CHAP, i.] TISSUES AND MECHANISMS OF DIGESTION. 493 



former by the great, the latter by the lesser splanchnic nerve, with the solar 

 plexus ; impulses along these rami find their way by the branches a to the 

 stomach, small intestine and upper part of the large intestine. 



The rami communicantes of the II, III, and IV lumbar nerves are connected by 

 the sympathetic chain with the inferior mesenteric ganglion ; and impulses 

 along these rami find their way by the branches 6, or by the hypogastric nerve 

 Hyp. and its branches to the colon and rectum ; impulses from the lumbar nerves 

 I and V may occasionally take the same path, as indicated by the dotted lines. 



S. I. II. III. Sacral nerves. Branches from these nerves, n. e. the so-called nervi 

 erigentes, branches not connected with the sympathetic chain, cerebro-spinal 

 nerves, pass to the hypogastric plexus PL hyp., and so reach the colon and 

 rectum ; that belonging to sacral I is dotted to shew that the impulses to the 

 colon and rectum along this nerve are few or occasional. 



developed after section of both vagus nerves. The presence of 

 food in some way or other, by some direct action quite apart from 

 the central nervous system, seems able so to increase the, more or 

 less latent, spontaneous power of the canal of which we spoke 

 above that adequate peristaltic movements can, under favourable 

 circumstances, be carried out without any aid from the central 

 nervous system. Nevertheless in the normal course of events 

 satisfactory movements are still further secured by the reflex 

 action through vagus fibres just described. Moreover, the central 

 nervous system, probably through the same vagus mechanism, can 

 give rise to movements apart from the actual presence of food. 

 Thus, in the dog, the very act of swallowing food or even the mere 

 smell of food has been observed to increase the movements of a 

 piece of intestine which has been isolated from the rest of the 

 alimentary canal but still retains its connections with the central 

 nervous system. 



While the movements of the stomach, of the small intestine 

 and of the first part of the large intestine are thus governed by 

 the vagus and splanchnic nerves respectively, other nerves are 

 concerned in the movements of the descending colon and the 

 rectum. On the one hand, the movements of these parts are, in the 

 dog, cat and other animals, governed by sympathetic fibres which 

 leaving the spinal cord in the upper lumbar (a few perhaps in 

 lower thoracic) regions reach the alimentary canal (Fig. 70) by 

 the inferior mesenteric ganglia and lower mesenteric nerves or the 

 hypogastric nerves and hypogastric plexus. On the other hand, 

 the movements of these same parts are also governed by cerebro- 

 spinal fibres which running in branches of certain sacral nerves 

 {often called nervi erigentes for reasons which we shall see later 

 on) (Fig. 70) (in the dog and cat the 1st, 2nd and 3rd, in the 

 rabbit the 2nd, 3rd and 4th) reach the canal by the hypogastric 

 plexus. 



It has been stated by some observers that stimulation of 

 the sympathetic fibres gives rise to contractions of the circular 

 muscular coat of the colon and rectum but inhibits the con- 

 tractions of the longitudinal coat ; and that conversely stimulation 

 of the sacral fibres causes contractions of the longitudinal coat but 



