MECHANICAL PHENOMENA. 29 







7. The Movements of the Large Intestine. As will be 

 described in greater detail later the food passes from the 

 small intestine through the ileocsecal valve into the ascend- 

 ing colon, and under normal circumstances this valve is 

 competent to the food which has passed through it. From 

 differences in physiological function we must distinguish 

 between the first portion of the large gut which is com- 

 posed of the ascending and transverse colon, and the second 

 portion which is made up of the descending colon. Not 

 only do the intestinal movements differ in these two por- 

 tions of the large bowel, but also their contents. For while 

 palpation shows that the food in the caecum, ascending and 

 transverse colons is soft, so that the walls of the gut can 

 readily be approximated, it is found that the contents of 

 the descending colon, sigmoid flexure, and rectum are made 

 up of hard, incompressible lumps. 



By far the commonest normal movement of the ascending 

 and transverse portions of the large bowel is that of anti- 

 peristalsis, that is to say, the peristaltic waves visible in 

 this.portion of the intestinal tract occur in a direction toward 

 the stomach and away from the rectum. The first food 

 which enters the colon from the small intestine is carried 

 by these anti-peristaltic waves into the caBcum. The con- 

 tents of the colon are not, therefore, as is generally believed, 

 carried forward toward the rectum by a slow peristalsis, 

 but are instead pushed backward a large number of times 

 by these anti-peristaltic waves. These anti-peristaltic waves 

 begin at the most advanced portion of the food, or, if con- 

 siderable is present, at the splenic flexure, from which they 

 sweep backward toward the caecum. The average dura- 

 tion of a period of anti-peristalsis is four to five minutes, 

 and the periods recur every ten to twenty minutes. Between 

 the periods the colon is quiet. 



As the ileocaecal valve is competent under normal circum- 

 stances, the anti-peristaltic waves do not force the food 

 out of the large intestine back into the small intestine, 



