376 VETERINARY PHYSIOLOGY 



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. Stimulation of the sympathetic 

 fibres which inhibit the peristalsis causes contraction of the 

 sphincter between the small and large intestine. 



As the contents of the small intestine are forced through the 

 ileo-caecal 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. 



Defseeation 



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 vertical anal canal. It is prevented from escaping into this 

 by the sharp fold which the last part of the bowel makes, and 

 by the contraction of the strong sphincter ani muscle. 



Defaecation 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 dent 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 possible. 

 In some diseases of the cord this centre is stimulated 

 and cannot be inhibited, and thus defaecation is interfered 

 with, while in other diseases, when this centre has been 

 destroyed, the sphincter does not contract, and faeces may 

 escape continuously. 



Normally the act of defaecation is partly voluntary and partly 

 involuntary. The voluntary part of the act consists in closing 

 the glottis, and making a forced expiration so as to press upon 

 the contents of the abdomen ; while at the same time the 

 perineal muscles are relaxed, and the rectum straightened, and 

 thus the contents are allowed to pass into the anal canal. The 



