FOOD AND DIGESTION. 361 



(NaH 2 P0 4 + KaCl = Na 2 HP0 4 + HC1) ; the disodic orthophosphate is 

 then reconverted by the action of carbonic acid and water (Na a HPO -f- 

 CO, + H 2 = NaH 2 P0 4 -f NaHCOJ : all these salts are found in the blood. 



The ferment Pepsin can be procured by digesting portions of the mucous mem- 

 brane of the stomach in cold water, after they have been macerated for some time 

 in water at a temperature 27 37.8 C. (80 100 F.). The warm water dissolves 

 various substances as well as some of the pepsin, but the cold water takes up little 

 else than pepsin, which is contained in a grayish-brown viscid fluid, on evaporating 

 the cold solution. The addition of alcohol throws down the pepsin in grayish-white 

 flocculi. Glycerine also has the property of dissolving out the ferment; and if the 

 mucous membrane be finely minced, and dehydrated by absolute alcohol, a power- 

 ful extract may be obtained by macerating it in glycerine. 



Functions. The chief function of gastric juice is such alteration of 

 proteid food-stuffs as will lead to their ready absorption and such modifi- 

 cation as will favor their further digestion (as far as necessary) in the 

 intestines ; gastric digestion is thus both a complete and a preliminary 

 process. Less important functions are the antiseptic action, coagulation 

 of milk, and inversion of disaccharides into monosaccharides. The chief 

 digestive power of the gastric juice depends on the pepsin and acid con- 

 tained in it, both of which are, under ordinary circumstances, necessary 

 for the process. 



The general effect of digestion in the stomach is the conversion of the 

 food into chyme, a substance of varying composition according to the 

 nature of the food, yet always presenting a characteristic thick, pulta- 

 ceous, grumous consistence, with the undigested portions of the food 

 mixed in a more fluid substance, and a strong, disagreeable acid odor and 

 taste. 



This action on proteids may be shown by adding a little gastric juice 

 (natural or artificial) to some diluted egg-albumin, and keeping the mix- 

 ture at a temperature of about 37.8 C. (100 F.); it is soon found that 

 the albumin cannot be precipitated on boiling, but that if the solution be 

 neutralized with an alkali, a precipitate of acid-albumin is thrown down. 

 After a while the acid-albumin disappears, so that no precipitate results 

 on neutralization, and proper analysis will show that all the albumin has 

 been converted into other proteid substances, viz., proteases and peptones. 

 The process, as is the case in salivary digestion, is never complete and 

 the final result is always a mixture of peptones with proteoses which can- 

 not be further peptonized : the relative proportions, of course, depend on 

 the duration of the process. A side product is found (as an insoluble 

 residue) in artificial gastric digestion which gives practically all the pro- 

 teid reactions and is soluble in dilute alkali, though insoluble in water, 

 sodium chloride, or dilute acid. This is known as anti-allumid and may 



