The digestive canal or gastrointestinal tract or is one long continuous tube from mouth to anus. The first section, the mouth, is the food mill – tongue, cheek walls, palate, and teeth all taking part in the operation of mastication. The salivary glands placed in the cheeks and beneath the tongue provide a secretion that lubricates and exerts a chemical action on the starchy foods.
Gastrointestinal Tract Parts:
Stomach:
The esophagus (gullet) is the part of the tube designed to convey the masticated foods and the beverages from the mouth through the chest cavity to the stomach. The stomach is an
enlargement of the tube, a sac; it is situated in the upper part of the abdomen and crosswise in normally stout individuals; in thin people it is placed vertically and may hang down low in the abdomen. Its size depends on the size and body form of the person. It averages a capacity of three pints.
Small Intestine:
The small intestine comes next. The first part of this section, the duodenum, is important as the small ducts leading from the liver and pancreas enter it and there deliver their secretions. The small intestine is twenty-odd feet long and about one inch wide.
Large Intestine:
The last main diversion of the digestive tube is the large intestine, the colon. This is about five feet long; it varies in width from two to three inches. It begins in the lower right abdomen, ascends on that side, then crosses over to the upper left and down the left side to the lower abdomen. Its last part is called the rectum. This is some-thing of a sac; it is used to store the feces until it is convenient for the material to be expelled in the act of defecation.
Wall of Gastrointestinal Tract:
The wall of the tube throughout is made up three layers; the innermost is called the mucous membrane. This is a pavement of cells arranged not unlike the cobblestones of a city street. Many of these cells are for simple paving purposes, but most of them are secreting cells of various kinds. The material secreted varies in the different portions of the tube. Mucus for lubricating purposes or for protection must be constantly poured out.
The esophagus is particularly rich in this kind of cell so that food that has not been rendered entirely soft in the mouth will slip down into the stomach without causing mechanical irritation. The colon is even more plentifully supplied with these mucus cells. The work of moving the contents of the colon along is something of a strain on the lining membrane, and mucus is here essential.
In the stomach, there are several varieties of cells. One secretes the hydrochloric acid; others manufacture pepsin and rennin. The small intestine has its cells for the production of its own peculiar digestive juices.
The middle layer of the wall is made up of muscle fibers arranged in a circular manner around the tube. It is the contracting and elongating of these fibers that impart to the tube its motion. This is the force that churns the contents as they are being digested and which moves them along in a rhythmical manner. This motion is called peristalsis – an exceedingly important function. The outside layer, the serous membrane, is a fibrous protecting coat somewhat elastic but strong enough to limit the expansion of the tube of gastrointestinal tract.
Absorption from the digestive canal takes place directly into the fine blood vessels with which the mucous membrane is richly provided. In the small intestine, however, where most of the food elements are absorbed, there is a special absorbing system provided. Small open mouthed tubules called lacteals project slightly into the tube here; they empty into larger tubes, and these finally into a duct – thethoracic duct – which communicates directly with a large vein where the food materials thus collected are thrown into the blood stream to be taken to all parts of the body for its uses.