Download Rat Dissction Instructions - Digestion

Rat Dissection Guide – Digestive system
1. Open the rat’s mouth. This is where the food enters and digestion begins. Observe the
arrangement and type of teeth. The rat is omnivorous and has teeth for both cutting and
grinding.
2. Lay the rat on its back. Is the rat male or female? How can you tell?
4. Secure your rat with string from all 4 limbs around the dissecting pan
5. Take a midventral pinch of the skin of the lower abdomen and make an incision with scissors.
Cut upward midventrally as far as the bottom of the rib cage (do not cut open the rib cage today).
Locate the diaphragm, which is a thin layer of muscle that separates the thoracic cavity from the
abdominal cavity. Do not cut above the diaphragm for this lab. Extend the cut back to the genital
openings.
6. Push the skin laterally to each side of the rat, freeing any connection between the skin and the
abdominal muscles of the body wall by sliding your finger between them. Then, lift the
abdominal muscles and make a midventral incision with your scissors – extending it anteriorly to
the rib cage and posteriorly to the genital openings.
You have just entered the abdominal cavity or coelom. Make at least one lateral slit to each side
of the body wall (perpendicular to the long midventral slit), being careful not to puncture the
intestines. Pin the skin flaps down.
7. Observe the packing of the abdominal contents; using your fingers, explore gently, noting
connections and relative positions. Do not be afraid to lift organs away from one another. If you
pull the liver gently posteriorly, you can observe the muscular diaphragm, which separates the
abdominal cavity from the thoracic cavity.
8. Examine the contents of the abdominal cavity. First examine the liver, the large red lobed
organ posterior to the diaphragm. The stomach lies just posterior and dorsal to the liver on the
rat’s left. Find the esophagus, following it from its entrance into the stomach anteriorly to where
it emerges from the diaphragm. The general arrangement of the digestive system is the same in
all mammals, including rats and humans. After being swallowed, food passes down the
esophagus to the stomach. Following preliminary digesting in the stomach, it moves to the small
intestine for final digestion and the absorption of nutrients. The unabsorbed materials move into
the large intestine (colon) where much of the water is reabsorbed, and finally to the rectum and
anus for elimination.
9. Examine the intestinal tract. Spread it out gently (lift it up and fan it out), noting the
mesentery that attaches the intestine to the body wall (all the organs in the body cavity are
surrounded and supported by mesenteries). Note the fanlike arrangement of blood vessels in the
mesentery. The veins draining the capillary beds in the walls of the intestine collect into the
hepatic portal vein. Follow this vein on its superior course towards the liver; it ultimately will
enter the liver at the porta hepatis. Glucose from the venous blood will enter the liver cells and
be stored as glycogen; the liver also metabolizes some poisons that may have entered the body
via the intestines. If the animal has eaten something poisonous that was then absorbed into the
blood, the liver can attempt to detoxify the blood before it enters the general circulation. A few
substances (alcohol, aspirin, and some water) are absorbed through the stomach wall and enter
the general circulation first and therefore are detoxified more slowly. This accounts, for example,
for the rapid effects of alcohol ingestion.
10. At the end of the small intestine is a large blind pouch, the caecum. This structure is usually
well developed in herbivores but minimal in most carnivores. Bacteria living in the caecum can
digest cellulose thus aiding the rat in obtaining nutrition from plant material. The colon (or large
intestine) extends beyond the caecum and ends with the terminal segment (the rectum) at the
anus.
11. Lastly, find the pink, diffuse pancreas, buried in the mesenteries, just posterior to the
stomach, and look at the dark red spleen, located posterior and to the right of the stomach. The
pancreas secretes digestive enzymes (amylase, lipase, trypsin) directly into the small intestine.
Embedded within the pancreas are clusters of cells, known as the islets of Langerhans, that
release the hormones insulin and glucagon in the blood stream to control blood glucose levels.
The spleen is a part of the immune system and also serves as a storage area for red blood cells.
Fig 1: Rat cutting guide.
NOTE: You will be making cuts # 1, 2 and 3 ONLY in the main body for this digestion lab. DO
NOT MAKE CUTS 4 and 5 TODAY!
#5
#4
#2
#1
#3