DEFJECATION. 1 29 



can squeeze back the approaching matters. Any one familiar 

 with the digital examination of the unevacuated rectum, knows 

 that no faeces are met with for about two inches. 



Considerable accumulation may take place in the sigmoid flex- 

 ure without much discomfort ensuing, but when the rectum is dis- 

 tended, an urgent sensation of wanting to empty it is experienced, 

 and the voluntary movements mentioned above are performed by 

 the levator ani and the neighboring perineal muscles, with the 

 object of preventing any substance reaching the part of the rectum 

 immediately above the sphincter. 



If the rectum be distended with fluid, the occasional anal ele- 

 vation does not suffice to keep it back, and a continuous and 

 combined action of the sphincters and levator ani, etc., is neces- 

 sary to ward off the expulsion of the contents. 



When the lower bowel is habitually emptied at the same hour 

 daily a habit which should be carefully exercised the sensations 

 of requirement to go to stool occur with great punctuality, or can 

 be readily induced by the will, so that normal defalcation is 

 reputed to be, and practically is, a voluntary act. But not com- 

 pletely so, for, somewhat like swallowing, the later stages of defae- 

 cation consist essentially of a series of involuntary reflex events 

 which we can initiate by the will, but when it is once started, are 

 powerless to modify until the reflex sequence is completed. 



Under ordinary circumstances, the evacuation of the faeces is 

 commenced by the voluntary pressure exercised on the abdominal 

 contents by the respiratory muscles. The diaphragm is depressed, 

 the outlet of the air passages firmly closed, and the expiratory 

 muscles thrown into action, while at the same moment the muscles 

 which close the pelvic outlet relax, and allow the anus to descend, 

 so that the inferior angle of the rectum is straightened and a vol- 

 untary inhibition of the sphincter is brought about. This volun- 

 tary expiratory effort seldom requires to be continued for more 

 than three or four seconds before some faecal matter reaches the 

 part of the rectum just above the sphincter. When this has 

 occurred, no further abdominal pressure is necessary (except when 

 the masses of faeces are large and hard), for the local stimulus 

 starts a series of reflex acts which carry on the operation. These 



