30 PHYSIOLOGY OF ALIMENTATION. 



The constrictions as they pass over the colon therefore 

 force the food into a blind pouch. As the food does not 

 burst through the caecum while subjected to the ever-in- 

 creasing pressure of the contractile ring it can only escape 

 through the advancing ring. We have already become 

 familiar with this phenomenon in the stomach when the 

 peristaltic waves pass over it against a closed pylorus. 

 In the colon, therefore, we have the food again subjected 

 to a thorough mixing, and another opportunity is pre- 

 sented for absorption. 



The movements characteristic of the descending colon are 

 a series of tonic constrictions which pass downwards toward 

 the rectum. As the food accumulates in the ascending 

 colon it is at first confined to the region of anti-peristalsis. 

 As more food enters the ascending colon the contents of 

 the large bowel are forced over more "and more into the 

 transverse and descending portions. After a time, as the 

 food .approximates the splenic flexure a deep constriction 

 appears which pinches off a globular mass from the main 

 body of the food as indicated in A, Fig. 8. This constric- 

 tion persists, in other words, is 

 a tonic one. The globular mass 

 now moves slowly forward toward 

 the rectum, and as it passes 

 onward down the descending 

 colon new constrictions appear 

 ( Copied from CANNON : Amer- which again pinch off globular 

 ican Journal of Physiology, masses from the advancing food 

 1902, VI, p. 268.) column in the transverse colon. 



This is shown in Fig. 9. These rings slowly move down 

 the descending colon, pushing the rounded masses of food 

 before them until they reach the sigmoid flexure and rectum. 

 Even during the short passage from the splenic flexure into 

 the rectum some absorption occurs, for the food masses 

 which are pinched off in the transverse colon are much 

 softer than the dry faeces which collect in the rectum. 



