CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 465 



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, 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 

 oesophagus 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 

 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. 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. The afferent im- 

 pulses 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 one which we may picture to ourselves some- 

 what 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 some way or other, by some direct action quite 

 apart from the central nervous system, is able to increase this 

 power so that, without any aid from the central nervous system, as 

 after section of the vagi, adequate peristaltic movements can, 

 under favourable circumstances, be carried out. Nevertheless in 

 the normal course of events satisfactory movements are still 

 further secured by the reflex action through vagus fibres just 

 described. Thus, in the dog, the act of swallowing food or even 

 the mere smell of food has been observed to increase the move- 

 ments of a piece of intestine isolated from the rest of the alimen- 

 tary canal but retaining its connections with the central nervous 

 system. Under this view the peristaltic movements produced by 

 centrifugal stimulation of the vagus in the neck are comparable 

 not so much with the contraction of a skeletal muscle when its 



F. 30 



