302 THE TISSUES AND MECHANISMS OF DIGESTION. 



Defecation then appears to take place in the following manner : The 

 large intestine and sigmoid flexure becoming more and more full, stronger 

 and stronger peristaltic action is excited in their walls. By this means the 

 feces are driven into the rectum, and so, by a continuance of the movements, 

 increasing in vigor, against the sphincter. Through a voluntary act, or 

 sometimes at least by a simple reflex action, the lumbar sphincter centre is 

 inhibited and the sphincter relaxed. At the same time the contraction of 

 the abdominal muscles presses firmly on the descending colon, and thus, 

 contractions of the levator ani assisting, the contents of the rectum are 

 ejected. 



It must, however, be remembered that, while in appealing to our own con- 

 sciousness, the contraction of the abdominal walls and the relaxation of the 

 sphincter seem purely voluntary efforts, the whole act of defecation, includ- 

 ing both of these seemingly so voluntary components, may take place in the 

 absence of consciousness, and indeed, in the case of the dog at least, after 

 the complete severance of the lumbar from the dorsal cord. In such cases 

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

 rectum. 



237. 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 movements ; 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 mechanisms 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 

 namely, those 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 oesophagus but also in the 

 stomach, in the small intestines, and even in the large intestines by stimula- 

 tion 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 produces 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 similar 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. 89, JR. V., L. K, and reach the intes- 

 tines by the portion of the right or posterior vagus, Fig. 89, E'. 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 movements in question, is 

 shown by the fact that these continue to be developed after section of both 

 vagus nerves. Probably the whole action is a mixed one which we may 

 picture to ourselves somewhat as follows : The alimentary canal possesses a 

 power of spontaneous movement, feeble it is true, very inferior to that of the 

 heart and very apt to be latent, but still existing. The presence of food in 



