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The Digestive System
____ The first internal system you will study will be the digestive system. Remember the warning about
excessive cutting. Remember to write all bold faced words in your glossary. Answer all questions on
your own paper. To start the internal dissection you will need to secure your fetal pig to the dissection
tray. Follow the directions below.
____ Place your pig in the dissecting tray so its ventral side is up.
____ Cut two pieces of string about 120 cm. long.
____ Tie a piece of string securely around the ankle of each right leg.
____ Run the two pieces of string under the dissecting tray and tie them tightly to the ankles of the left
legs, thus holding the pig securely in place in the dissecting tray.
____ If you stretch the leg as far apart as possible at this point, it will be easier to locate the inner organs
during your dissection. The digestive system of the pig consists of the mouth, oral cavity, pharynx,
esophagus, stomach, small intestine, large intestine, rectum and anus. The salivary glands, liver, and
pancreas are accessory organs.
____ The mouth and pharynx. The mouth opens into the oral cavity. Use scissors to cut through the
corners of the mouth to the back of the jaw, so the mouth opens widely. Note the hard palate, which
makes up the anterior part of the roof of the mouth. It is made of bone and covered with folds of mucous
membrane. The hard palate separates the oral cavity from the nasal cavities. Note that the membrane
continues posteriorly, forming the soft palate. This does not contain bone. In humans there is a fleshy
protuberance from the soft palate called the uvula.
____ Locate the uvula on your neighbor. Not your pig.
____ Notice the sensory papillae on the surface of the tongue. Taste buds in the papillae are responsible
for taste sensations. We sometimes call them just taste buds.
____ Ducts from the salivary glands enter near the rear of the oral cavity. These glands secrete saliva
containing the enzyme salivary amylase, which breaks down starch during chewing.
____ Check the gums for teeth. Cut into the gum of the pig along one side, and extract one or more of
the developing teeth. Compare your pigs teeth to your own as well as from the jaws of other animals.
Animals have is an incisor (for biting), canine (for tearing), or molar (for grinding). Canines are
important for carnivores. Incisors and molars are important for herbivores. Omnivores tend to use all
three types. People are omnivores and so are pigs.
____ In the region of the lower jaw, remove the skin just in front of the ear and locate the salivary
glands. Use the figure below as a cutting guide. The salivary glands are the color and texture of used
chewing gum. The parotid salivary gland is the one that swells up during mumps in humans
____ Now return to the oral cavity. At the base of the tongue, the oral cavity ends and the pharynx
begins. You may want the instructor to help you with this. The pharynx is the common passageway for
the digestive and respiratory tracts. Both food and air must pass through this structure. You may need
to extend your cuts to make the mouth wide enough to see all structures.
____ At the posterior end of the tongue, find a trough shaped fold of tissue, the epiglottis. You may
have to reach deep in the pharynx with your probe to locate the epiglottis and pull it into view. Posterior
to this opening you will find the glottis, which leads into the larynx, which is part of the respiratory
tract. During swallowing, the epiglottis forms a lid over the glottis and deflects food out of the
respiratory tract.
____ Dorsal to the glottis find the second, wider opening into the esophagus. The esophagus is a
collapsible muscular tube connecting to the stomach. Food enters the esophagus during swallowing,
then moves down to the stomach by waves of muscular contraction called peristalsis. Pass the probe
through the glottis and into the esophagus. Be sure that you know the difference between them.
11. Label bold faced structures listed above on figure 2.
____ Now it is time to examine the abdominal cavity. Use the umbilical cord as a landmark. Be careful
to leave the strip of tissue caudal to the umbilical cord. Avoid cutting structures in the midline. With
a razor blade, make a shallow incision about 5 mm. anterior to the umbilical cord, forward to the most
anterior pair of mammary glands. Cut around the umbilical cord on both sides, and continue the two
parallel cuts posteriorly, keeping them outside the mammary glands. See the figure below. Deepen the
cuts carefully until you reach the abdominal cavity. You will know you are there when you see dark
colored organs underneath the transparent membrane called the peritoneum. Locate the posterior end of
the sternum, or breastbone, and cut laterally on each side following the posterior edge of the rib cage.
Make similar cuts just anterior to the hind legs.
____ Now fold out the flaps of the body wall, exposing the abdominal cavity. It s anterior end has a
transverse partition, the diaphragm. Pull up and back on the umbilical cord and you will see the
umbilical vein going to the liver. This vein carries food and oxygen to the fetus.
____ Once you have identified the umbilical vein, you may cut it so that you can finish pulling back the
flap of skin to expose the internal organs.
____ If the body cavity contains much fluid, wipe it out with a paper towel. During the dissection, use
paper towels to soak up any remaining fluids.
____ Examine the shiny membrane that lines the abdomen. This structure is called the peritoneum.
____ Now find the transparent sheets of tissue that support the internal organs. These are called the
mesenteries. It is filled with many blood vessels.
____ Examine the large dark brown liver, located just posterior to the diaphragm. Notice that the liver
consists of lobes. Lift the posterior edge of the liver and find the cut end of the umbilical vein - anterior
to the right side of the liver. Just below the entrance of the umbilical vein into the liver you should see
the gall bladder as a greenish sac embedded in the under surface of the liver. The liver secretes bile.
____ Using projected images as an aide, locate the following abdominal organs. With the liver still
elevated, identify the stomach on the left side of the abdominal cavity. Locate the point near the midline
where he esophagus enters the stomach. This is the lower esophageal sphincter.
____ At the other end of the stomach, you will find another sphincter muscle, the pyloric sphincter.
This marks the beginning of the small intestine.
____ Cut open the stomach lengthwise. Notice the ridges inside the stomach. These are called rugae.
____ Note the tongue shaped spleen attached to the stomach mesentery.
____ Identify the anterior end of the small intestine leading from the stomach. It is the C-shaped
duodenum. This is an important part of the digestive system because the ducts from the liver,
gallbladder and pancreas enter the small intestine here. The ducts from the liver and gallbladder join as
the common bile duct, which is the white cord leading from the gallbladder into the duodenum.
____ If you lift the stomach and press the intestinal mass posteriorly, you will expose the light-colored,
granular pancreas. The ducts of the pancreas are too little to see.
____ Trace the rest of the small intestine posteriorly from the duodenum. Push the intestines to the
right, and relocate the point at which the small intestine enters the large intestine. It enters from the
right side, making a blind pouch called the caecum. The caecum of herbivores is usually quite large,
and contains enzymes to break down plant material. In humans, but not pigs, there is an organ called the
appendix at the end of the caecum.
____ Notice the great length of the small intestine crowded into the small space. Intestines tend to be
short in carnivores, and long in herbivores and omnivores. Free the small intestine from where it joins
the pyloric sphincter. Dissect it free of the mesentery. Unravel the small intestine until you reach the
large intestine.
12. Measure the length of the entire small intestine. Record its length in centimeters. Compare this
length to the length of your pig from snout to rump. Using this ratio as a guide, how long would your
intestines be? (assume that you are 2 meters tall)
____ Cut off a portion of the small intestine. Slit it lengthwise and examine it under the dissecting
microscope. The velvet like ridges are called villi.
13. Draw a detailed picture of the villi. Use the picture in your text for assistance. What do the villi do
to the surface area of the small intestine? Why might this be important?
____ Locate the main portion of the large intestine, the colon. It forms a tight double coil, goes to the
dorsal abdominal wall and continues posteriorly along the midline. This type of coiling is uncommon in
mammals. The colon stores food waste and prepares it for discharge. Excess water is absorbed back into
the body.
____ Remove a short segment of the colon and cut it open. Examine this under the microscope.
14. Make a detailed drawing of the microscopic view of the colon in your notes. How is this different
than the small intestine? What could account for this difference?
15. You will need to pass the oral test with your instructor before you proceed. Be able to identify all
bold faced organs.
Additional Questions about the Digestive System
16. Make sure that you have labeled and colored all parts on the pig digestive system diagram. Color
corresponding parts on the and human digestive system diagram. Use the same color for corresponding
16. Use your book to determine the products of starch digestion.
17. What kinds of teeth do you have? Why? What kinds of teeth would you expect a pig to have since
they also are omnivores.
18. Use your book to determine the two functions of salivary glands in the digestive
19. Considering that "itis" means inflamation, why does a doctor call a sore throat "pharyngitis"?
20. Why is the epiglottis important? What happens if the epiglottis fails to cover the glottis during
21. What are the functions of the liver? Why does the food from mom go straight into the baby's liver
before it enters the rest of its system?
22. What are the functions of the liver? Why does the food from mom go straight into the baby's liver
before it enters the rest of its system?
23. Why are the mesenteries filled with blood vessels.
24. What types of food will you have trouble digesting if the gall bladder malfunctions?
25. Why might the lower esophageal sphincter (also caller the cardiac sphincter) be useful if you, say
decide to do a cartwheel right after lunch?
26. After looking at the function of both the esophageal and pyloric sphincter, what can you generalize
about the role of sphincter muscles?
27. What does the role of rugae in the stomach appear to be.
28. Frequently the stomach of a fetus is not empty. Considering that a fetus receives nourishment
through the placenta, why might there be anything in the stomach? Where does it come from?
29. What would the role of the spleen be if you had a major cut?
30. What is a general description of the functions of the gallbladder and pancreas? Where do their
fluids enter the digestive system? What do you suppose happens in the duodenum?
31. Look in the book. How would you describe the shape of the human colon? How is this different
from the pigs?