CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 383 



It must however be remembered that, while in appealing to 

 our own consciousness, the contraction of the abdominal walls and 

 the relaxation of the sphincter seem purely voluntary efforts, the 

 whole act of defsecation, including both of these seemingly so 

 voluntary components, may take place in the absence of conscious- 

 ness, and indeed, in the case of the dog at least, after the complete 

 severance of the lumbar from the thoracic cord. In such cases 

 the whole act must be purely reflex, excited by the presence of 

 faeces in the rectum. 



226. The nervous mechanisms of gastric and intestinal 

 movements. Both the stomach and intestines when removed 

 from the body and thus wholly separated from the central nervous 

 system may, by direct stimulation, be readily excited to move- 

 ments ; and indeed in the absence of all obvious stimuli, movements 

 which seem to be spontaneous may at times be observed. The 

 movements of which we are speaking are orderly movements of a 

 peristaltic nature, not mere local contractions of a few bundles of 

 plain muscular fibres. The alimentary canal therefore, like the 

 heart, though to a less degree, possesses within itself such mechan- 

 isms as are requisite for carrying out its own movements ; and, 

 as in the case of the heart, there is no adequate evidence that the 

 ganglia scattered in its muscular walls, those namely forming 

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

 movements. 



On the other hand, powerful movements of a peristaltic kind 

 may be induced, not only as we have already seen in the oesoph- 

 agus but also in the stomach, in the small intestine, and even in 

 the large intestine by stimulation of the vagus nerve. 



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 pro- 

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

 mucous membrane, sends up afferent impulses, and that these 

 give rise by reflex action to efferent impulses which descend the 

 vagus fibres to successive portions of the canal, in a manner simi- 

 lar to that already described in reference to the oesophagus. If 

 this be so the efferent impulses reach the stomach and upper part 

 of the duodenum by the terminal portions of the two vagi, Fig. 84, 

 R. V. Z. V., and reach the intestines by the portion of the right or 

 posterior vagus, Fig. 84, R'. V ., which passes into the solar plexus 

 and thence by the mesenteric nerves. The afferent impulses from 

 the stomach travel also apparently by the vagus ; the paths of 

 those from the intestines have not yet been determined. 



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

 only means by which the presence of food brings about the move- 

 ments in question, is shewn by the fact that these continue to be 

 developed after section of both vagus nerves. Probably the whole 

 action is a mixed on3 wh : ch we may picture to ourselves somewhat 



