Stomach

The stomach is not an important absorptive organ. A certain amount of absorption, however, takes place through its walls into the capillary loops tributary to the gastric veins. As already indicated above, the foods ready for absorption in the stomach are dissolved monosaccharids and the peptones resulting from the gastric digestion of proteins. Recent researches have also shown that certain fats, particularly milk fats, taken in the natural emulsion represented by milk, may undergo such a change in the stomach as to fit them for partial absorption. However, the total bulk of absorption from the stomach is very small as compared with that which takes place in the small intestine.

Small Intestine

The small intestine, fitted, as above described, with a special absorptive organ, the villus, is the seat of the most active absorption. By the time the food has reached the jejunum the digestion of carbohydrates, proteins, and fats has reached a point where practically all of these foodstuffs are ready for absorption. Furthermore, the earlier they are absorbed after they are ready for absorption, the better it is for the system, because a retention in the intestinal canal after they have reached the position ready for absorption is likely to result in further fermentative changes, due in part to the action of the trypsin, on proteins particularly, and in part to the action of bacteria, always present in the intestinal tract.

Through the walls of the jejunum and ileum the monosaccha-rids, the peptones, and the saponified fats are rapidly absorbed, leaving the undigested portions and indigestible portions, together with a large part of the still unabsorbed water, a large part of the bile and mucus, and most of the products of bacterial action.

Large Intestine

The large intestine receives this soupy mass of substance just described through the ileocecal valve. Received into the cecum, the mass passes slowly up the ascending colon across the transverse colon, down to the descending colon. As it makes its way slowly through this segment of the intestinal tract, it loses a large part of the water, which is absorbed by the bloodvessels of the colon. The purpose of this late absorption of water is easy to understand. If the water were absorbed early in the process the contents of the intestinal tract would be reduced to a pasty consistency, which would make their passage along the lumen of the canal difficult and slow. The late absorption of water is evidently a special adaptation to insure the fluidity of intestinal contents up to as late a period in the process as possible.