FETAL PIG DISSECTION
OBJECTIVE 1. Dissect a fetal pig and identify the structures listed in Step 1.
OBJECTIVE 2. Give the function of each organ or structure listed in Step 1.
OBJECTIVE 3. Trace the path of food through the digestive tract of the pig.
NOTE: The fetal pigs that are used for the dissection are from pregnant females that
are brought to market by the producers; these animals are not killed solely
for use in dissection.
PROCEDURE:
1. Before you come to lab for the fetal pig dissection, review the function of each structure
listed below. You should know the location of the structure, any associated enzymes, and the
kind of food that is digested there (if it is part of the digestive system).
tongue
hard & soft palates
teeth
lungs
nasopharynx
glottis
epiglottis
esophagus
thymus gland
larynx
thyroid gland
diaphragm
heart
mesenteries
stomach
pyloric sphincter
small intestine
spleen
ureter
liver
pancreas
trachea
large intestine
rectum
kidneys
gonads (ovaries, testes)
external genitals
urinary bladder
urethra
gall bladder
2. Please note:
A. Some students prefer to use gloves for the dissection. We will not provide these;
please bring your own if you wish to use them.
B. Follow the directions in the lab manual carefully.
C. Do as little cutting away and removing of tissue as possible.
D. Your fingers or a probe are usually the best tools.
E. Do not remove tissue until you have identified it.
F. Carefully follow the instructions in the lab for disposal of materials and clean up of
dissecting equipment. Do NOT dispose of any materials in the sinks!!
3. Dissection Instruments: You will be assigned a dissection kit to be used in your study of the
fetal pig. These are the instruments, and their appropriate use:
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Scalpel: Use for your initial incisions and for the removal of skin.
Scissors: Use for further cutting of tissue.
Probe: Use for tracing tubes or passageways and for separating tissue.
Forceps: Use for pulling apart, separating, lifting, and removing tissues.
Teasing needle: Use for lifting and pushing aside tissue.
4.
The following anatomical reference terms are used in describing the location of structures in
relation to the midpoint of the body: dorsal, ventral; anterior, posterior; medial, lateral.
(See Figure 1)
5.
To hold your specimen in position, attach a length of string or a rubber band to each ankle
on one side of the pig. Pull each string underneath the dissection pan and attach to the
opposite ankle.
dorsal
medial
anterior
posterior
lateral
ventral
Figure 1.
I. THE ORAL CAVITY
1A. Open the jaws as wide as you can without cutting, and identify the oral cavity. The
tongue is on its ventral surface, and the hard palate forms its dorsal surface. The hard palate
separates the oral cavity from the nasal cavities. Examine the jaws on a demonstration skeleton
of a mammal and note how they fit one another. Are both jaws movable?
1B. Make the first incisions on your fetal pig from the corner of the mouth to the bottom of
the ear on both sides of the head (Figure 2). Use scissors to cut through the tissue and bone
including the jawbone to the base of the tongue. Do NOT cut through the tissues at the back of
the throat.
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1C. Spread the jaws so the oral cavity is completely exposed (Figure 3). Examine the
tongue carefully and answer the following questions.
a. How is the tongue attached?
b. What is the role of the tongue in feeding in the pig?
c. Is the tongue adapted for other functions in other animals?
d. Locate the papillae scattered over the tongue’s surface; these contain taste buds.
Figure 2. First Incision
incisor
canine
hard palate
nasopharynx
soft palate
esophagus
glottis
tongue
epiglottis
papillae
Figure 3. Oral Cavity
1D. The soft palate, located at the posterior border of the hard palate, contains no bone. In
humans, an extension of the soft palate, the uvula, hangs down into the throat. Locate the uvula
on your lab partner.
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1E. Locate the upper and lower gums and the few teeth that may have erupted. Those that
have erupted will probably be the third pair of incisors and canines. Cut into the gums of your
pig to locate the molar teeth. Remove them and observe their characteristics. Determine how
many embryonic teeth are present.
What is the function of each of these tooth types: incisors, canine, and molars? How is each
tooth type especially adapted for its function?
1F. The posterior region of the oral cavity is known as the pharynx. Both food and air pass
through this area. The cavity is defined ventrally by the base of the tongue and dorsally by the
rear border of the soft palate.
1G. Locate the epiglottis, a small flap of tissue located at the base of the tongue. The
epiglottis protects the opening to the trachea by diverting food or liquid directly to the
esophagus. The opening is the glottis. The first portion of the trachea is the larynx. What is a
common name for the larynx, and what is its function? Feel the cartilaginous rings that mark the
larynx and the trachea. This is to keep the tube open tube at all times. Why should this be
necessary?
A second opening, which leads to the esophagus, lies posterior to the glottis. The esophagus
is the muscular tube that leads from the mouth to the stomach. Note that it has no cartilage rings
as does the larynx. Suggest why this is so. What process moves the food through the esophagus?
1H. Locate the opening to the nasal passageway on the roof of the mouth to the rear of the
soft palate. This is called the nasopharyngeal opening. With your mouth closed, inhale through
your nose. You will feel the air entering the back of your mouth. How does that compare to the
area you are observing in the fetal pig?
II. THE NECK REGION
For the second incision, turn your pig ventral side up and remove a large segment of skin
that runs posteriorly from the middle of the lower jaw to the chest and laterally to the sides of the
throat (Figure 4). Use care when cutting the skin.
Figure 4. Neck Region - Second Incision
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2A. The first tissues that will be visible when the skin is removed will be thin fibers that run
anterior-posterior (Figure 5). These fibers are muscles. They must be removed to expose the
thymus glands which run down both sides of the throat in the neck region and into the chest
cavity. The glandular tissue is very different in appearance from the muscle fibers, and it appears
cheesy in consistency. The thymus glands are not part of the digestive system. They function in
the young mammal to process lymphocytes, a kind of white blood cell that serve as one of the
animal's defenses against infectious disease.
muscles
thymus gland
thyroid
Figure 5. Neck Region - Exposed Tissue
2B. You will have to go a bit deeper to see a second smaller gland, the thyroid gland,
located ventral to the trachea. It is usually dark reddish-brown in color which makes it rather
easy to identify. What is the function of the thyroid gland?
2C. Using a probe, separate the superficial muscles and glands from one another without
cutting them. Probe down into the deeper layers of the neck. Medially, beneath several strips of
muscle, you will find the hard-walled larynx and trachea, parts of the respiratory passageways
(Figure 6). Dorsal to the trachea, probe for the esophagus. One way to be certain that you have
located the esophagus is to open the mouth, probe well down into the esophagus, and then feel in
the neck for the tube containing the hard probe. Be sure to understand the relationships of the
food and air passages in the head and neck.
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glottis
larynx
trachea
esophagus
bronchus
lung
diaphragm
Figure 6. Thoracic Cavity
III. THE THORACIC CAVITY
Continue the incision that you made in the neck region posteriorly until you reach the
diaphragm. Then cut flaps laterally to expose the heart and lungs. You have now exposed the
thoracic cavity.
3A. Examine the thoracic cavity. The heart is located in the center of this cavity. It is also
wrapped in thin tissue, the pericardium. The dark flap-like structures on top of the heart are the
right and left auricles which are extensions of the atria. The greatest portion of the heart consists
of the ventricles.
3B. Also in the thoracic cavity, on either side of the heart, are the lungs (Figure 6). These
are contained in the pleural cavity. This cavity is lined by a thin outer membrane that lies
against the rib cage and there is another such membrane that encases the lungs. Together, these
membranes form a sac filled with fluid that helps reduce friction as the lungs expand and
contract. Examine each lung. Also follow the trachea as it branches into right and left bronchi
which lead to the right and left lung. Both the trachea and the bronchi are prevented from
collapsing by the presence of cartilaginous rings. The heart and the lungs are protected by the rib
cage. Note how the diaphragm forms the lower limit to the thoracic cavity.
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IV. THE ABDOMINAL REGION
The abdominal region is that area, posterior to the ribs, in which the ventral body wall has
no body support. The body wall in this region encloses a large peritoneal cavity. Most of the
digestive, excretory, and reproductive organs are found in the peritoneal cavity. Each of these
systems has one or more ducts which pass to the exterior through a ring of bones called the
pelvic girdle. What are these ducts?
Cut through the body wall carefully, beginning with a scalpel and continuing with scissors.
Open the abdominal cavity following the lines indicated in Figure 7. Use the umbilical cord as
a point of reference. Leave a median strip of tissue posterior to the umbilical cord. Do NOT
cut structures that lie in the midline!
umbilical
cord
urogenital
opening
Figure 7. Abdominal Region - Third Incision
4A. The thin tissue which lines the inside of the entire abdominal cavity is called parietal
peritoneum. All the visceral organs in the cavity are covered by visceral peritoneum. The two
tissues are connected by thin sheets of tissue called mesentery. Several mesenteries suspend and
support the visceral organs and serve as bridges for the passage of blood vessels and nerves.
Inflammation of these tissues is known as peritonitis.
4B. Locate the liver, the largest organ in the abdomen. Its anterior surface is smoothly
convex and fits snugly into the concavity of the diaphragm, the muscular partition that separates
the thoracic and abdominal cavities (Figure 8). Push the liver aside with your fingers, and
examine the diaphragm. (Note: If the abdominal cavity is filled with dark brown material, wash
it out in the sink. This material is clotted blood from a burst abdominal blood vessel. It does not
harm the specimen.)
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4C. Push the liver aside and identify the stomach, a large sac dorsal to the liver on the left
side of the pig. Locate the point at the anterior end of the stomach where the esophagus
penetrates the diaphragm and then almost immediately joins the stomach (Figure 9). At its
posterior end, the stomach makes a curve to the right and narrows to join the anterior end of the
small intestine. The constriction at the junction between the stomach and the small intestine is
called the pylorus. Attached to the stomach by mesentery tissue is a long, flat, reddish organ,
the spleen, which is not a part of the digestive system. What is the function of the spleen?
lobes of the lungs
diaphragm
lobes of the liver
spleen
umbilical cord
urinary bladder
large intestine
small intestine
Figure 8. Visceral Organs - Ventral View
4D. Examine closely the anterior end of the small intestine, called the duodenum. The bile
duct (from the liver) empties into the duodenum just below the stomach. On its way from the
liver, the bile duct gives off a branch to the gall bladder, a small greenish sac embedded in the
liver on the underside of one of the right lobes.
4E. Lift the stomach and locate the pancreas, a light colored diffuse gland lying in the
mesentery between the stomach and the small intestine. The pancreas has a duct which empties
into the small intestine near the pylorus which you can see with very careful dissection.
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4F. With a scalpel or razor blade, make an incision into the posterior end of the stomach
and carry the cut through the pylorus a short distance into the duodenum. Find the pyloric
sphincter muscle.
Although the embryonic animal is not feeding, small bits of epithelial tissue on its surface
become dislodged and wash into the mouth. These bits of tissue, along with small amounts of
bile from the liver, pass through the digestive tract during embryonic development. Thus, you
may notice a considerable amount of greenish material in the stomach and duodenum.
4G. From the pyloric region, the small intestine runs posteriorly for a short distance and is
then thrown into an irregular mass of bends and coils held together by a mesentery. If you look
closely at the mesentery, you can see the blood vessels that deliver blood to and from the small
intestines. Recall that most digestion, as well as almost all absorption of digested food into the
blood, occurs in the small intestines. These, then, are the blood vessels that absorb nutrients from
the small intestine. Where the distal end of the small intestine joins the large intestine (also
known as the colon), a blind sac, the cecum, can be found. The cecum contains bacteria which
are important in digestion of the cellulose in plant material for some herbivores.
esophagus
thoracic cavity
diaphragm
outline of the liver
stomach
bile duct
gall bladder
pylorus
pancreas
duodenum
spleen
colon
rectum
ileum
caecum
urinary bladder
anus
Figure 9. Digestive System - Ventral View
4H. Follow the main portion of the large intestine, as it runs from the point of junction with
the small intestine into a tight coil. In the pelvic region, along the midline of the dorsal wall of
the abdominal cavity, the alimentary canal is called the rectum. It empties to the outside through
the anus.
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Animal digestive tracts frequently include modifications that serve to increase the internal
surface areas. Such modifications are common in most vertebrates. Of what value is this
increased surface area? A common method of increasing the effective digestive and absorptive
area in the intestine is the presence of finger like extensions of the inner layer of the small
intestine. These projections are called villi. In some animals, such as reptiles, no villi are present.
However, the same effect is achieved by the innumerable folds of the intestinal lining. Cut open
and examine the interior of the small intestine.
V. UROGENITAL SYSTEM
5A. Note the kidneys on the dorsal wall of the abdominal cavity. They are separated from
the cavity by the peritoneum.
5B. Determine the sex of your pig. Males have the urogenital opening just posterior to the
umbilical cord; females have a small nipple-like clitoris near the anus.
5C. If your pig is a male, locate the testes as shown in Figure 10. If it is a female, locate the
ovaries and fallopian tubes in the abdominal cavity, posterior to the kidneys (Figure 11).
peritoneum
kidney
ureter
bladder
ductus deferens
umbilical cord
penis
seminal vesicle
prostate gland
urethra
process vaginalis
testis
anus
Figure 10. Male Urogenital System
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dorsal aorta
peritoneum
mesentery
kidney
ovary
ovary
funnel of uterine tube
broad ligament
umbilical cord
umbilical artery
ureter
urethra
pubis
clitoris
Figure 11. Female Urogenital System
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