252 HANDBOOK OF PHYSIOLOGY. 



fluid is due to free hydrochloric acid, although other acids, e.g., lactic, 

 acetic, butyric, are not unfrequently to be found therein as products of 

 gastric digestion or abnormal fermentation. The amount of hydro- 

 chloric acid varies from 2 to .2 per 1000 parts. In healthy gastric juice 

 the amount of free acid may be as much as .2 per cent. 



As regards the formation of pepsin and acid, the former is produced 

 "by the central or chief cells of the peptic glands, and also most likely by 

 the similar cells in the pyloric glands ; the acid is chiefly found at the 

 surface of the mucous membrane, but is in all probability formed by the 

 secreting action of the parietal cells of the peptic glands, as no acid is 

 formed by the pyloric glands in which this variety of cell is absent. 



The ferment Pepsin can be prepared by digesting portions of the mu- 

 cous membrane of the stomach in cold water, after they have been mace- 

 rated for some time in water at a temperature 80-100 F. (27.-37.8 

 C.). The warm water dissolves various substances as well as some of the 

 pepsin, but the cold water takes up little else than pepsin, which is con- 

 tained in a grayish-brown viscid fluid, on evaporating the cold solution. 

 The addition of alcohol throws down the pepsin in grayish-white flocculi. 

 Glycerin also has the property of dissolving out the ferment ; and if 

 the mucous membrane be finely minced, and the moisture removed by 

 absolute alcohol, a powerful extract may be obtained by throwing into 

 glycerin. 



Functions The digestive power of the gastric juice depends on the 

 pepsin and acid contained in it, both of which are, under ordinary cir- 

 cumstances, necessary for the process. 



The general effect of digestion in the stomach is the conversion of the 

 food into chyme, a substance of various composition according to the na- 

 ture of the food, yet always presenting a characteristic thick, pultaceous, 

 grumous consistence, with the undigested portions of the food mixed in 

 a more fluid substance, and a strong, disagreeable acid odor and taste. 



The chief function of the gastric juice is to convert proteids into pep- 

 tones. This action maybe shown by adding a little gastric juice (natural 

 or artificial) to some diluted egg-albumin, and keeping the mixture at a 

 temperature of about 100 F. (37.8 C.) ; it is soon found that the albu- 

 min cannot be precipitated on boiling, but that if the solution be neu- 

 tralized with an alkali, a precipitate of acid-albumin is thrown down. 

 After a while the proportion of acid-albumin gradually diminishes, so 

 that at last scarcely any precipitate results on neutralization, and finally 

 it is found that all the albumin has been changed into another proteid 

 substance which is not precipitated on boiling or on neutralization. 

 This is called peptone. 



Characteristics of Peptones. Peptones have certain characteristics 

 which distinguish them from other proteids. 1. They are diffusible, i.e., 

 they possess the property of passing through animal membranes. 2. 



