DIGESTION. 675 



The acidity of gastric juice is due chiefly to hydrochloric acid, 

 present in quantities varying from 0.1 to 0.3, or even 0.4 per cent.; 

 to a slight extent also to organic acids and acid salts. The presence 

 of free acid in gastric juice cannot be demonstrated until about 

 twenty minutes after the swallowing of food ; this is due to the 

 power of proteins to form compounds with hydrochloric acid. During 

 this time the ptyalin of saliva is active in the hydrolysis of starch. 

 The gastric juice is the only secretion of the body containing free 

 acid. The mode of production of the hydrochloric acid is not under- 

 stood. While it is known that it is derived from the chlorides of the 

 blood, the details of the process are not yet worked out. The function 

 of the acid is to activate the enzymes of the stomach which are se- 

 creted in the zymogen state, and to aid in peptic digestion. It also 

 has a marked antiseptic action upon the contents of the stomach and 

 upper intestine. Organic acids, chiefly lactic, are frequently found in 

 the stomach, but these are not secreted in the gastric juice itself, but 

 are produced by some fermentative action from the food after it has 

 entered the stomach. Lactic acid is never found in a normal stomach 

 unless it was present in the food before ingestion. It is often present 

 in cases of gastric stagnation with a decreased hydrochloric output. 

 These cases may be either benign or, more often, malignant in origin. 

 Thus, the persistent occurrence of lactic acid with a diminution or 

 absence of hydrochloric acid is an indication of serious disturbances, 

 possibly of cancer of the stomach. 



The origin of the lactic acid is the carbohydrate food ; other food 

 material may of course produce other organic acids (e. g., butyric acid 

 from butter). 



The enzymes of gastric juice. The two important enzymes of the 

 stomach are pepsin and rennin, which are secreted in an inactive or 

 zymogen form, and are activated by the hydrochloric acid of the gas- 

 tric juice. In addition to these there is probably a lipolytic enzyme 

 (gastric lipase) present. In the cardiac end of the stomach the reaction 

 does not become acid for some time after digestion commences, and the 

 ptyalin of the saliva continues its action on the starches during this time. 



Digestive action of gastric juice. The conversion of proteins into 

 peptone is a progressive reaction due to the action of pepsin in hydro- 

 chloric acid solution. Simple proteins are first changed into syn- 

 tonin, an acid albuminate ; this is split into compounds known as 

 primary proteoses (proto-proteoses, protalbumoses). By further action 

 these primary proteoses form secondary proteoses (deutero-proteoses, 

 deutero-albumoses), which are finally split, forming peptones. 



