DIGESTION 291 



lumbar nerves, in the same way as the higher parts of the alimentary 

 canal, and particularly the small intestine, are influenced by the 

 vagus and the splanchnics. Stimulation of these sacral nerves 

 within the spinal canal causes contraction, tonic or peristaltic, of 

 the descending colon and rectum ; stimulation of the lumbar nerves 

 or of the portions of the sympathetic into which their visceral fibres 

 pass (lumbar sympathetic chain from second to sixth ganglia, or the 

 rami from it to the inferior mesenteric ganglia) causes inhibition of 

 the movements, preceded, it may be, by a transient increase. 



Stimulation of the sacral nerves causes or increases the contraction 

 of both coats of the descending colon and rectum ; stimulation of 

 the lumbar nerves inhibits both. And in the small intestine the 

 same law holds good ; the two coats are contracted together by the 

 action of the vagus, or inhibited together by that of the splanchnics. 

 With the establishment of these facts an ingenious theory, originated 

 by v. Basch, falls to the ground. He supposed that the same nerve 

 which causes contraction of the circular coat in all tubes whose walls 

 are made up of two layers of muscle also contains fibres that bring 

 about inhibition of the longitudinal coat, and vice versa. It was 

 suggested that in this way antagonism between the two coats was. 

 prevented. 



Some drugs, such as strychnia, stimulate peristaltic move- 

 ments by acting through the central nervous system ; others, 

 like muscarine, by acting directly on the local nervous 

 mechanism of the intestine. Atropia antagonizes the action 

 of muscarine by local influence. Morphia, which is much 

 used in medicine to diminish intestinal movements, also 

 appears to act locally, although it is said that after this 

 drug the intestinal walls are steadily contracted, not relaxed. 

 Venous blood in some animals, but apparently not in all, 

 has a stimulating effect, causing movements which much sur- 

 pass the normal movements, both in their vigour and in the 

 speed with which they travel. For this reason the peristaltic 

 contractions seen on opening the abdomen in a recently 

 killed rabbit give an exaggerated picture of what actually 

 occurs in the intact body, 



, frefsecation is partly a voluntary and partly a reflex act. 

 But in the infant the voluntary control has not yet been 

 developed ; in the adult it may be lost by disease ; in an 

 animal it may be abolished by operation, and in each case 

 the action becomes wholly reflex. In the normal course of 

 events, the rectum, which is empty and quiescent in the 

 intervals of defaecation, is excited to contraction as soon as 



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