490 DIGESTION. 



that digestion consists in the combination of this acid with 

 albuminous bodies, and explains the fact that digestion can 

 be renewed by the addition of hydrochloric acid after it has 

 ceased, by supposing that the pepto-hydrochloric acid, thus 

 liberated, is enabled to begin to digest anew. 



The combination of pepsin and hydrochloric acid to form a 

 new arid is supported by several facts, and is very generally 

 believed, but Schmidt's hypothesis regarding its mode of 

 net ion is open to the objection that it is not merely a com- 

 pound of albumin with acid which is formed during digestion, 

 but peptones. It therefore seems more probable that the pep- 

 sin acts as a ferment only in acid solutions, causing the albu- 

 minous bodies to take up water and split up. 1 



That pepsin and hydrochloric acid mutually combine when 

 mixed, as in digestive liquids, is rendered probable, not only 

 by the fact already shown that they produce effects together 

 which neither is capable of producing separately, but that in 

 such mixtmv> the characters of both are modified. 



This is seen by comparing the action of dilute hydrochloric 

 acid alone with that of hydrochloric acid pepsin. The former 

 extracts all the salts and leaves a gelatinous substance, while 

 the latter extracts this substance and leaves a brittle mass 

 containing a large proportion of inorganic salts. As regards 

 pepsin, a modification of property is shown in Yon Wittich's 

 observation, that, although pepsin alone does not diffuse 

 through vegetable parchment, pepsin with hydrochloric acid 

 does so readily. Thai the decomposition of albuminous sub- 

 stances is essentially connected with their taking up water, is 

 rendered probable by the fact that digestion does not take* 

 place in its absence, and that products similar to those of 

 digestion can he obtained by boiling albuminous bodies with 

 water for a very long time, or for a shorter time with dilute 

 acid. 



The former of these facts can be easily demonstrated by 

 treating fibrin which has been soaked in glycerin and not 

 washed at 40 C. with a glycerin solution of pepsin undiluted 

 with water, acidulated to the proper degree by the addition of 

 a few drops of strong acid ; under these circumstances the 

 fibrin is not digested. The latter ma}* be shown by boiling 

 fibrin with dilute sulphuric acid for an hour or two, and then 

 neutralizing the liquid, filtering and testing the filtrate for 

 peptones. 



* 127. Secretion of Gastric Juice. Pepsin is contained 

 in all parts of the peptic glands, but free acid is only formed 

 near their orifices. To show this, kill a pigeon, open it imrne- 



1 For a clear account of the probable mode of action of ferments, see 

 " Betrechtnngen niter die Wlrkongsweiae der ungeformten Fermente," 

 by Dr. (i. lliifner; Uarth, Leipzig, 1872. 



