450 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



not been thoroughly cleared up. The closure of the aorta may act 

 through the intermediation of the spinal cord, either by the stimulation 

 of motor or the paralysis of inhibitory apparatus ; or it may act directly 

 on peripheral, intra-muscular, ganglionic cells ; or directly on the mus- 

 cular fibres themselves. That the central nervous system is not solely 

 concerned in the production of peristalsis is proved by its occurrence in 

 an intestine removed from all connection with the central nervous 

 system, and it has been found that ligation of the mesenteric artery pro- 

 duces in the main the same effects as ligation of the aorta. We have 

 therefore, to look to the periphery for the mechanisms which maintain 

 peristalsis. It is not permissible to assume that the state of affairs here 

 is analogous to the connections between nerves and striped muscular fibre, 

 even although intestinal peristalsis may be proved to be influenced by 

 nerve impulses. The state of affairs is more analogous to the action of 

 the pneumogastric nerve on the heart ; for while the peristaltic action 

 may occur independently of the central nervous system, as has been 

 proved by its occurrence in excised intestine, it also is influenced by 

 nervous impulses passing along the splanchnic and pneumogastric nerves. 

 We have, therefore, to infer that the movements are mainly due to nervous 

 impulses starting in the ganglia found in the walls of the intestine, — the 

 ganglia of Auerbach and Meissner's plexus, — and that these ganglia may 

 be influenced by impressions traveling along these nerves. When the 

 splanchnic nerve is cut, the peristaltic contractions are temporarily 

 arrested, probably through the large supply of arterial blood which then 

 passes through the walls of the intestine. On the other hand, if the 

 splanchnic nerve be stimulated while active movement is going on, 

 peristalsis is arrested, in this case probably through the consequent 

 constriction of the intestinal blood-vessels. If the pneumogastric he 

 stimulated, the intestinal movements are increased, especially when the 

 splanchnic nerve has been cut, and it is probably through the pneumo- 

 gastric that the movements of the intestine are reflexly influenced 

 through the emotions. 



Temperature is also of influence on the intestinal movements. 

 Contact of the exterior with cold air, or the introduction of cold fluid 

 into the interior, accelerates peristaltic movements; so, also, the more 

 fluid the contents of the intestine, the more active are the intestinal move- 

 ments, thus perhaps explaining the action of various cathartics which 

 lead to the transudation of considerable quantities of fluid into the 

 interior of the bowels. 



The movements of the large intestine are the same as occur in the 

 small intestine, but are less marked, owing to the modified, sacculated 

 shape of this portion of the alimentary canal. Just as the peristalsis of 

 the small intestine commences at the pylorus, the contractions of the walls 



