DEFMCATION 145 



lower end of the pelvic colon upwards to the splenic flexure, 

 the rectum meanwhile being empty. The process of 

 defsecation is initiated by a peristaltic wave, which pushes 

 the distal end of the faecal mass into the rectum. In 

 different individuals various stimuh bring this about— 

 the taking of food or a cold bath. It is a reflex which is 

 developed by habit. The rectum is specially sensitive to 

 distension — this being interpreted subjectively as a desire 

 to defsecate. The rectal distension causes reflexly a strong 

 wave of contraction, which travels downwards from the 

 splenic flexure. This is accompanied by the inhibition of 

 the internal sphincter ani. The efferent path for both 

 these actions is by the sacral autonomic. This reflex 

 action is reinforced by the voluntary act of contracting 

 the diaphragm, the thoracic and abdominal muscles with 

 the glottis closed. 



If the call to defsecation — that is to say, the sensation 

 aroused by distension of the rectum — is not obeyed, the 

 sensation passes away, the result being that the rectum 

 becomes filled with an accumulation of faeces to which it 

 is insensitive. The reflex mechanism is thus thrown out 

 of gear. After normal defsecation the bowel should be 

 empty from the splenic flexure downward. 



10 



