DIGESTION IN THE STOMACH 707 



collected in vessels connected with the fistulous opening. If taken 

 from a fasting animal, such a juice is perfectly free from admixture, 

 and can be regarded as pure gastric juice. It is quite clear, strongly 

 acid, without smell. It contains about 0-3 to 0-6 per 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 - 049 to 0-62 



Total solids - 043 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 suspect, 



