Rat Digestive System

Oral cavity

With a pair of heavy scissors or bone cutters, cut the muscles, skin and mandible at the angle of the mouth on both sides and depress the lower jaw with your fingers. Locate the following structures: large incisor teeth, molar teeth, the tongue, and the bony roof of the mouth or hard palate (Figure 3). The palate separates the nasal and oral cavities. Unlike carnivorous mammals (and yourself) whose teeth stop growing shortly after they emerge, the teeth of many herbivorous mammals (including rat) continue to grow throughout life. Note that the rat does not have canines and premolars.
What is the reason for the absence of these two types of teeth in the rat?
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Figure 3: Picture of the oral cavity of the rat.

Salivary glands

To identify the salivary glands. Carefully remove the skin from one side of the face and neck to expose an area as shown in Figure 4. When you are removing the tissues to expose the glands, be certain not to cut too deep. There are three pairs of salivary glands. The first is the parotid gland, which lies just beneath the ear and extends over the ventro-lateral surface of the neck to the shoulder. The submaxillary glands are just ventral to the slightly larger parotids and are inseparable from the more anterior overlying sublingual glands. Also locate a fourth gland, the extraorbital lacrimal gland, which forms tears and other eye secretions. Examine Figure 4 to find the relative location of the aforementioned glands.
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Figure 4: Diagram of the salivary glands of the rat.
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Internal organs

Follow the incision lines indicated in Figure 5 and the description below to expose the abdominal and thoracic cavities of the rat. A pair of scissors (as opposed to a scalpel) should be used to make the incisions so that damage to the visceral organs is avoided.
Make a small incision with your scissors just caudal to the posterior end of the sternum. Do not cut too deeply into the body cavity. Snip through the abdominal wall toward the pelvic area. If your specimen is a male, cut posteriorly and to the right of the penis until you reach the scrotum. To expose the thoracic cavity, cut in an anterior direction through the ribs, just to the right of the mid-ventral line. Cut through the body wall laterally, just anterior to the diaphragm (Figure 5) and make two more lateral incisions in the posterior region of the abdomen. Gently rinse the thoracic and abdominal cavities with running water.
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Figure 5: Incision lines for dissection of the rat.
The coelom is the body cavity within which the viscera (internal organs) are suspended. The abdominal cavity and viscera are covered by a membranous tissue called the peritoneum, which is formed from mesoderm. Recall that the coelomic cavity of evolutionarily advanced invertebrates and vertebrates is always lined by mesodermal tissue. The peritoneum is extensive and forms the following delicate membranes. It is crucial to observe the membranes at this point because they are very delicate and usually get damaged during the examination of the internal organs.
a) Mesentery Proper - A double layer of peritoneal membrane extending from the dorsal body wall to the viscera.
b) Falciform Ligament - A layer of tissue extending from the ventral body wall and diaphragm, to the liver.
c) Parietal Peritoneum - This is the membrane which lines the body wall of the abdominal cavity.
d) Greater Omentum - This is a double walled peritoneal sac which extends from the greater curvature of the stomach over the intestines.
e) Lesser Omentum - This membrane joins the lesser curvature of the stomach to the liver.
After finding all of the above membranes, locate the following organs and glands associated with digestion (Figure 6):
a) Stomach - As in other animals, this sac-like structure serves as a storage site for ingested foods. The stomach is comprised of three areas, which are morphologically similar but histologically different. These are the cardiac portion (entrance from the esophagus), the fundic portion (large middle area) and the pyloric portion (constricted posterior portion) (Figure 6). The stomach opens to the duodenum through the pyloric sphincter. Cut open the stomach with a median incision and wash out its contents. Note the folds (rugae) on the walls. Observe the pyloric sphincter at the point of junction between the stomach and duodenum.
b) Small Intestine - The small intestine is comprised of three regions, which can be differentiated histologically. The anterior portion (the duodenum) receives the ducts from the digestive glands, pancreas and liver. The second portion is the jejunum and the most posterior is the ileum. The small intestine is the site of most chemical digestion and the absorption of nutrients. Digestive enzymes from the pancreas and the intestine itself are secreted into the lumen of the small intestine where the chemical breakdown of food occurs. Note the length of the small intestine. Cut a piece (1 -2 cm long) of the small intestine. Open it and wash it clean. Examine it under the dissecting microscope. The velvet texture of the lining of the small intestine is created by numerous minute projections, the villi. The absorptive surface is greatly increased by the presence of villi.
c) Large Intestine - The ileum opens to the large intestine via the ileocolic valve. The large intestine consists of four areas; a large caecum or blind sac near the ileocolic valve; an ascending colon on the right side; a short transverse colon; and a descending colon heading posteriorly.
The caecum is quite large in herbivores, like the rat, and may contain microorganisms, which further breakdown the plant material not already digested by the enzymes of the small intestine.
The major function of the large intestine itself is the re-absorption of the large quantities of water secreted into the gut during digestion. Thus, as undigested material moves along the colon, water is removed from it, resulting in a mass of waste material, the feces. Feces are stored in the rectum (the terminal portion of the colon) until eliminated through the anus.
d) Rectum - The rectum is the continuation of the descending colon through the pelvic region. It terminates with the anus, which opens externally.
e) Liver- It is divided into several lobes. The bile that is produced in the liver passes directly to the duodenum, via the bile ducts. The gall bladder, which stores the bile in most mammals, is not present in the rat. Bile is needed for proper digestion of fats. The liver has many functions such as detoxification of certain chemicals and production of glycogen (a carbohydrate storage material).
f) Spleen - This is an elongate flattened organ, which is attached to the greater curvature of the stomach. The spleen and the liver remove old red blood cells from circulation and break them down. The products of the breakdown are secreted into the stomach by the spleen and into the duodenum by the liver.
g) Pancreas - The pancreas is a diffuse gland, which is embedded in the mesentery proper and greater omentum. It is found along the anterior edge of the duodenum, just after the stomach. The pancreas secretes many digestive enzymes into the small intestine as well as hormones (insulin and glucagons) into the blood.
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Figure 6A: Diagram of the internal organs of the rat.
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Figure 6B: Close-up view the stomach and caecum of the rat.
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