GASTRIC GLANDS 89 



These stimulating chemical products are not developed alike 

 from all foods; and the conclusion is warranted that some 

 substances do not undergo gastric digestion so readily as 

 others. Ordinary bread and the whites of eggs, for example, 

 are said not to develop them. It has been further shown that 

 fats, oils, etc., actually develop substances which chemically 

 inhibit gastric secretion. There appears also to be a kind of 

 chemical regulation of the amount and quality of juice, ac- 

 cording as much or little, or varying acidity, is needed in the 

 digestion of the substance in the stomach. 



Conditions influencing digestion operate mainly by produc- 

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

 these changes in turn are largely effected through the ner- 

 vous system. Fever, overeating, depressing emotions, stren- 

 uous physical or mental exercise, etc., decrease the secretion 

 and correspondingly 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 gran- 

 ules 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 supposed to exist in the cells as some pre- 

 liminary material corresponding to pepsinogen. This ma- 

 terial 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 chlor- 

 ides of the blood and the union of the chlorine thus liber- 

 ated with hydrogen, but how or why this occurs is not ex- 

 plained by phenomena so far observed. 



Secretory Nerves. While it has been impossible to de- 

 monstrate secretory fibers to the cells of the gastric glands, 



