326 A MANUAL OF PHYSIOLOGY 



only necessary to repeat that pepsin alone cannot digest proteins 

 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 proteins like 

 boiled fibrin, or does so only with extreme slowness. Uncoagu- 

 lated proteins, however, are readily changed by it into acid- 

 albumin ; and by the prolonged action of acids, especially at a 

 high temperature, further changes of much the same nature 

 as those produced in peptic digestion may be caused in all 

 proteins. But under the ordinary conditions of natural 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. The acid enters into a temporary 

 combination with the protein, the more highly hydrolysed pro- 

 teins, such as peptone, combining with a greater proportion 

 of acid than such proteins as fibrin or albumin. These com- 

 pounds so easily undergo hydrolytic dissociation that, in spite 

 of its union with the proteins, the hydrochloric acid is able to 

 act along with the pepsin, so that peptic digestion goes on even 

 when enough protein is present to combine with all the acid. 

 There is evidence that in the gastric juice the pepsin exists in 

 the form of an unstable compound with hydrochloric acid, and 

 it is probably this pepsin-hydrochloric acid compound which is 

 actual catalytic agent in peptic digestion. Although hydro- 

 chloric acid acts most powerfully, other acids, such as nitric, 

 phosphoric, sulphuric, or lactic (arranged in the order of their 

 efficacy), can replace it. 



The milk-curdling ferment, rennin, or chymosin, is contained in 

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

 which, as rennet, has long been used in the manufacture of 

 cheese. It exists in the healthy gastric juice of man, but dis- 

 appears in cancer of the stomach and in chronic gastric catarrh. 

 It is doubtful whether the properties of rennin are ever found in 

 gastric juice or any preparation obtained from it or from the 

 gastric mucous membrane unless pepsin is present. This has 

 suggested that there is no separate milk-curdling ferment, but 

 that the clotting of caseinogen is merely an associated action of 

 the pepsin. Proteolytic ferments of the most varied origin will 

 curdle milk. Pawlow has recently shown that it is highly prob- 

 able that the milk-curdling property not only of the gastric 

 juice, but also of the pancreatic juice and of the secretion of 

 B runner's glands, is associated with the proteolytic ferment. 



