ALIMENTARY TRACT, or GASTRO INTESTINAL TRACT, is the whole digest ive tract in animals concerned in the processes of digestion and nutrition. It is in man a highly developed and differentiated tube or canal from 25 to 30 feet in length extending from the mouth to its posterior end, the anus. The mouth opens into the pharynx and this in turn into the esophagus, a muscular tube leading into the stomach. This is a dilated and pouchlike portion of the canal. From the stomach the tube is narrowed into the small intestine, which occupies from 22 to 25 feet in length and is divided into three parts histologically distinguished from each other. These di visions are the duodenum, jejunum and il eum. The ileum opens at its termination into a blind sac, the cecum, which extends down some two or three inches from the juncture and from the posterior left surface of which the appendix vermiformis is given off. The ascending colon, the first division of the large intestine, begins here and passes up the right side of the abdomen, crosses over under the liver high up in the abdomen, about at the level of the umbilicus, constituting the transverse colon. This turns at the left into the descend ing colon which passes down in front of the left posterior abdominal wall, continues into the iliac colon which in turn extends to the pelvic colon. This forms a loop in the true
pelvis and at its distal end it turns sharply downward to form the rectum which bends with a forward concavity and ends in a dilata tion which terminates in the anal canal, the final termination of the alimentary tract. This is about an inch in length and is guarded at its external opening by two circular muscles known as the sphincters.
The number of accessory glands and organs that empty their secretions into the alimentary tract is very great. The most important are the salivary glands in the mouth, the secretory glands of the stomach, the liver and pancreas, the secretions of which enter by a common duct just below the stomach, and the secretory glands of the intestines.
The structure of the different portions of the tube is similar, but variations in function produce some modifications in the muscular coats. In general there is a layer of mucous membrane on the interior of the canal; this is surrounded by a supporting framework of con nective tissue, and is further strengthened by a varying amount of unstriped muscular tissue. For details of structure•see INTESTINE; STOm ACII. For the more complicated chemical proc esses of digestion, the work of the alimentary tract, see DIGESTION; METABOLISM; NUTRITION.