130 HUMAN PHYSIOLOGY 



The individual proteids, including those of vegetable 

 origin, yield proteo-ses differing but slightly from each other. 



The combined proteids are first split up by gastric juice 

 into their components; after this, the proteids thus split off 

 are digested like simple proteids. Nudeins are not dissolved 

 by gastric juice; hence, in the digestion of caseinogen, the 

 insoluble paranuclein remains. 



Of the albuminoids, only collagen is digested by pepsin. 

 It is first changed into its hydrate, gelatin, and from this, by 

 hydrolytic splitting up, gelatoses, corresponding to proteoses, 

 are formed. 



The process of proteid digestion by gastric juice depends 

 upon: 



1. The amount of free hydrochloric acid, the amount 

 most favorable for digestion being 0.2-0.4^. 



The acidity of the stomach does not indicate the amount of free 

 hydrochloric acid present because, first, the gastric juice may 

 contain free organic acids, especially lactic acid, and, secondly, 

 the proteoses-chlor-hydrate has also an acid reaction (see page 129). 

 Free hydrochloric acid is present in the gastric juice only when it 

 gives the Giinzburg reaction (see page 95). 



As artificial experiments in digestion show, the hydrochloric 

 acid can be replaced by other acids, but they are all inferior to it. 

 The lactic acid frequently formed in the stomach by the fermenta- 

 tion of carbohydrates also has digestive action; it is, however, 

 not a normal constituent of gastric juice and therefore its part in 

 digestion is only incidental. 



2. Upon the amount of pepsin. The intensity of the 

 digestion increases with the amount of pepsin present till it 

 reaches a certain limit. The increase is proportional to the 

 square root of the concentration of the pepsin. 



3. Upon the kind of proteid present and its power of in- 

 bibition. Fibrin, which is strongly swollen, is sooner 

 digested than coagulated white of egg, which swells but little. 

 Native proteids are more easily digested than the coagulated, 

 and animal proteid more easily than plant proteid. 



4. Upon the temperature. The gastric juice acts best at 

 37-40 C. At o digestion stops, and at 80 the pepsin is 

 destroyed. 



