i 7 o TEXT-BOOK OF PHYSIOLOGY. 



connective tissue, it dissolves and acidifies the proteins, thus making possible 

 the subsequent action of pepsin. 



The inorganic salts of the gastric juice are probably only incidental and 

 play no part in the digestive process. 



Mode of Secretion. The observations of Dr. Beaumont and the experi- 

 ments of many physiologists have made it certain that the secretion of the 

 gastric juice is intermittent and not continuous, that it is only on the intro- 

 duction and digestion of the food that the normal amount is poured out. 

 During the intervals of digestive activity the stomach is practically free from 

 all traces of the juice. The mucous membrane is pale and covered with a 

 layer of mucus having an alkaline or neutral reaction. The introduction, 

 however, of small portions of food or irritation with a glass rod causes a 

 change in the appearance of the mucous membrane. At the points of 

 irritation the membrane becomes red and vascular and in a few minutes 

 small drops of a secretion make their appearance; these coalesce and run 

 down the sides of the stomach. 



The statements of Beaumont and many subsequent investigators that 

 the secretion thus obtained is gastric juice have been apparently disproved 

 by Pawlow, who asserts that it is only an alkaline mucous the function of 

 which is protective in character. According to this investigator, mechanic 

 stimulation is incapable of exciting the secretion. 



The primary stimulus to gastric secretion, according to Pawlow, is a 

 psychic state induced, on the one hand, by the sight or the odor of food 

 especially if the animal is hungry and the food appetizing; and on the other 

 hand by the mastication of food which is agreeable to the animal. Thus 

 when a dog was tempted by the sight of food, the secretion made its appear- 

 ance at the end of six minutes and during the time of the experiment, which 

 lasted for an hour and a half, 80 cubic centimeters of the juice were obtained. 

 This is known as psychic or appetite juice. The character of a psychic 

 state, however, greatly influences the amount of the juice secreted. Agree- 

 able emotions increase, depressing emotions inhibit it. Again when a dog 

 with a divided esophagus and a gastric fistula was subjected to sham feeding, 

 mastication continued for five or six hours during which time 700 cubic 

 centimeters of juice were obtained from the stomach. Similar results have 

 been obtained in human beings with an occluded esophagus and a gastric 

 fistula. It is evident from these facts that the secretion of gastric juice is 

 favorably influenced by the sight and odor of appetizing food, by exhilarating 

 emotional states and thorough mastication. 



As a result of the psychic states induced by the sight and odor of food and 

 of the taste of food during mastication, nerve impulses not only descend from 

 the brain but are also transmitted from the mouth through afferent nerves, 

 to some central mechanism; and that from this mechanism, nerve impulses 

 must in turn be discharged to be transmitted through efferent nerve fibers 

 which are distributed to the epithelium of the gastric glands. Experimental 

 investigations render it probable that the central mechanism is located in the 

 medulla oblongata and that the efferent path for the secretor fibers lies in the 

 trunk of the vagus nerve. Though this nerve has been the subject of much 

 experimentation, the results which have been obtained have not been uni- 

 form. The investigations of Pawlow seem to be the most reliable. He 



