684 PHYSIOLOGY 



cent, total solids ; it contains no peptone, but traces of protein. The 



following Table represents its average composition : 



Hydrochloric acid . . . 046 to 0-58 per cent. 



Chlorine 0-49 to 0-62 



Total solids .... 0-43 to 0-60 



Ash . . . . 0-09 to 0-16 



If the juice be allowed to stand in the ice chest for a day it becomes cloudy 

 and deposits a fine granular precipitate, which apparently represents the 

 active agent of the juice, and may perhaps be regarded as pepsin in a pure 

 form. 



The actions of gastric juice are due partly to the acid, partly to the 

 combined action of the acid and the ferments. The acid of the gastric juice, 

 when obtained free from admixture, is entirely hydrochloric acid. Dog's 

 juice contains on the average about 0-6 per cent. HC1 ; human gastric juice 

 probably contains less, about 0-2 per cent. When, however, we examine the 

 gastric contents, composed of a mixture of gastric juice and semi-digested 

 food, we always find, besides the hydrochloric acid, other acids present, 

 among which the most prominent is lactic acid. So constantly is this latter 

 acid present that it was formerly thought by some physiologists to be the 

 chief acid of the gastric juice. It is produced by processes of fermentation 

 occurring in the food. Whenever we take carbohydrates we swallow at the 

 same time micro-organisms, and these in the warm moist mass quickly 

 attack the carbohydrates, converting them into sugar and then into lactic 

 acid. As the gastric juice gradually soaks into the food and renders it acid, 

 it stops this lactic acid fermentation, so that whereas in the early stages of 

 gastric digestion both acids are present in considerable quantity, towards the 

 end of gastric digestion lactic acid is almost entirely absent. 



In some pathological conditions free hydrochloric acid may be entirely wanting from 

 the gastric juice, and the detection of this acid in gastric juice becomes therefore a 

 matter of considerable clinical importance. For this purpose we can employ various 

 indicators, which change colour in the presence of a free strong acid such as HC1, but 

 are unaffected by weak acids such as lactic acids, or the fatty acids. Chief among 

 these indicators are Congo red, which turns blue with mineral acids, and a slaty colour 

 with lactic acid ; and tropaolin 00, which turns a brilliant red in the presence of a free 

 mineral acid, but is unaltered by lactic acid. The reagent which is most employed is 

 Gunzberg's reagent. This is a mixture of phloroglucin and vanillin dissolved in absolute 

 alcohol. A drop of this is evaporated to dryness in a porcelain capsule. A drop of 

 the fluid suspected to contain free acid is then added, and also evaporated to dryness. 

 If free HC1 be present, the residue on drying becomes a brilliant red colour, an effect 

 which is not produced by the presence of free lactic or free fatty acids. 



Great stress has been laid on the determination of the actual amount of 

 free H ions present, and for this purpose the acidity of gastric juice or of 

 digestive mixtures has been tested by determining its inverting power on 

 cane sugar, or its power to hasten the saponification of ethyl acetate. The 

 acidity estimated in this way is diminished considerably by the presence of 

 albumens, and still more by the presence of albumoses or peptones. But 

 it does not seem that the adjuvant action of the acid on the proteolytic 



