356 HUMAN PHYSIOLOGY 



with, while in other diseases, when this centre has been 

 destroyed, the sphincter does not contract, and faeces may 

 escape continuously. Normally the act of defecation is 

 partly voluntary and partly involuntary. The voluntary part 

 of the act consists in closing the glottis, and making a forced 

 expiration so as to press upon the contents of the abdomen, 

 while at the same time the perineal muscles are relaxed, and 

 the rectum straightened, and thus the contents are allowed 

 to pass into the anal canal. The act is completed by the 

 emptying of the rectum by the contraction of the levatores 

 ani muscles. 



