DIGESTION IN THE STOMACH. 65 



pared from the pyloric region possesses no digestive activity, for 

 in this region no hydrochloric acid is secreted. The activation 

 of this pepsinogen can also be accomplished by tissue extracts 

 and by the products of micro-organismal growth. Because of 

 such growth in the stomach contents, it is often found, in dis- 

 eased conditions in which there is no acid secretion, that active 

 pepsin is present. Accompanying the pepsin, if indeed not iden- 

 tical with it, the gastric juice contains the milk-curdling ferment, 

 rcnnin. It also contains a fat-splitting ferment, lipase, whose 

 activities are, however, limited to emulsified fats. 



The most remarkable constituent of the gastric secretion is 

 hydrochloric acid, which in some animals, such as the dog, may 

 attain a percentage of 0.6, being usually about 0.2 in the case of 

 man. It is derived from the parietal cells of the glands in the 

 cardiac region of the stomach, none being present in the secre- 

 tion of the pyloric region, where there are no parietal cells. 



The source of the acid is of course the blood, for although this 

 is practically neutral, yet it contains, on the one hand, substances 

 such as sodium bicarbonate which readily yield hydrogen ions, 

 and on the other, chlorides which, by dissociation, make chlorine 

 ions readily available. Although it is thus possible, in the light 

 of modern physico-chemical teaching, to formulate an equation 

 for the reaction, yet we are at a loss to explain why just at this 

 particular place (i. e., in the gland cells of the stomach) in the 

 animal body and nowhere else the Cl- and H-ions should be 

 picked out of the blood and secreted as HC1. 



Little as we know about the cause and mechanism of the secre- 

 tion of hydrochloric acid, we do know something regarding its 

 value and use in the process of digestion, and in general we may 

 state that this is partly regulatory and partly digestive. It is 

 regulatory in that it serves as the exciting cause of subsequent 

 events in the digestive process, and digestive not Qnly in that it 

 actually assists in the break-down of protein, but also because it 

 may cause a certain amount of acid hydrolysis of sugar after it 

 has neutralized all the alkali of the swallowed saliva. Its action 

 on protein is, however, the most important, for it initiates pro- 

 teolytic break-down by producing so-called acid protein on 



