DIGESTION 303 



while those of fibrin are called fibrinoses ; of globulin, globuloses ; of 

 casein, caseoses ; and so on. Probably the peptones produced from 

 different proteids are also not absolutely identical. 



Beyond peptone gastric digestion does not go. Indeed, 

 in no case does the whole, and only under very favourable 

 conditions even the half, of the original proteid in an 

 artificial digestion ever reach the stage of peptone. The 

 pancreatic juice, as we shall see later on, not only effects a 

 more complete conversion into peptone, but can split up 

 peptone itself into substances which are no longer proteid. 

 Since the subject of proteid digestion must come up again, 

 it will be well to postpone any closer discussion of the 

 process till we can view it as a whole. In the meantime it 

 is only necessary to repeat that pepsin alone cannot digest 

 proteids at all. Its action requires the presence of an acid ; 

 in a neutral or alkaline medium peptic digestion stops. As 

 in the case of other ferments, there is a certain temperature 

 at which pepsin acts best, an 'optimum ' temperature (35 to 

 40 C., or about that of the body). At o C. it is inactive, 

 except in cold-blooded animals (frog). Boiling destroys it. 



Dilute acid alone does not dissolve coagulated proteids 

 like boiled fibrin, or does so only with extreme slowness. 

 Uncoagulated proteids, however, are readily changed by it 

 into acid-albumin ; and by the prolonged action of acids, 

 especially at a high temperature, further changes may. be 

 caused in all proteids, apparently of much the same nature 

 as those produced in peptic digestion. But under the 

 ordinary conditions of natural or artificial gastric digestion, 

 it may be said that the acid alone does little until it is aided 

 by the ferment, just as the ferment alone does nothing 

 without the aid of the acid. One striking difference, how- 

 ever, there is : the acid is used up during the process ; the 

 ferment is little, if at all, affected. Although hydrochloric 

 acid acts most powerfully, other acids, such as lactic, phos- 

 phoric, or sulphuric, can replace it. 



The milk-curdling ferment, rennin, is contained in large 

 amount in an extract of the fourth stomach of the calf, 

 which has long been used in the manufacture of cheese. 

 It exists in the healthy gastric juice of man, but disappears 

 in cancer of the stomach and in chronic gastric catarrh. 



