GASTRIC GLANDS 89 



and quality of juice, according as much or little, or a varying 

 acidity, is needed in the digestion of the substance in the 

 stomach. 



Conditions influencing digestion operate mainly by producing 

 changes in the quantity or quality of gastric juice, and these 

 changes in turn are largely effected through the nervous system. 

 Fever, overeating, depressing emotions, strenuous physical or 

 mental exercise, etc., decrease the secretion and correspond- 

 ingly interfere with digestion. 



Changes During Activity. Like the salivary cells, the cu- 

 boidal peptic cells can be shown to undergo changes during 

 secretory activity. When at rest they contain abundant gran- 

 ules, but during secretion these granules disappear, first from the 

 base and later from well-nigh the whole cell. The granules are 

 supposed to contain pepsin, or rather pepsinogen, for it is thought 

 that pepsin is not formed by the cell directly, but is made out of 

 pepsinogen, which is the product of the peptic cells, probably 

 under the action of hydrochloric acid. The rennin is also sup- 

 posed to exist in the cells as some preliminary material cor- 

 responding to pepsinogen. This material may be termed 

 rennin zymogen. 



Changes in the acid cells during activity also occur, but are 

 more obscure than those in the peptic cells. The source of 

 hydrochloric acid is a decomposition of the neutral chlorides of 

 the blood and the union of the chlorine thus liberated with hydro- 

 gen, but how or why this occurs is not explained by phenomena 

 so far observed. 



Secretory Nerves. While it has been impossible to demon- 

 strate secretory fibers to the cells of the gastric glands, such fibers 

 must exist in the vagus. Section of it (and the sympathetic), 

 however, does not entirely stop the secretion, but incidents re- 

 ferred to in a preceding section, such as secretion at sight 

 of food, or when food is chewed and not swallowed, certainly 



