DIGESTION IN THE STOMACH 685 



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

 acidity caused by the presence of peptone. The coloured indicators men- 

 tioned 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 following pro- 

 cedure 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 therefore 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 Bad 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, hydrochloric 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 O4 per cent, hydrochloric acid, it swells up to a gela- 

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

 or any preparation of pepsin, the gelatinous mass rapidly undergoes solution. 

 If the mixture be boiled and neutralised, immediately after solution has 

 occurred, nearly the whole of the protein is thrown down in a coagulated 

 form. The first effect therefore is the production of coagulable soluble 

 proteins from the insoluble fibrin. If the action be allowed to proceed for 

 some hours, a whole series of products of hydrolysis are found in the mixture. 

 On neutralising the fluid, a precipitate may be thrown down consisting chiefly 

 of acid albumen. The greater proportion of the protein remains in solution. 

 This remainder may be purified from any unaltered coagulable protein by 

 boiling in slightly acid solution and filtering. The filtrate contains a 

 mixture of bodies belonging to the class of hydrated proteins, viz. proteoses 

 and peptones. 



By means of fractional precipitation with ammonium sulphate or zinc 

 sulphate, these mixtures can be subdivided into various substances, although 

 in no case can we be certain that we are dealing with chemical individuals. 

 The Table on p. 686 represents the chief bodies obtained by Pick by this 



