THE BIOCHEMICAL PROCESSES OF DIGESTION 483 



siderable quantity 0.05 per cent or more of an organic acid is neces- 

 sary, whereas it requires only a trace of hydrochloric acid. Normal 

 human gastric juice, when titrated with one of these indicators, gives 

 a figure which corresponds to about 0.03 N. hydrochloric acid (see page 

 22). For the accurate determination of the hydrogen-ion concentration, 

 it is necessary to use the gas-chain method (see page 29). 



When gastric juice is collected through a fistula from an empty 

 stomach, very little difference will be found between the free hydro- 

 chloric acid and the total acid; that is, between th'e results obtained by 

 the second and the first of the methods described above. This is because 

 in such juice there is no organic matter capable of, combining with the 

 hydrochloric acid, and there are no other acids, such as lactic or butyric, 

 which might be produced by fermentative processes. The difference 

 between the two titrations, however, becomes quite marked when pro- 

 tein food is undergoing digestion in the stomach, because at its different 

 stages of digestion protein combines with increasing quantities of the 

 hydrochloric acid. The pathologic condition in which there is most 

 definitely a diminution of the hydrochloric acid is cancer, either of the 

 stomach itself or occasionally of some other part of the body. An in- 

 crease is particularly marked in ulcer of the stomach. A considerable 

 variation in hydrochloric acid may however be the result merely of func- 

 tional (neurotic) conditions. 



THE SOURCE OF THE ACID 



A question that has puzzled physiologists for many years concerns the 

 mechanism by which hydrochloric acid is secreted. The percentage of 

 hydrochloric acid in the gastric juice is considerably above that at which 

 any animal cells can live, and yet this acid is secreted by the lining 

 membrane of the stomach, its source being, of course, the ' sodium 

 chloride of the blood plasma. How then do the cells of the gastric 

 glands bring about the separation of this powerful acid from the per- 

 fectly neutral blood plasma ? In the first place, it is significant that the 

 mucous membrane of the stomach contains a higher percentage of 

 chlorine than the average of other organs and tissues, indicating that it 

 has the power of abstracting chlorine from the blood. The excess of 

 chlorine in the mucosa must, moreover, be but a very small fraction of 

 that actually secreted into the the gastric juice. The chlorine content 

 of the mucosa of the cardiac end is considerably greater than that of the 

 pyloric. These facts indicate that chlorine is attracted by the gastric 

 cells, but they throw no light on the question as to where the hydro- 

 chloric acid is really formed. Is it in the cells, or only in the lumen of 

 the gland tubes? That is to say, is it formed before or after the gastric 



