CHAP. L] TISSUES AND MECHANISMS OF DIGESTION. 491 



that the ganglia scattered in its muscular walls, those namely 

 forming the plexus of Auerbach, play any prime part in developing 

 these movements. 



Its movements are however governed by the central nervous 

 system. Stimulation of the vagus nerve gives rise to powerful 

 movements of a peristaltic kind, or increases movements previously 

 going on, not only as we have already seen in the oesophagus but 

 also in the stomach, the small intestine, and upper part of the 

 large intestine. The impulses thus exciting movements reach the 

 stomach and upper part of the duodenum by the terminal portions 

 of the two vagus nerves, Fig. 70, R. V. L. V., and reach the intestines 

 by the portion of the right or posterior vagus, Fig. 70, R'. V'., which 

 passes into the solar plexus, and thence by the mesenteric nerves. 



On the other hand, if while lively peristaltic action is going on 

 in the bowels the splanchnic nerves be stimulated the bowels are 

 brought to rest, often in a very abrupt and striking manner ; and 

 the same may be observed in the stomach. We may therefore 

 speak of fibres inhibitory of peristaltic movements of the stomach 

 and intestines, as passing from the spinal cord through the splanch- 

 nic nerves and reaching those organs through the abdominal 

 plexuses, Fig. 70 SpL, Spl. mi., Sol.pl. We may thus perhaps com- 

 pare the movements of the stomach and intestine with those of the 

 heart. We may consider that the alimentary canal possesses the 

 power of spontaneous movement, a power feeble it is true as com- 

 pared with that of the heart, and often latent for relatively long 

 periods but still existing. This spontaneous power is like that of 

 the heart under the dominion of the central nervous system ; it 

 may on the one hand be augmented and when latent called into 

 visible play by augmentor impulses reaching the canal along the 

 vagus nerves ; on the other hand, it may when in play be inhibited 

 by inhibitory impulses reaching the canal along the splanchnic 

 nerves. It should be added that inhibition of peristaltic move- 

 ments by stimulation of the splanchnic nerves is by no means so 

 clear, distinct and certain as is inhibition of the heart's beat by 

 stimulation of the vagus nerves. In some cases stimulation of the 

 splanchnics may even call forth peristaltic movement instead of 

 inhibiting it. 



In actual life the chief and usual cause of the movements of 

 the stomach and intestines is the presence of food in their interior. 

 But we do not know definitely the exact manner in which the food 

 produces the movement. It may be that the food, by stimulating 

 the mucous membrane, sends up afferent impulses reaching the 

 central nervous system by afferent fibres of the vagus or in some 

 other way, and that these give rise by reflex action to efferent im- 

 pulses which descend the vagus fibres to successive portions of the 

 canal, in a manner similar to that already described in reference to 

 the oesophagus. 



But that such a reflex action through vagus fibres is not the 



322 



