Hydrochloric acid exists in the human gastric juice in the average proportion of 0.2 per cent. Experimentally it is found that the best proteolytic digestion results with 0.1 to 0.2 per cent of hydrochloric acid. The acidity is reduced upon a vegetable diet.

The acid is secreted during active digestion but not normally at other times. It is often diminished in quantity, and less often it may be abnormally increased. Its action upon proteid foods like egg albumin, blood fibrin, and meat, is to cause them to swell and become somewhat translucent. It contributes the necessary acid reaction in which the ferment pepsin causes solution of these and other proteids and converts them into albumoses.

The quantity of free acid obtainable from the stomach is not great when much proteid food has been eaten, although its secretion may have been considerable. The secretion of the acid begins shortly after the ingestion of food, and reaches a maximum within about an hour. After an hour or two longer, according to the nature and quantity of the food present, it lessens and finally ceases.

According to the views of some writers, the first acid formed in the stomach is lactic acid, which joins the alkali of chlorides in the blood, and liberates chlorine to form hydrochloric acid. Maly has advocated the theory that the hydrochloric acid was formed from the action of phosphates in the blood serum, splitting up the chlorides so that the highly diffusible hydrochloric acid passes readily out upon the free surface.

Roberts furnishes the following table illustrating the effect of varying quantities of hydrochloric acid on the speed of peptic digestion:

2 grammes beef-fibre; i c. c. glycerin extract of pepsin; varying proportions of hydrochloric acid; water to 100 c. c

Proportion of dry HC1 in the digesting mixture.

Time in which digestion was completed.

0.05 per cent

... 500 minutes, almost no digestion.

0.08 " "

... 200 "

0.1 " "

... 130 "

0.15 " "

... 115 "

0.2 " "

... 100 "

0.3

... 115 "

0.4 " "

..160 "

0.6 " "

... 350 minutes, embarrassed.

The acid has an antiseptic influence, preventing the decomposition of food, and even checking it if already begun. It is also believed to possess an influence over the formation of pepsin. Briicke claims, contrary to many observers, that the acid of the stomach makes starch more soluble, and that much of it is also converted by lactic-acid fermentation in the stomach into erythrodextrin.