420 A TEXTBOOK OF PHYSIOLOGY 



The glottis is closed after an inspiration, and the action of the dia- 

 phragm and the abdominal muscles forces faeces past the pelvi-rectal 

 flexure. 



When the desire to defaecate is obeyed, the rectum is further dis- 

 tended by faeces by raising the intra-abdominal pressure in the above- 

 mentioned fashion. The contraction of the diaphragm after inspira- 

 tion is the most effective agent in raising the abdominal pressure. 

 This is aided by the crouching posture assumed. The contraction 

 of the abdominal muscles, the flexion of the spine, the pressure of 

 the thighs against the belly wall, and the contraction of the muscles 

 of the pelvic floor, serve to sustain the increased abdominal pressure. 



When the rectum is sufficiently distended, there ensue strong 

 peristaltic contractions of the whole colon, which, in conjunction 

 with continued contraction of the abdominal muscles and the relaxa- 

 tion of the anal sphincters, force the faeces out, the final expulsion 

 being aided by the contraction of the levator ani muscles, which 

 draw the anal canal upwards, and also constrict the lowest part of 

 the rectum. 



Although normally a voluntary process, defaecation may take place 

 in involuntary fashion when the rectum becomes sufficiently distended 

 with faeces. The faeces may become hard and dry when the voluntary 

 aids to defaecation are lacking, and it may be difficult to expel the 

 hardened masses. 



Defaecation is stated to be under the control of a centre in the 

 lumbo -sacral region. The effector nerves run to the rectum in the 

 sympathetic system by way of the inferior mesenteric ganglion and the 

 hypogastric nerves, and in the pelvic nerves (nervi erigentes) from 

 the third sacral nerve to the inferior hsemorrhoidal plexus. Stimula- 

 tion of both sets of nerves leads to contraction of the rectum. It is 

 probable, however, that the pelvic nerves are the more effective, and 

 that the haemorrhoidal plexus may be regarded as a subordinate 

 centre for defaecation, since a somewhat incomplete reflex act of 

 defaecation can occur in the dog even when the lumbo-sacral cord is 

 destroyed. The levator ani and the external sphincter muscles are 

 supplied by the fourth sacral nerve. The action of these muscles, 

 controlled from the spinal centre, is essential for the complete reflex. 

 The lumbo-sacral centre is under control of excitatory or inhibitory 

 impulses from the cerebrum, and when this control is withdrawn, as 

 after division of the spinal cord, incontinence of faeces results. 



