CHEMICAL BASIS OF THE ANIMAL BODY. 1191 



The secretion and extracts of the small intestine possess to 

 a slight extent the power of slowly hydrolyzing starch into 

 maltose, the conversion being more rapid if portions of the 

 mucous membrane of the intestine be finely divided and 

 immersed in the starch solution. The tissue and its extracts 

 on the other hand possess to a very marked extent the power of 

 rapidly effecting a conversion of maltose into dextrose; this is 

 of great physiological significance inasmuch as it points to 

 the probability that the carbohydrates are absorbed from the 

 intestine as dextrose and not as maltose, a view which is sup- 

 ported by the fact that maltose does not appear to be capable 

 of direct assimilation, but is excreted largely unchanged if in- 

 jected into the blood. The intestinal mucous membrane also 

 yields an inverting enzyme which, similarly to the well-known 

 * invertin ' of yeast, decomposes cane sugar into a mixture of 

 dextrose and laevulose (see p. 1221). 



Pepsin. 



This is the characteristic proteolytic enzyme of gastric 

 juice. 



Preparation of peptic digestive fluids. If a few drops of a 

 glycerin extract of gastric mucous membrane be added to dilute 

 (2 p.c.) hydrochloric acid or if the tissue be simply extracted 

 for a short time with the dilute acid and the extract be filtered 

 a solution is obtained which suffices for demonstration and 

 ordinary purposes. When however a peptic extract is required 

 for research purposes it is essential to adopt some more elabo- 

 rate method which yields a^product as free as possible from ad- 

 mixed substances ; one of the best is as follows. The mucous 

 membrane is digested with phosphoric acid and the fluid pre- 

 cipitated with lime-water. The precipitate of calcium phos- 

 phate is then filtered off, washed, and dissolved in dilute 

 hydrochloric acid, and this solution is then dialyzed until it is 

 free from chlorine and phosphates, and on acidulating with 

 hydrochloric acid is read)' for use. 



Pepsin does not exist preformed in the cells of the gastric 

 glands but as a zymogen to which the name of pepsinogen is 

 given ; this is readily converted into pepsin by the action of 

 hydrochloric acid. 



The hydrolytic activity of pepsin is manifested only in 

 presence of an acid. The most efficient acid in this respect 

 for artificial digestions is hydrochloric of a strength of -2 p.c. 

 The average percentage of this acid may be stated as -2 p.c. 

 in normal gastric juice, but it varies slightly in the case of 

 different animals. Other acids may be substituted for the 

 hydrochloric, the optimal percentage varying for the several 

 acids. 



