DEFECATION. 451 



of the large intestine commence at the ileo-csecal valve, which is the 

 terminal point of the peristaltic movements of the small intestine. The 

 movements of the large intestine are said not to be influenced by stimu- 

 lation of the splanchnic nerves. 



XIV. DEFECATION. 



The contents of the alimentary tube, -which are forced onward bv 

 the peristaltic movements of the walls of the intestine, are arrested at the 

 lower extremity of the large intestine through the tonic contraction of 

 the sphincters of the anus. 



By defaecation is meant the mechanisms, partly voluntary and partly 

 reflex, which are concerned in the evacuation of the contents of the lower 

 bowel. The anus is closed by two muscles — an inner sphincter which 

 consists of unstriped involuntary fibres, and an external sphincter com- 

 posed of voluntary red striped muscles ; both of these muscles are in a 

 state of tonic contraction due to the constant transmission of impulses 

 from a special centre located in the lumbar portion of the spinal cord, 

 and which is only inhibited during the act of defaecation. The contact 

 of the fasces with the mucous membrane of the rectum leads, as it is 

 ordinarily described, to the desire to defaecate, and inhibits the contrac- 

 tion of the sphincter muscles. The peristaltic contraction of the larger 

 bowel is then sufficient alone to evacuate the contents of the rectum, 

 while a simultaneous elevation of the anus, through the contraction 

 of the levator ani muscles, raises the floor of the pelvis and pulls the 

 anus, to a certain extent, up over the descending faecal mass, at the same 

 time preventing distention of the pelvic fascia. "As the fibres of both 

 levatores converge below and become united with the fibres of the exter- 

 nal sphincter, they aid the latter during the energetic contractions of 

 the sphincter." Defaecation is partly a voluntary movement and may be 

 aided voluntarily through pressure produced by the contraction of the 

 abdominal muscles. When the contact of the faecal mass with the mu- 

 cous membrane of the upper portion of the rectum originates a desire to 

 defaecate, the impulse is transmitted to the brain and from this to the 

 spinal coi'd, inhibits the centre of defaecation in the lumbar portion of the 

 cord, and b'y this means relaxes the anal sphincter ; a deep inspiration is 

 then made and the glottis is closed ; powerful voluntary contractions of 

 the abdominal muscles press upon the abdominal contents and force the 

 intestinal mass down into the pelvis, thus mechanically aiding peristalsis 

 in causing the downward passage of the faeces. 



The contraction of the anal sphincters is kept up through the action 

 of a nervous centre situated in the lumbar spinal cord. If the connec- 

 tion of this centre with the sphincter is divided relaxation of the anus 

 takes place, but section of the cord in the dorsal region only temporarily 



