DIGESTION IN THE STOMACH 133 



appearance. Gastric juice does not flow continuously. There 

 are two factors which cause a flow to begin, nerve impulses 

 and chemical stimulation of the glands. The former may have 

 two origins, psychic or reflex. The mere thought or sight of 

 food may cause a flow of gastric juice. This is known as the 

 psychic or appetite secretion. The presence of food in the 

 mouth gives rise to a copious flow by reflex action. The flow 

 produced by nervous stimulation is large in amount, and con- 

 tinues for some time. It does not, however, account for all of 

 the gastric juice poured out, for if a dog is allowed to swallow 

 the food which he chews, the amount of juice obtained is greater 

 than when the food is caused to drop out of an opening made 

 into the esophagus. The presence of the food in the stomach 

 thus is responsible for a portion of the secretion. Some investi- 

 gators, among them Beaumont, have reported that mere mechan- 

 ical stimulation of the stomach walls by means of a feather or 

 a glass rod would cause secretion. Pavlov believes, however, 

 that mechanical stimulation causes no flow. The secretion re- 

 sulting from the presence of food in the stomach is not due to 

 reflex nervous impulses acting on the gastric glands, for it oc- 

 curs even after the nervous connections of the stomach have 

 been severed. All foods will not cause a flow, therefore it can- 

 not be the result of mechanical stimulation. It is evident that a 

 chemical substance must be involved, which stimulates the gas- 

 tric glands to action. Injecting a partially digested mixture 

 into the blood does not cause a flow. From this and other 

 evidence is has been concluded that by the action of the food on 

 the pyloric mucous membrane a substance is produced which is 

 given off to the blood stream. When this substance, to which 

 the name of gastrin has been given, reaches the gastric glands, 

 it incites them to secrete gastric juice which is poured out into 

 the stomach cavity. This subject is further discussed in the 

 chapter on Intestinal Digestion. Some foods, such as meat 

 broth, produce this effect directly. Other foods do so only after 

 they are partly digested. This chemical secretion, though smaller 

 in amount than the nervous secretion, persists for a much 



