THE MECHANISMS OF DIGESTION 503 



sphincter, as it is called, opens when food is pressed against it from the 

 ileum, but remains closed when food is pressed against it from the cecum. 

 It therefore obeys the law of the intestine. That it is physiologically 

 distinct from the musculature of the rest of the ileum is indicated by the 

 fact that the splanchnic and vagus nerves do not affect it in the same 

 way; thus, stimulation of the splanchnic causes a strong contraction of 

 the sphincter, whereas it is unaffected by stimulation of the vagus. 



Peristalsis is much more rapid in the duodenum than in other parts of 

 the small intestine. During the first stages of digestion, the food ordi- 

 narily lies mainly in the right half of the abdomen, and later in the left 

 half. There is considerable variation in the time that elapses before it 

 enters the colon. In the cat, carbohydrates reach this part of the gut in 

 about four hours. 



Movements of the Large Intestine 



On account of the great differences which we have already seen to 

 exist in the size and relative importance of the colon as a digestive organ 

 in different classes of animals, it is not surprising that the movements 

 observed are very different according to the dietetic habits of the animal. 

 Apparently the movements are much the same in the cat as in man. As 

 the food passes through the ileocolic sphincter into the cecum and 

 accumulates there, it gradually sets up, by its pressure, a contraction of 

 the muscular walls of the gut somewhere about the junction between 

 the ascending and transverse colon. This wave of contraction then 

 begins to travel slowly toward the cecum, without, however, being pre- 

 ceded by any relaxation of the wall of the gut, as is the case with a true 

 peristaltic wave. This first wave is soon followed by others, with the 

 result that the food is forced up into the cecum, against the blind end 

 of which it is crowded, being meanwhile prevented from passing into 

 the ileum by the operation of the ileocolic sphincter and by the oblique 

 manner in which the ileum opens into the cecum. 



As the result of the distention of the cecum set up by these so-called 

 antiperistaltic waves, a true coordinated peristaltic wave is occasionally 

 initiated, and passes along the ascending colon preceded by the usual 

 wave of inhibition. These waves, however, disappear before they reach 

 the end of the colon, so that the food is again driven back by the so- 

 called antiperistaltic w r aves. The effect of the movements is to knead 

 and mix the intestinal contents, and thus encourage the absorption of 

 water from them. The resulting more solid portions then collect toward 

 the splenic flexure, and become separated from the remaining more fluid 

 portion by transverse waves of constriction, which develop into peri- 

 staltic waves carrying the harder masses into the distal portions of the 



