768 PHYSIOLOGY 



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 saponi- 

 fication 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 powers of 

 the gastric ferment is in any way affected by the diminution of its 

 acidity causad by the presence of peptone. The coloured indicators 

 mentioned above, however, serve as trustworthy indications of the 

 amount of free acid present, considered with regard to its digestive 

 functions. 



In order to determine quantitatively the amount of free HC1, the follow- 

 ing procedure is employed (Morner and Sjoqvist): Ten cubic centimetres of 

 the gastric juice are neutralised with barium carbonate (litmus being employed 

 as an indicator). The mixture is dried in a platinum dish, incinerated, and 

 the ash extracted with warm water and filtered. In this process the organic 

 acids are destroyed and converted into barium carbonate. The solution there- 

 fore contains merely the barium which was taken up to combine with the free HC1. 

 Estimation of the barium in the filtrate gives the amount of BaCl 2 present, and 

 therefore the amount of hydrochloric acid in the gastric juice. The barium is 

 determined by titrating in presence of sodium acetate and acetic acid with 

 potassium bichromate solution, ' tetra paper ' being used as an indicator. This 

 turns deep blue in the presence of free bichromate in solution. 



THE ACTIONS OF GASTRIC JUICE ON FOOD-STUFFS 

 By the action of the hydrochloric acid certain changes are induced 

 in the food-stuffs. Cane sugar is inverted to glucose and fructose ; 

 some proteins, such as blood fibrin, are swollen up to form a jelly-like 

 mass. The caseinogen of milk is precipitated, the collagen of the 

 connective tissues is swollen up. It is possible that a certain small 

 amount of hydrolysis also takes place in the dextrins and maltose 

 produced by the action of ptyalin on starch. 



The chief digestive function of the gastric juice is dependent 

 on the action of the ferment pepsin. This substance, which is 

 inactive in neutral medium, needs the co-operation of an acid, hydro- 

 chloric acid being the most effective, though its place may be taken 

 by phosphoric, sulphuric, or lactic acid. Its main effect is on the 

 proteins of the food. The stages in its action may be best studied 

 by taking as an example its action on blood fibrin. If fibrin be 

 immersed in 0-4 per cent, hydrochloric acid, it swells up to a gelatinous 

 mass. On then stirring in an extract of gastric mucous membrane, 



