CH. VIII.] PEPSIN. 2OI 



added so that the test is just negative. With a little experience 

 one can judge how near one is to the end point, from the rapidity 

 with which the red colour develops and from the intensity obtained. 



Calculation, x cc. of soda, together with y cc. of HC1, just fail 

 to give a positive reaction. 



(x y) cc. o-i N. soda are required to neutralise the free HC1 

 in 10 cc. of the fluid. 



Free HC1 = (x y) x 0-0365 gram, per cent. 



D, Pepsin, 



Pepsin is the proteolytic enzyme secreted by the chief 

 or peptic cells of the gastric glands. These cells elaborate 

 the zymogen, pepsinogen, which is converted to pepsin by 

 hydrochloric acid. 



Pepsin is remarkable in only acting in a decidedly 

 acid medium, the optimum P H being about i -4. Not only 

 is it inactive in neutral or alkaline solutions, but the latter 

 rapidly destroy it. Pepsinogen is much more stable to 

 alkaline solutions, and can thus be distinguished from 

 pepsin. Acids other than hydrochloric can be employed 

 in artificial digestion experiments, but not so successfully. 

 With such weak acids as acetic and butyric the digestive 

 action, even in 4 per cent, of the acid, is very feeble, since 

 the requisite hydrogen-ion concentration cannot be ob- 

 tained. Neutral salts inhibit the action of pepsin, this 

 being in marked contrast to the influence of sodium chloride 

 on ptyalin. 



The chemical nature of pepsin has not yet been estab- 

 lished, it being almost impossible to separate it from 

 substances on to which it is adsorbed. It may eventually 

 transpire that the fact that all preparations contain iron 

 and chlorine may not be without significance. 



Commercial pepsin is prepared by extracting the 

 mucous membrane of hogs' stomachs with dilute hydro- 

 chloric acid, filtering, and concentrating under reduced 

 pressure at low temperatures, a large surface for evapora- 

 tion being provided. A more active product can be 



