230 A MANUAL OF VETERINARY PHYSIOLOGY 



maintain peristalsis. Bunge has shown that if cellulose be with- 

 held from the diet of rabbits they die from intestinal obstruction. 

 It is the cellulose and lignin in the diet of herbivora which largely 

 provide the needful stimulus to peristalsis. 



Regulation of the Digestive Processes. — Under normal con- 

 ditions, excepting for hot fluids, there is no sensation in the 

 alimentary tract posterior to the fauces ; speaking generally, 

 once the food has passed the point mentioned, we are no 

 longer conscious of its existence. The intestines and stomach 

 may be handled without causing pain, though under disease 

 when they are inflamed, tightly contracted, nipped, or immensely 

 distended, the pain produced is acute, and, judging by the 

 violence shown by horses, intolerable. 



With the absence or apparent absence of an afferent nerve- 

 supply, it is remarkable in what way the supreme function of the 

 gastro-intestinal tract is kept in working order. There are no 

 impulses passing to the brain or medulla directing operations, 

 and it can be shown, experimentally, that extirpation of the 

 abdominal sympathetic in the dog does not cause any inter- 

 ference with digestion, nor with the movements of the intestines. 

 In fact, as we have just seen, movements of the intestines 

 occur after all the nerves have been divided. It is more than 

 likely that the key to the position is that furnished by Bayliss 

 and Starling, and that the regulation of the gastro-intestinal 

 tract is a chemical regulation by means of hormones. The exist- 

 ence of these substances has already been mentioned, and more 

 will be said later, but it is well here to emphasize the fact that 

 they are capable of acting after all nervous connections have 

 been severed. They act through the medium of the blood. 



Gases of the Intestines.- — The largest amount of gas found in 

 the intestinal canal is in the caecum and colon ; the small intestines 

 naturally contain very little, frequently none, whatever is formed 

 there being probably rapidly passed into the large bowels. In 

 the large intestines marsh-gas commonly exists, forming with 

 carbonic acid the bulk of the gases present. The pathological 

 conditions arising in the large bowels of horses, and in the rumen 

 of cattle, as the result of fermentation — particularly of green 

 food — and the enormous size to which these animals may in 

 consequence be distended, are matters of common clinical 

 experience. In both horse and ox the gas may generally be 

 ignited a short distance away from the cannula which has been 

 passed to give relief, the marsh-gas igniting readily on meeting 

 with the proper proportion of oxygen. The whole of the chemical 

 changes in the intestinal canal are carried on in the absence of 

 oxygen ; the gases which are produced depend mainly on the 

 nature of the food, green material producing marsh-gas and 



