354 VETERINARY PHYSIOLOGY 



hour, persists for an hour and then rapidly falls, while with 

 bread it reaches its maximum at the end of one hour, rapidly 

 falls, but persists for a much longer period than in the case 

 of flesh. (3) The digestive activity of the juices varies with 

 the kind of food and with the course of digestion. It is higher 

 and persists longer after a diet of bread, which is difficult to 

 digest, than after a diet of flesh, which is more easily digested. 

 <4) The percentage of acid does not vary markedly. When 

 more acid is required more gastric juice is secreted. (5) The 

 work done by the gastric glands is greater in the digestion of 

 bread than in the digestion of flesh. 



(g) Nervous Mechanism of Gastric Secretion. It has been 

 proved that in the dog the secretion of gastric juice can go on 

 after the nerves to the stomach have been divided, and this has 

 been ascribed to a reflex stimulation of the nerve plexus in the 

 submucosa. But this mechanism plays a small part compared 

 with the influences of the central nervous system through the 

 vagus. Pavlov finds that, when the vagus is cut below the 

 origin of the cardiac nerves so that they are not acted upon, 

 and the animal left undisturbed for some days, stimulation of 

 the nerve with a slowly interrupted induced current causes, after 

 a long latent period of a minute or two, a flow of gastric juice. 



This vagus action may be called into play either by the contact 

 of suitable food with the mouth or by the sight of food. This 

 he demonstrated by making an oesophageal fistula in a dog 

 with a gastric pouch, so that food put in the mouth escaped 

 from the gullet and did not pass into the stomach (fig. 150). 

 Mere mechanical or chemical stimulation of the mouth 

 produces no effect, but the administration of meat produced 

 it. The sight of food in a fasting dog produces after a latent 

 period of five minutes a copious flow of gastric juice. Pavlov 

 calls this "psychic" stimulation. It is an example of how 

 the " distance receptor " in the eye reflexly brings about an 

 appropriate reaction just as the " non-distance receptor " in 

 the wall of the stomach under certain stimuli brings about 

 an appropriate reaction. It is somewhat rash of a physiologist 

 who can know nothing of the relation of the psychic state 

 the actions with which it is associated to affirm, as Pavlov does, 

 that the psychic change is causal. 



There is some evidence that the formation of gastric juice 



