We have seen that in the small intestine the conditions are very favorable both for digestion and for absorption, and that very much the greater part of the available nutrients has been absorbed before the food mass reaches the ileocaecal valve. Hertz has observed, however, that often the ileum is still full at the end of four or five hours after the last trace of chyme has left the stomach. Consequently there may be an accumulation of incompletely digested food and active digestive enzymes in the last few inches of the ileum, where it remains and undergoes digestion for perhaps a longer period than in the stomach. During all this time there is active segmentation, but very little peristalsis.

Beginning at infrequent intervals some time after the chyme first reaches it, the ileocaecal valve relaxes each time a peristaltic wave passes along the last few inches of the ileum. Cannon finds that the ileocaecal valve is physiologically" competent" for food which passes through it normally from the small intestine. This means that the food which has reached the large intestine in the natural way is ordinarily never forced back into the small intestine again. This is important because in the anterior portion of the large intestine the waves which appear most frequently are those of antiperistalsis - i.e. tend to force the food back toward the small intestine. Since the ileocaecal valve prevents the food passing back, these antiperistaltic waves result in thoroughly churning the food in this part of the large intestine and constantly bringing fresh portions in contact with the intestinal wall so that the conditions here are quite favorable for absorption. Moreover, the walls of the large intestine furnish an alkaline secretion which further aids the completion of the digestive changes already begun. So far as known the large intestine secretes no digestive enzyme of its own.

With the passage of material from the ileum into the caecum, the caecum and ascending colon become gradually filled. Recent observations show that this passive filling takes place very slowly except during and immediately after meals (Hertz). The material remains in the large intestine for a comparatively long time (generally about a day, often longer); for the peristaltic movements which carry the material onward, while stronger than the waves of antiperistalsis, are of less frequent occurrence, at least in the first part of the large intestine. During this time there is a marked absorption of water, along with the remaining products of digestion. The residual material gradually becomes more solid and takes on the character of feces.