Fetal Pig Dissection
INTRODUCTION
Like humans, the pig is a placental mammal. Because the anatomy of the fetal pig
resembles that of other placentals, this laboratory serves two important
functions: It provides a representative overview of vertebrate anatomy and
provides the framework for understanding functioning body systems. The objective of this investigation is to
study the external anatomy, the organ systems of the abdominal cavity, the
thoracic cavity, and the urogenital system of the fetal pig.
MATERIALS
PROCEDURE
Part I: External Anatomy
- Place your pig in a dissecting tray. Using a ruler, measure the length of the
pig from the snout to the tail. Use
Figure 1 to estimate the age of the fetal pig.
a)
Estimate and record the age of your fetal pig.
- Identify the four regions of the pig’s body: the head, the neck,
the trunk, and the tail.
- Place the pig on its dorsal surface and observe the umbilical cord.
Locate the paired rows of nipples
along the ventral surface of the pig. Both males and females have these
nipples.
b) What is the function of the umbilical cord?
- Use the diagrams to determine the sex of your pig. In females, the urogenital opening is
located immediately below the anus. A small, spiked tissue called the genital
papilla projects from the urogenital opening (see Figure 3). In males, the scrotum containing the
testes can be located just below the anus. The urogenital opening of the male is
found immediately posterior to the umbilical cord (see Figure 4).
c) What is the sex of your pig?
- Examine the feet of the fetal pig.
d) Indicate the number of toes.
Part II: The Abdominal Cavity
During the dissection you will be directed to examine specific organs as
they become visible. Remove only those
organs indicated by the dissection procedure. Proceed cautiously to prevent damaging
underlying structures.
- With the pig still on its dorsal surface, attach one piece of
string to the pig’s ankle and another to its opposite wrist. Pull each piece under the dissecting pan
and tie together. Repeat the
procedure for the other wrist and pull once again so that the ventral
surface is exposed.
- Using scissors, pinch the skin and make an incision just in front
of the umbilical cord. Cut toward
the anterior of the pig. Follow the
incision shown in Figure 5, which is indicated as #1.
- Follow the incision markers, indicated in the diagram as #2, toward
the posterior of the pig. Make an
incision near the neck, indicated by #3, and then at the posterior portion
of the abdominal cavity, indicated by #4. Make a lateral incision near the
posterior portion of the ribs, indicated by #5. This incision runs parallel to the
diaphragm, which separates the thoracic cavity at the anterior of the
animal from the abdominal cavity, near the posterior of the animal.
- Pull apart the flaps along incision #5, exposing the abdominal
cavity. Use the scalpel to tear the
connective tissue, the peritoneum, which holds the internal organs to the
lining of the body cavity. Now pull
apart the flaps of the skin covering incision #4 to expose the posterior
portion of the abdominal cavity. Use
pins to hold back the flaps of skin.
- Locate the liver near the anterior of the abdominal cavity. Using a probe, lift the lobes and locate
the gall bladder. Follow the thin
duct from the gall bladder to the coiled small intestine. Bile salts, produced in the liver, are
stored in the gall bladder. The bile duct conducts the fat emulsifying
bile salt to the small intestine.
e) How many lobes does the liver have?
f) Describe the location of the gall bladder.
- Locate the stomach beneath the liver. Using forceps and a probe, lift the
stomach and locate the esophagus near the anterior junction. Locate the small intestine at the
posterior junction of the stomach. The small intestine is held in place by
the mesentery, a thin, somewhat transparent connective tissue. Note the
blood vessels that transport digested nutrients from the intestine to the
liver.
- Using the scalpel and forceps, lift the junction between the
stomach and small intestine, removing supporting tissue. Uncoil the junction and locate the gland
called the pancreas. Recall that the pancreas produces a number of
digestive enzymes, hormones, and sodium bicarbonate.
g) Describe the appearance of the pancreas.
- Locate the spleen, the elongated organ found around the outer
curvature of the stomach. The
spleen stores red and white blood cells. The spleen also removes damaged red blood
cells from the circulatory system.
- Using a scalpel, remove the stomach from the pig by making
transverse cuts near the junction of the stomach and the esophagus, and
near the junction of the stomach and small intestine. Make a cut along the mid-line of the stomach,
open the cavity, and observe. Finally,
rinse the stomach and view the lining under a dissecting microscope.
h) Describe the contents of the stomach.
i) Describe the appearance of the inner lining
of the stomach.
Part III:
The Thoracic Cavity
- Carefully fold back the flaps of the skin that cover the thoracic
cavity. You may use dissecting pins
to attach the ribs to the dissecting tray.
j) What organs are found in the thoracic
cavity?
k) How is the abdominal cavity separated from
the thoracic cavity?
- Locate the heart. Using
forceps and a dissecting probe, remove the pericardium from the outer
surface of the heart. The large
blood vessel that carries blood from the liver to the right side of the
heart is called the inferior vena cava (recall that the right side refers
to the pigs right side). Blood from
the heart enters the right side of the heart thought the superior vena
cava. Both the superior and
inferior vena cavae are considered to be veins because they bring blood to
the heart.
l) Describe how the blood flows through the
heart, beginning at the vena cavae.
- Make a lateral incision across the heart and expose the heart
chamber.
m) Compare the size of the wall of a ventricle
with that of an atrium. Why does the left ventricle contain more muscle than
the right ventricle?
- Locate the spongy lungs on either side of the heart and the trachea
leading into the lungs.
n) Why do the lungs feel spongy?
- Place your index finger on the trachea and push downward.
o) Describe what happens.
p) What function do the cartilaginous rings of
the trachea serve?
Part IV: The Urogenital System
- Using scissors, remove the intestines and what remains of the
stomach.
- Refer to the diagram of the thoracic cavity and urogenital system
and locate the kidney. Using
forceps and a scalpel, carefully remove the fat deposits that surround the
kidney.
q) Describe the shape and color of the kidneys.
- Locate the thin tube leading from the kidneys. The ureter carries urine from the kidney
to the bladder. Cut into the kidney and note the large number of tubules.
If your pig is a male, refer to steps 23 and 24. If your pig is a
female, refer to steps 26 and 27. Make sure that you also view the organs of a
pig of the opposite sex from your specimen.
- Use the diagram of the male urogenital system to locate the testes.
If your fetal pig is advanced in
development, the testes may have descended into the scrotum; however, they
will probably be found in the inguinal canal. Like the ovaries, the testes develop
inside the body cavity. The lower
temperatures of the scrotum promote the proper development of sperm cells.
- Use the diagram to locate the vas deferens, which conducts sperm
cells from the testes to the urethra.
r) Indicate why the male reproductive system is
often referred to as the urogenital system while the female’s is not.
- Use the diagram of the female reproductive system to locate the
ovaries. The ovaries can be found
immediately posterior to the kidneys.
- Locate the fallopian tubes leading from the ovaries. The fallopian tubes are supported by the
broad ligaments. Follow the fallopian
tubes, which meet to form the uterus.
The fallopian tube is the site of fertilization. Once fertilized, the egg travels to the
uterus, the site of the embryo and fetal development. Locate the vagina and follow the canal
into the uterus. The constriction that marks the division between the
vagina and uterus is known as the cervix.