DIGESTION 835 



ments seem to be replaced by rhythmic contraction of the 

 saccules. In the cat [)eristalsis in a backward direction — 

 an anti-peristalsis — starts in the middle of the colon and 

 passes to the caecum. 



On several occasions, usually after taking food, a rapid 

 movement of the contents downwards in mass has been 

 observed. This must be due to the rapid passage of a very 

 powerful contraction, and it appears to be originated as a 

 reflex from the stomach — a gastro-colic reflex. 



The result of the movement of the large intestine is to 

 pass the contents onwards towards the rectum. In this 

 passage the contents become less fluid from the absorption 

 of water. 



3. Defsecation. 



By the peristalsis of the intestine the matter not absorbed 

 from the wall of the gut is forced down and accumulated in 

 the rectum. It is prevented from escaping by two sphincter 

 muscles, viz. the internal sphincter, which is merely a 

 thickening of the circular muscular coat of the colon, and 

 the external sphincter, which is of skeletal muscle. 



(1) The mechanism of the act of defalcation is a 

 local reflex, similar to that controlling the rest of the in- 

 testine. In a dog, with all the lower part of the spinal cord 

 removed, defsecation can, after some time, take place 

 normally. 



(2) The peripheral mechanism is controlled by a centre 

 in the lumbar enlargement of the spinal cord. When this is 

 destroyed, the sphincters are for a time relaxed, and faeces 

 are passed whenever tbey are driven down by intestinal 

 contractions. The centre thus seems to exercise a tonic 

 influence on the sphincters. Its action is inhibited by 

 impulses sent up the sacral nerves from the distended 

 rectum. 



(3) The lumbo-spinal centre is dominated by higher centres 

 in the cerebrum, by the action of which defalcation may be 

 inhibited for a time or the reflex may be liberated. 



The local centre is stimulated when distension of the 

 rectum is produced by the accumulation of material — 



