THE FOOD AND DIGESTION 355 



part drives its contents into the relaxed part below. These 

 peristaltic movements go on after the nerves to the gut are 

 cut, but they are stopped when the ganglia in the wall of the 

 intestine are poisoned with nicotine. It has therefore been 

 concluded that the nerve ganglia in the intestinal wall form 

 a local reflex mechanism, which is stimulated by the presence 

 of foreign matter in the intestine, and which brings about 

 the co-ordinated contraction and relaxation, which together 

 constitute a true peristalsis. 



But while peristalsis is thus independent of the central 

 nervous system, it is nevertheless controlled by it. The 

 splanchnic nerves inhibit, while the vagus to the small intes- 

 tine and upper part of the large gut, and the nervi erigentes 

 to the lower part of the large gut are augmentor nerves, 

 increasing the peristalsis. 



As the contents of the small intestine are forced through 

 the ileo-csecal valve, the large intestine relaxes to receive 

 them, and then, a series of contractions passing from below 

 upwards an anti-peristalsis sets in by which the contents 

 are very thoroughly churned. Afterwards they are forced 

 downwards by tonic peristaltic waves. 



The intestinal movements are inhibited by emotions. 



Defsecation. 



By the peristalsis of the intestine, the matter not absorbed 

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

 the part of the rectum which passes horizontally forward 

 to end in the anal canal, into which it is prevented from 

 escaping by the sharp fold which the last part of the bowel 

 makes, and by the contraction of the strong sphincter ani. 



Defsecation depends primarily on the intestinal peristalsis, 

 without which it cannot be performed. When faeces accumu- 

 late in the rectum, the mucous membrane is stimulated, and 

 impulses are sent up to inhibit a centre in the lumbar region 

 of the cord which keeps the sphincter ani contracted, and 

 the sphincter is relaxed, and the escape of faeces made pos- 

 sible. In some diseases of the cord this centre is stimulated 

 and cannot be inhibited, and thus defaecation is interfered 



