268 APPLIED PHYSIOLOGY 



Defseeation. 



In evacuating the large intestine the material in the 

 lower part of the descending colon is first expelled by 

 the combined action of peristalsis and pressure by the 

 abdominal muscles and diaphragm. The material 

 higher up is then carried down into the cleared area 

 and the process of evacuation repeated. 



During the straining which normally accompanies the 

 act of defaecation, the diaphragm descends to its lowest 

 point, carrying with it the hepatic and splenic flexures 

 of the colon. The hepatic flexure, indeed, may nearly 

 reach the level of the umbilicus, so that the ascending 

 colon is compressed to an almost globular form (Hertz). 

 The transverse colon also descends from about an inch 

 above to an inch below the umbilicus. 



As faeces are forced into the rectum and anal canal, 

 afferent impulses are set up which produce strong peri- 

 staltic contractions, involving the whole length of the 

 colon. 



In normal circumstances the stool passed to-day is 

 probably derived in chief measure from the food of the 

 day before yesterday. 



The functions of the colon as an organ of excretion 

 will be considered in another chapter (p. 287). 



Keviewing the whole trend of recent work on the 

 physiology of digestion, and considering its bearings 

 upon practical medicine, one is struck by the great 

 delicacy and complexity of the processes by which the 

 digestion of the food is accomplished, and one ceases to 



