THE MECHANISMS OF DIGESTION 



469 



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- 



Fig. 165. Diagram of time it takes for a capsule containing bismuth to reach the various parts 



of the large intestine. 



called antiperistaltic waves. 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 

 colon, where they collect chiefly in the sigmoid flexure. The descending 

 colon itself is never distended with contents and merely serves as a tube 

 for transferring the masses from the transverse colon to the sigmoid 

 flexure. The time taken for a capsule of bismuth to reach the various 

 parts of the large intestine is shown in Pig. 165. 



After a certain mass has collected in the sigmoid flexure and rectum, 

 the increasing distention causes a reflex evacuation of this portion of the 



