i86 TEXT-BOOK OF PHYSIOLOGY. 



central nervous system. Though the mechanism involved is ob- 

 scure, it has frequently been observed that the sight of food or the 

 chewing of food without its passage into the stomach is attended by a 

 dilatation of the blood-vessels and a copious flow of gastric juice 

 within a few minutes, showing the cooperation of vaso-motor and 

 secretory nerve influences, a ^sult similar to that which occurs when 

 the food comes into contact with the mucous membrane itself. It 

 was also observed by Dr. Beaumont that mental emotions, such as 

 fear and anger, will arrest the normal secretion. 



Many attempts have been made with varying degrees of success 

 to determine the paths of the efferent impulses to the glands. As the 

 vagus is the only cranial nerve connecting the stomach with the 

 central nervous system, it has been the subject of much experimenta- 

 tion. The results obtained, however, have not been uniform. The 

 recent investigations of Pawlow are most reliable. ' He found that 

 division of both vagi was followed by a loss of reflex action. Stimu- 

 lation of the peripheral ends with induction shocks, one per second, 

 after a latent period of about seven minutes, caused a flow of gastric 

 juice. 



Physiologic Action of Gastric Juice. In the study of the 

 physiology of gastric digestion as it takes place under normal con- 

 ditions it is important to bear in mind that the foods introduced into 

 the stomach are heterogeneous compounds consisting of both nutritive 

 and non nutritive materials, and that before the former can be digested 

 and utilized for nutritive purposes they must be freed from their 

 combinations with the latter. This is accomplished by the solvent 

 action of the gastric juice, which in virtue of the chemic activity of its 

 constituents on proteids, gradually disintegrates the food and reduces 

 it to the liquid or semi-licjiid condition. 



^ The nature of thos-ehange-and the respective influence which the 

 acid and pepsin exert can be studied with almost any form of proteid. 

 The most suitable form, however, is coagulated fibrin obtained from 

 blood by whipping and thoroughly freed from blood by washing 

 under a stream of water. The chemic features of proteids, as well as 

 the typical- forms contained in the different articles of food, have . 

 been considered in connection with the chemic composition of the body 

 and the composition of foods (see pages 31 and 136). For purposes of 

 experimentation artificial gastric juice may be employed. This is as 

 effective as the normal secretion and in no essential respect differs 

 from it. A glycerin extract of the mucous membrane acidulated 

 with 0.2 per cent, hydrochloric acid is probably the best. 



If small pieces of fibrin be suspended in clear gastric juice and 

 kept at a temperature of 104 F. (40 C.) for an hour or two, they will 

 be dissolved and will entirely disappear, giving rise to a slightly 

 opalescent mixture. In the early stages of the process the fibrin be- 



