DIGESTION. 29 



gastric juice, begin to exude upon its surface, which gradually run together 

 and trickle down the sides of the stomach. 



The secretion of gastric juice is a reflex act, taking place through the 

 central nervous system and called forth in response to the stimulus of food in 

 the stomach. That the central nervous system also directly influences the 

 production of the secretion is shown by the fact that mental emotion, such 

 as fear and anger, will arrest or vitiate the normal secretion. The reflex 

 nature of the process can be shown by experimentation upon the pneumo- 

 gastric nerve. If during digestion, when the peristaltic movements are active 

 and the gastric mucous membrane flushed and covered with gastric juice, the 

 pneumogastric nerves are divided on both sides, the mucous membrane 

 becomes pale, the secretion is arrested and the peristaltic movements become 

 less marked. Stimulation of the peripheral end produces no constant effects ; 

 stimulation of the central end, however, is at once followed by dilatation of the 

 vessels, flushing of the mucous membrane and a re- establishment of the 

 secretion. It is evident, therefore, that during digestion afferent impulses are 

 passing up thepneumogastrics to the medulla ; efferent impulses, in all proba- 

 bility, pass through the fibres of the sympathetic nervous system to the blood 

 vessels and glands concerned in the elaboration of the gastric juice. After 

 all the nervous connections of the stomach are divided, a small quantity of 

 juice continues to be secreted for several days. This has been attributed 

 to the action of a local nervous mechanism and to the direct action of the 

 food upon the protoplasm of the secreting cells. 



The Gastric Juice is a secretion of the true peptic glands, and when 

 obtained from the stomach through a fistulous opening, is a clear, straw- 

 colored fluid, decidedly acid, with a specific gravity of 1.005 t i.oio. 



COMPOSITION OF GASTRIC JUICE. 



Water 975o 



Pepsin 15.00 



Hydrochloric acid 4.78 



Inorganic salts 5.22 



1000.00 



The water forms the largest part of the fluid, and holds in solution the 

 other ingredients. It results from a transudation from the blood vessels 

 under the increased blood supply. Of the inorganic salts the chlorides of 

 sodium and potassium are the most abundant. 



Pepsin is the organic nitrogenize.d ferment of the gastric juice, and is 

 formed, during the intervals of digestion, by the peptic cells. In the 



