Some time after the food mass reaches the valve, at certain intervals, the valve relaxes every time a peristaltic wave passes over the lower end of the ileum. This allows the food to pass into the large intestine. In contrast to the small intestine, the large intestine is only about five feet in length, but, as we have said, its diameter is of much greater width. Its width is variable, however. It decreases from the cecum to the sigmoid flexure, where it expands, and then it becomes smaller again in the rectum.
The large intestine, like the small intestine, is divided into three parts. The first, it will be recalled, is called the cecum, meaning “blind”; the second is called the colon which comes from the Greek word meaning the lower part of the bowels; and the third, rectum, signifying “straight”.
The cecum is located in the lower right hand corner of the abdomen and is about two and one-half inches long and slightly wider across. The colon is divided into three sections which correspond to their respective positions. The ascending colon begins at the cecum and extends upward to the lower side of the liver. Here it turns left and becomes the transverse colon, passing across the abdomen to the left side of the body. It then turns downward as the descending colon to the lower left-hand corner of the abdomen where it turns inward and backward to form the sigmoid or “S-shaped” flexure. Here it continues as the rectum which ends in the guarded opening called the anus. The anus remains tightly closed at all times, except when it is stimulated to open in order to permit the collected waste matter to pass out. The opening and closing of the anus is accomplished by circular bands of muscles called sphincters which are controlled by sensory and motor nerves. Under normal conditions the waste or fecal matter remains in the sigmoid until there is an accumulation large enough for it to move into the rectum which is normally empty. When the waste matter accumulates in the lower part of the rectum, it stimulates the sensory nerves of the mucous membrane, and by a reflex action the sphincter muscles relax and the contents is expelled through the anus.
But our food isn’t ready to be expelled as yet it hasn’t even been digested completely. When the food enters the ileum, reverse or anti-peristaltic waves tend to force the food back towards the small intestine. The ileocecal valve, however, prevents the return of the food to the small intestine under normal condition. As a result the food mass is again thoroughly churned and fresh portions are constantly brought into contact with the intestinal walls to be absorbed. In addition, although it does not secrete a digestive enzyme of its own, the large intestine furnishes an alkaline fluid which aids the completion of digestion.
The cecum and the ascending colon become gradually filled with the passage of the food mass from the ileum. The filling takes place slowly except during and immediately after meals and the material remains in the large intestine for a day or longer. This is because the peristaltic movements which send the food mass onward, while they are stronger than the anti-peristaltic waves, they are less frequent. During this time, a large part of the water and the remaining digested food products are absorbed, and with absorption the residual matter becomes more and more solid and finally assumes the character of feces or the “finished” waste matter of digestion.
We have now followed the course of food from the time it enters the mouth until the digested portions have been absorbed and the residue is ready to be evacuated. One of the principal reasons why we have described the process of digestion and absorption in detail is to impress the reader with the all important role the intestines play in these functions. Most people are under the impression that the stomach is the principal organ of digestion and absorption and that the intestines merely serve to evacuate the waste. This is far from the truth. The stomach does little more than to continue the beginning stages of digestion started in the mouth and to act as a storehouse for the great mass of food. By far the greater part of digestion and absorption takes place in the small intestine and is completed by the large intestine. It is of utmost importance, therefore, that the intestines be kept in a clean, vigorously healthy state, not only that they may perform their function of elimination of waste adequately, but and in many respects this is more important in order that digestion and absorption may be optimally completed. Unless the body is sufficiently supplied with the proper amounts of proteins, carbohydrates, fats, mineral matter and vitamins to be used to build and repair tissue and supply energy, life cannot go on. And we cannot repeat often enough that if one part of the digestive tract is not functioning properly, this malfunctioning will be reflected in the other parts. You cannot expect to digest and absorb your food properly if your colon is clogged with waste.