40 PHYSIOLOGY OF ALIMENTATION. 



is evident that approximately five hours are required by the 

 food to traverse the small intestine. 



The food about to pass into the large intestine is directed 

 toward the latter from some distance back in the small gut (see 

 Fig. 8, B). From here it moves slowly along the ileum and is 

 pushed through the valve into the colon to fall into the 

 caecum. This passage of food through the ileocsecal valve 

 seems to act as a stimulus which excites the colon to activity. 

 As the food approaches the ileocecal valve the large intestine 

 is quiet and relaxed. As soon, however, as the food has 

 entered it a strong contraction takes place along the cecum 

 and lower portion of the ascending colon, which is followed 

 immediately by the anti-peristaltic waves which have already 

 been described and which continue running for two or three 

 minutes. 



Under ordinary circumstances the succeeding masses of 

 food force the older portions onward through the large 

 bowel, but even in starvation most of the contents of the 

 large bowel are gotten rid of. But a complete emptying 

 of the large intestine seems never to occur. The time 

 which the food spends in the large intestine varies of course 

 with the intervals elapsing between succeeding defecations. 

 The time which the intestinal contents spend in the ascend- 

 ing and transverse portions of the large bowel is certainly to 

 be measured in hours. During all this time absorption is 

 actively going on. As the food passes into the descending 

 colon, sigmoid flexure, and rectum this absorption is probably 

 considerably diminished. 



The voluntary part of the act of defalcation is preceded 

 by an involuntary act of preparation which takes an hour 

 or more. This is the time required for the intestinal con- 

 tents to pass from above into that portion of the lower 

 bowel which has been emptied by the last act of defecation. 

 The voluntary part of the act of defecation takes only a 

 few seconds, depending upon the amount and consistency 

 of the defecated mass. 



