heart. The phrenic nerves
that control the contraction of the diaphragm during breathing will be present below the pericardium and will interfere with its removal. The nerves are white thread like structures that are not easily broken, but in order to completely clean away the pericardium you may have to break some of them.
3. Heart — This a conical organ located in the center of the thorax in the space between the lungs. Locate the following parts: two atria and two ventricles, coronary arteries
and veins on the surface of the heart, pulmonary artery, aortic arch, and the ductus
a. Pulmonary artery — This is the large blood vessel located between the two atria.
It will be the most easily seen of the vessels.
b. The aortic arch is located behind the pulmonary artery. Place your finger on the
pulmonary artery and the pig’s left atrium (your right side). Now press down and
a little to your left. Remove any pericardium that is covering the aortic arch and
pulmonary artery as you trace them lower down behind the heart. You should be
able to see where the pulmonary artery enters the aorta (figure 9). The
pulmonary artery transports blood to the lungs; however, in a fetus, the lungs are
almost completely bypassed until just before birth. What you are observing is the
bypass called the ductus arteriosus. This shunt must close at or soon after birth.
If you roll the heart a little farther over and remove the membranes and break
some of the nerves just below the ductus arteriosus, you can see the much smaller
part of the pulmonary artery going downward toward the lungs.
4. Lungs — Note the spongy lungs. The right lung is the largest, and is divided into four lobes. The left lung is divided into three lobes (in humans the right lung has three and the left lung has two). The lungs are covered with membranous sacs, the pleurals.
There are two different types of pleural linings. One lines the thoracic cavity, the other lines each lung. Remove any remaining pleural membrane from the lungs.
5. Dorsal aorta — Roll the left lung so you can see behind it. You may have to break loose more pleural sac and nerves. With the left lung rolled to the right and up, you will be able to see three tubes. Starting at the ductus arteriosus, follow the tube down the back— this is the dorsal aorta. Toward the center and a little ventral is the esophagus.
The other tube is the posterior vena cava, the large vein that brings the blood back to
the heart from the lower body.
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coronary arterythymus gland
inner right lobe lung
left lobe of lung
location of diaphragm
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Animal Biology 81
Find the following structures:
A. Female: (Fig. page 81)
1. Ovaries — These are oval or bean-shaped structures at the tips of a Y-shaped pair of tubes
coming up from behind (dorsal to) the urinary bladder.
2. Oviducts — These are very tiny tubes that are located on the backside of the ovaries. These
are called the fallopian tubes in humans.
3. Horns of the Uterus — These are the two larger tubes of the Y. The region where the horns
of the uterus join and disappear behind the urinary bladder is the body of the uterus. In humans
the uterus is not split into horns, and the oviducts would look much like the horns of the uterus.
The split uterus is an adaptation for multiple births.
4. Body of Uterus — The region just below where the two horns join together.
5. Cervix — Located at the end of the uterus, this is a hard and slightly enlarged region. 6. Vagina — This starts just below the cervix and is the tube down to the connection to the
7. Urethra — This is part of the excretory system. The urethra is the tube that leads to the
outside of the body from the bladder. Notice how the vagina and urethra join in the pig as a
urogenital sinus. The vagina and the urethra remain separated in humans.
B. Male reproductive system: (Fig. Page 81)
1. Urogenital opening — Just below the umbilical cord (Figure 2, page X-2)
2. Penis — The tube that has been separated from the ventral body wall and muscle. The penis
continues as a U-shaped tube to just above the anal opening.
3. Urethra — It continues with the penis and begins at the enlarged area on the U-shaped tube
and ends at the bladder.
4. Scrotal sacs — There are two scrotal sacs. Each contains a testis.
5. Testes — The oval shaped structures inside the scrotal sacs. One testis has been exposed by
opening one sac. When the fetal pigs are very young, the testes are in the body cavity. They
will descend into the scrotal sac through the inguinal canal. Some of your pigs may have the
testes either still in the body cavity, or just beginning to descend into the scrotum. The opening
into the inguinal canal is often associated with a hernia in man. The intestine may push down
into the scrotum through the canal.
6. Epididymus — These are the small coiled tubular structures on the testes.
7. Vas deferens — These are the tubes that lead from each testis out of the scrotal sac and into
the urethra. The two vas deferens can be seen at the testis or where they from a Y and enter
the urethra (dorsal side at the posterior end of the bladder).
8. Glands that produce the seminal fluid—
a. Bulbourethral glands (Cowper’s) — These are located just anterior to where the penis and
urethra join. They are two large pill-shaped glands on each side of the urethra.
b. Seminal vesicles — These are located on either side of the vas deferens as it becomes a
single tube and enters the urethra.
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c. Prostate gland — A tiny bump (in an adult human, the prostate is about the size of a
walnut) just below where the vas deferens becomes a single tube. The vas deferens
appears to pass through the prostate as it goes toward the urethra. The seminal vesicles
are on both sides of the prostate (figure 12).