468 DIGESTION 



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. 



Fig. 164. The effect of stimulation of right vagus nerve on the intestinal contractions. (From 



Starling.) 



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 



