After food has been acted upon by the digestive juices a certain amount of indigestible residue remains which is useless for any further purpose in the body. Mucus from the bowel walls and other cellular matter is mixed with this. Inadequate elimination of this indigestible material is spoken of as constipation. Other forms of waste products which result from the metabolism of foods which have been absorbed, are eliminated through the kidneys, skin, and lungs.
ELIMINATION THROUGH THE INTESTINES
Anatomy of Alimentary Tract. The alimentary or digestive tract consists of the mouth, the esophagus, the stomach, the small intestine, the colon, and the rectum.
Food taken by mouth passes down the esophagus into the stomach. The stomach is normally situated just beneath the diaphragm. In adults it holds approximately three pints, but soon accommodates itself to any amount of food taken, so that it may hold one or many pints.
From the stomach the food passes into the small intestine, which is a small tube averaging about twenty-two feet in length. In it the digestive processes are practically completed, so far as the calorie-yielding foods are concerned, and through its walls nearly all of these calorie-yielding foods are absorbed.
From the small intestine the unabsorbed residue in a liquid or soft pasty state passes into the colon or large bowel. The colon is a large tube varying from one to three inches in diameter, averaging about five feet in length. It passes up the right side of the abdomen, crosses in close proximity to the duodenum, gall bladder, liver, pancreas, and stomach, descends on the left side, and terminates in the rectum centrally. A knowledge of the location of the colon and its relation anatomically to other organs of the body is important, because colon disturbances are often wrongly interpreted.
The rectum is a small tube, about five or six inches in length, closed at the lower end by the small but strong sphincter muscles of the anal canal. The bowel turns so sharply above the rectum that it acts functionally as a valve, holding the fecal matter from passing into the rectum until just before elimination should occur. During the day and night the indigestible residue less most of its water, together with the bodies of dead and living bacteria, accumulates in the lower portion of the colon, known as the sigmoid flexure, being pushed there by the periodic gastrocolic contractions which follow the ingestion of a meal and by colonic movements. The lower portion of the colon and rectum are under the direct control of certain nerve centers in the spinal cord. When a degree of filling of the lower bowel has taken place there is a reflex stimulation of the muscular walls which results in a shortening, followed quickly by a contraction. This forces the residue into the rectum, and in turn the emptying of this organ takes place assisted by the diaphragm and the muscles in the abdominal wall and pelvic floor. A pressure within the rectum stimulates this final reflex, but if the reflex is inhibited, the rectal wall relaxes and the fecal matter becomes dry. Continued distension of the rectal wall in time results in a change in its tone and sensitivity, so that it does not respond normally to stimulation. A “movement of the bowels” empties the tract from the latter part of the transverse colon on through, an average weight of residue being 150 grams, or five ounces. The natural urge should be promptly attended to, otherwise the soft fecal matter may remain in the rectum and dry out, becoming more difficult to pass.
The rectum should not be filled constantly with feces. This interferes with the normal elimination of gas. It should be emptied when it spontaneously fills up, and remain comfortably empty during the remainder of the twenty-four hours. Children who are eating the large amounts of food which they usually eat, may have sufficient bulk to produce two spontaneous movements per day. Adults, however, usually have but one movement per day.
The Composition of. the Stool. If a stool is examined which is passed while no food is being eaten, so that no vegetable fiber or cellulose is present, it is found to consist largely of desquamated cells from the wall of the digestive tract and bacteria (mostly dead). Some mucus which facilitates passage through the colon will be observed, with residues of intestinal secretions. When food is eaten, in addition to these findings, cellulose will be found in the stool in the same form as eaten.
The Intestinal Rate. The normal intestinal rate, or the elapsed time from the taking in of food and the elimination of the undigested residue, is from forty-eight to seventy-two hours. Portions at least of an ordinary mixed meal may remain in the stomach for about five hours, and of a heavy meal, such as Thanksgiving or Christmas dinner, for as long as seven hours. The major portion of the first twenty-four hours is required to complete the digestive processes in the small intestine. The processes which take place in the colon usually require from twenty-four to forty-eight hours. Hence, the food taken on any particular day very frequently furnishes the residue for the movement of the third or fourth day. On a diet largely residue-containing, this time may be materially shortened.
Although this lapse of time which occurs between the taking of food into the stomach and the elimination of residue from the rectum, is not usually appreciated, it is a phenomenon which may be easily demonstrated. If a few charcoal tablets are taken following a meal, the char-coal provides a black marker in the otherwise yellow or brown stool. None will be found in the normal stool in the same or the next day. A trace may be found on the second day, but the greater part will be found in the stool on the third daythat is, after a lapse of sixty to seventy-two hours after its ingestion. Some may even be found in the normal stool of the fourth day.
Alvarez and Freedlander concluded from a long series of experiments in which they used small glass beads to test the rate of progress of food residue through the bowels, that the commonly accepted view that food residues from any one meal will be evacuated in the next twenty-four or forty-eight hours, is largely erroneous, and that it takes until the fourth day before eighty per cent of the beads are eliminated. Burnett concluded from a similar study in which he used the hard French millet seeds for a marker that the normal emptying time varies from sixty-three to one hundred thirty-four hours. He was especially interested in the relation of bowel absorption and skin diseases. Rates from fourteen to sixty-two hours were associated with malabsorption, whereas with slower rates the skin became healthy and remained so as long as they were maintained.