CHEMISTRY OF DIGESTION AND NUTRITION. 289 



The Acid of Gastric Juice. — The nature of the free acid in gastric juice 

 was formerly the subject of dispute, some claiming that the acidity is due to 

 HC1, since this acid can he distilled off from the gastric juice, others contend- 

 ing that an organic acid, lactic acid, is present in the secretion. All recent 

 experiments tend to prove that the acidity is due to HC1. This fact was first 

 demonstrated satisfactorily by the analyses of Schmidt, who showed that if, 

 in a given specimen of gastric juice, the chlorides were all precipitated by 

 silver nitrate and the total amount of chlorine was determined, more was 

 found than could be held in combination by the bases present in the secretion. 

 Evidently, some of the chlorine must have been present in combination with 

 hydrogen as hydrochloric acid. Confirmatory evidence of one kind or another 

 has since been obtained. Thus it has been shown that a number of color 

 tests for free mineral acids react with the gastric juice : methyl-violet solutions 

 are turned blue, congo-red solutions and test-paper are changed from red to 

 blue, 00 tropseolin from a yellowish to a pink-red, and so on. A number of 

 additional tests of the same general character will be found described in the 

 laboratory handbooks of physiology. 1 It must be added, however, that lactic acid 

 undoubtedly occurs, or may occur, in the stomach during digestion. Its pres- 

 ence is usually explained as being due to the fermentation of the carbohydrates, 

 and it is therefore more constantly present in the stomach of the herbivora. 

 The amount of free acid varies according to the duration of digestion ; that is, 

 the secretion does not possess its full acidity in the beginning, owing probably 

 to the fact (Heidenhain) that in the first periods of digestion, while the secre- 

 tion is still scanty in amount, a portion of its acid is neutralized by the 

 swallowed saliva and the alkaline secretion of the pyloric end of the stomach 

 (see the section on Secretion). Estimates of the maximum acidity in the 

 human stomach are usually given as between 0.2 and <).."> per cent. The 

 acidity of the dog's gastric juice is greater — 0.46 to <>.."}(; per cent. ( Pawlow). 



Origin of the HC1. — The gastric juice is the only secretion of the body con- 

 taining a free acid. The fact that the acid is a mineral acid makes this circum- 

 stance more remarkable, although other instances of a similar kind arc known; 

 for example, Dolium galea, a mollusc, secretes a salivary juice containing free 

 H 2 S0 4 and free HC1. When and how the IIC1 is formed in the stomach is 

 still asubject of investigation. Histologically, attempts have been made to show 

 that it is produced in the border cells of the peptic glands in the fundic end 

 of the stomach (see Secretion). It cannot be said, however, that the evidence 

 for this theory is at all convincing; it can be accepted only provisionally. 

 Ingenious efforts have been made to determine the place of production of the 

 acid by micro-chemical methods. Substance thai give color reactions with 

 acids have been injected into the blood, and sections of the mucous membrane 

 of the stomach have then been made to determine microscopically the part of 

 the gastric glands in which the acid is produced ; but beyond proving that the 

 acid is formed in the mucous membrane these experiments have given negative 

 results, the color reaction for acid occurring throughout the thickness of the 



1 Stirling : Outlines of Practiced Physiology. 

 Vol. I.— 19 



