a, 1, cylindrical or “sucking” cells; 2, goblet cell; 3, capillaries; 4, food particles ready for absorption by the cells; b, cylindrical and goblet cells seen from above.—Adapted from Landois.
The small intestine is peculiarly fitted for absorption. Its mucous lining is thrown up into folds to furnish a larger surface for this purpose. The folds hold the food as it passes toward the large intestine, until the villi have the opportunity to absorb it.
These transverse folds of the intestinal walls are called valvulæ conniventes.
The villi are tiny finger-like projections of the mucous lining of the intestines, which stand out of 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 an absorbable state called chyle, they are very promptly taken up by the villi.
If for any reason the chyle remains unabsorbed, it is liable to be attacked by the bacteria always present in the intestines, and gases form.
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 ileocecal valve into the large intestine.
The mass passes up the ascending colon, on the right side of the abdomen, across the transverse, and down the descending colon, on the left side, losing, by absorption, the small amount of foodstuffs not absorbed in the stomach and small intestine. That the large intestine is to some extent adapted to absorption is shown by clinical experiments with patients who cannot retain food in the stomach, the food in such cases being given through rectal injections.
While water and salt are absorbed in both the stomach and small intestine, the larger part of the water passes into the large intestine, that it may assist the passage of the intestinal contents.
Water also stimulates peristaltic movements.