DEFECATION. 



235 



the strongest voluntary impulses. Stimulation of the peduncles of the cerebrum and 

 of the spinal cord below this point causes contraction of the external sphincter. 



Defsecation. The evacuation of the faeces, which in man usually occurs at 

 certain times, begins with a lively peristalsis of the large intestine, which passes 

 downwards to the rectum. In order that the mass of faeces may not excite reflexly 

 the sphincter-muscles, in consequence of mechanical stimulation of the sensory 

 nerves of the rectum, there seems to be a centre which inhibits the reflex action 

 of the sphincters, which is called into play, owing, as it appears, to voluntary 

 impulses. Its seat is in the brain, perhaps in the optic thalami. When this 



Fig. 173. 

 Levator ani and sphincter ani externus. 



inhibitory apparatus is in action, the faecal mass passes through the anus, r without 

 causing it to close reflexly. The strong peristalsis which precedes defaecation can 

 be aided, and to a certain degree excited, by rapid voluntary movements of the 

 external sphincter and levator ani, whereby the plexus myentericus of the large 

 intestine is stimulated mechanically, thus causing lively peristaltic movements in 

 the large intestine. The expulsion of the faeces is also aided by the pressure of 

 the abdominal muscles, and most efficiently when a deep respiration is taken, so 

 as to fix the diaphragm, whereby -the abdominal cavity is diminished to the 

 greatest extent. The soft parts of the floor of the pelvis, during a strong effort at 

 stool, are driven downwards in the form of a cone, causing the mucous membrane 

 of the anus, which contains much venous blood, to be everted. The function of 

 the levator ani (figs. 172, 173) is to raise voluntarily the soft parts of the floor of 

 the pelvis, and to pull the anus to a certain extent upwards over the descending 

 faecal mass. At the same time, it prevents the distension of the pelvic fascia. As 



