350 PHYSIOLOGY OF THE DOMESTIC ANIMALS. 



In addition to tbese tests, a number of reagents, such as tropeolin, 

 methyl-violet, Congo-paper, etc., have been proposed to distinguish 

 mineral from organic acids. The best of the more recent tests is phloro- 

 gl ucin-vanillin , described by TViesner and Gunzburg. Two grammes of 

 phloroglncin and one gramme of vanillin form a reddish-yellow solution 

 with thirty grammes of absolute alcohol. A drop of this solution in the 

 presence of a trace of a free mineral acid forms a brilliant red color, at 

 the same time depositing bright-red cr3"stals. On the other hand, organic 

 acids, such as lactic or acetic acids, or even chlorides mixed with these 

 acids, produce no change in coloration. 



There is no doubt but that lactic, butyric, and other organic acids 

 may be present in gastric juice, but their origin is to be explained by 

 their respective fermentations, from articles of food, or from decompo- 

 sition of their salts. The amount of free acid found in gastric juice in a 

 state of health may vary from 0.02 per cent., as stated by Bidder and 

 Schmidt, which is probably a low estimate, to 0.5 per cent., as estimated 

 by Heidenhain, in the gastric juice of the dog. 



The degree of acidity of the gastric juice differs indifferent animals 

 and under different conditions. In the dog, Bidder and Schmidt found 

 that one hundred parts of filtered gastric juice required 0.390 grammes 

 potassium hydrate for neutralization ; in the sheep, one hundred parts of 

 gastric juice required only 0.2(54 grammes potassium hydrate, indicating 

 in a general way that the degree of acidit}' of this secretion is higher in 

 the carnivora than in the herbivora. 



The degree of acidity varies also with the stage of gastric digestion. 

 Rothschild found, after the administration of fifty grammes of rare 

 meat and three hundred and twenty-eight grammes of water through the 

 oesophageal sound in healthy individuals, that hydrochloric acid was the 

 only acid present. The degree of acidity was determined by removing 

 the contents of the stomach by the stomach-pump. The following table 

 shows the results of his investigations : — 



After J hour, 0.74 per mille, H CI. 



" 1 " 0.84 



" 14 hours, 0.99 



" 2 . 1.40 



" 24. " . 2.4(5 



" 3 " . stomach empty. 



There seems to be certain reasons for supposing that hydrochloric 

 acid is not entirely free, but in a state of partial combination with some 

 substance which does not entirely destroy its free acidity. Thus, gastric 

 juice has been found to dialvse differently from a solution of hydrochloric 

 acid of the same percentage. 



Numerous theories have been proposed to explain the formation of 

 the free acid of gastric juice. No one of these views has reached the 



