DIGESTION 173 



hand by the mastication of food which is palatable 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. It is stated by Carlson that 

 the amount of the psychic secretion in a man, due to the tasting and chew- 

 ing of palatable food, varies from three to four cubic centimeters per 

 minute. 



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 Pavlov seem to be the most reliable. He 

 found that after division of the nerve, secretion was arrested, and that stimu- 

 lation of the peripheral ends with induced electric currents at the rate of one 

 or two per second, caused after a latent period of several minutes' duration 

 a flow of gastric juice. Coincidently with the development of the psychic 

 secretion there is a dilatation of the gastric blood-vessels and an increase in 

 the supply of blood to the gastric glands. Whether this is due to the action 

 of vaso-dilatator fibers or to an inhibition of the action of vaso-constrictor 

 fibers is uncertain. 



Though the secretion of the gastric juice can be initiated by these means, 

 the amount secreted is but small compared with the quantity secreted after 

 digestion has begun. Then it is that the blood-vessels dilate to their full 

 capacity and furnish for several hours the requisite materials for the pro- 

 duction of the juice on a relatively large scale. That some factor is active 

 in keeping up the secretion in the stomach, is apparent from the in- 

 crease in the quantity and the change in the quality of the juice secreted by 

 the miniature stomach. 



The secondary stimulus to the gastric secretion is in all probability 

 chemic in character and developed in the stomach or in its walls during 

 digestive activity, inasmuch as the secretion takes place independent of 

 nerve influences and after division of all afferent and efferent nerves that 

 pass from and to the stomach. On the assumption that this factor might 

 be developed in the walls of the stomach itself, Edkins conducted a series of 



