It is now believed that a larger proportion of food is digested and absorbed than was heretofore realized, and that the excretions from the intestines are, in many cases, made up almost entirely of refuse, and of the catabolic waste of the system. In an ordinary, mixed diet, it is stated that about ninety-two per cent of the proteins, ninety-five per cent of the fats, and ninety-seven per cent of the carbohydrates are retained by the body.
In digestion, it is of the utmost importance that the muscular, mucous, and the sub-mucous coats, and the secreting glands of the stomach and intestines be kept thoroughly strong and active, that the digestive juices may be freely poured out, the nutriment be freely absorbed, and the food be moved along the digestive tract. The strength of any organ is gained through the nutriment in the blood; therefore, daily exercise, which calls the blood freely to these organs, is imperative.
Absorption of Food
The greater part of the food is absorbed through the intestines, yet some proteins, which have been fully digested by the gastric juice, and certain fats, particularly the fats in milk, which are in a natural state of emulsion, may be absorbed through the walls of the stomach. However, the absorption through the stomach is small compared to that through the intestines.
The small intestine is particularly fitted for absorption. Every inch or so along its course the mucous lining is thrown up into folds, as if to catch the food as it passes toward the large intestine, and to hold it there until the villi have the opportunity to absorb it. These transverse folds of the intestinal walls are called valvulæ conniventes. The villi are fingerlike projections of the mucous lining of the intestines, which stand out upon the lining somewhat as the nap on plush. They have been called “sucking” villi, because during the movements of the intestines they seem to suck in the liquid food. As soon as the foodstuffs,—proteins, carbohydrates, and fats, are put in a dissoluble state ready for absorption, they are very promptly absorbed by the villi. If, for any reason, they remain unabsorbed, they are liable to ferment by the action of the trypsin, or to be attacked by the bacteria always present in the intestines.
The peptones, sugars, and saponified fats are rapidly absorbed, while the undigested portion, together with the unabsorbed water, the bile, mucus and bacterial products, are passed through the ileo-cecal valve into the large intestine.
That the large intestine is also adapted to the absorption of fats is shown by clinical experiments with patients who cannot retain food in the stomach, the food in such cases being given through rectal injections.
In the large intestine, the mass passes up the ascending colon, across the transverse colon, and down the descending colon, losing, by absorption, foodstuffs not absorbed in the stomach and small intestine.
While water and salt are absorbed both in the stomach and in the small intestine, the evident purpose in leaving the larger part of the water to be absorbed in the large intestine is that it may assist the intestinal contents in passing along. The water also stimulates the peristaltic movement.
As the food is absorbed through the walls of the alimentary canal, it is picked up by the rootlets of the mesenteric veins[5] and by the lymph channels,—the latter, through the abdominal cavity, are called lacteals. Nearly all of the fats are absorbed through the lacteals. The whitish color given to the contents of the lacteals, by the saponified fats, gives rise to the term lacteal, meaning “whitish.”