The digestive system is a long tube inside the rat, with the mouth as the opening at the anterior end and the anus as the opening at the posterior end. The process of digestion, the enzymatic breakdown of complex food substances into their simpler components, occurs in the lumen (cavity) of the digestive tube. The small molecules resulting from digestion are then absorbed by the cells lining the gut and transferred to all the other cells of the body via the circulatory system. Within the cells, these molecules may be burned to release energy for cellular activity, built into the structural elements of the cell, or stored for later use. The undigested material passes along the gastrointestinal tract and out of the anus as feces.
The mouth is the most anterior part of the digestive system. Within the mouth, the food is ground up by chewing and mixed with saliva, which contains carbohydrate-splitting enzymes and lubricating mucus. Incisors are the four front most long, sharp teeth in the mouth of a rat. The incisors are especially designed for gnawing. If you make an incision on one side of the body from the region of the shoulder to the angle of the jaw, and continue cutting along the lower jaw you will reveal the salivary glands. There are three pairs of salivary glands. The largest lies just behind the ear and extends to the ventrolateral surface of the neck. The other glands are more ventral and extend anteriorly under the lower jaw. The saliva, as previously mentioned, contains enzymes, which begin the digestion of carbohydrates, and mucus, which moistens food and sticks it together to facilitate swallowing. The tongue plays a big role in the swallowing response. The food moves from the mouth into a chamber shared by the respiratory system called the pharynx and on into the esophagus. The esophagus can be seen under the trachea which is a tube recognized by its cartilage rings in the neck region. The other organs of the digestive system are located within the body cavities. All the organs of the body cavity, particularly those of the digestive system, are called the viscera. These organs are supported from the dorsal body wall by mesenteries. The wall of the body cavities and the organs are lined with a thin, moist membrane, the peritoneum.
The liver is a large, reddish brown mass that lies immediately posterior to the diaphragm, the muscle dividing the thoracic and abdominal cavities. The liver has a great number of functions. However, its role in digestion is to produce bile, a substance that emulsifies fats breaks them into minute droplets, making them easier to digest. In humans, the bile is stored in the gall bladder before being released into the small intestine. However, the rat lacks a gall bladder. Therefore, the bile is released through a duct directly into the small intestine, where it acts. The stomach is a muscular organ. It is located on the left side of the rat’s upper abdomen. Food enters through the mouth and travels to the stomach from the esophagus. The esophagus pierces the diaphragm and is next to the trachea. It is different from the trachea because it does not have cartridge rings. It looks like a tube and moves food from the mouth to the stomach. At the top, it receives food from the pharynx and at the bottom; it discharges it into the first portion of the stomach. As food reaches the end of the esophagus, it goes into the stomach through a valve called the lower esophageal sphincter. To digest food, the stomach secretes acids and enzymes that break the food down. The stomach is lined with layers of muscle tissue called rugae. The stomach muscles contract periodically, churning food to assist digestion. The pyloric sphincter is a muscular valve that opens to allow food to pass from the stomach to the small intestine. Most of the digestion and the absorption of the products of digestion take place in the small intestine. Glands in the wall of the small intestine secrete...
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