THE CHEMISTRY OF THE DIGESTIVE JUICES 353 



only effects a more complete conversion into peptones, but can split 

 up the whole or a very large proportion of the peptones themselves 

 into amino-acids and the other ' building-stones ' of the original 

 protein. Since the subject of protein 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 proteins at all. Its action 

 requires the presence of an acid; in a neutral or alkaline medium 

 peptic digestion stops. The precise mode of action of the acid is by 

 no means clear. 



Dilute acid alone does not dissolve coagulated proteins like boiled 

 fibrin, or does so only with extreme slowness. But it causes them to 

 swell up by imbibition of water, and probably in this way facilitates 

 the entrance of the ferment. Uncoagulated proteins are readily 

 changed by acid 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 proteins, such as 

 peptones, combining with a greater proportion of acid than such 

 proteins as fibrin or albumin. These compounds 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 some 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 the actual catalytic agent in peptic digestion. 

 Although hydrochloric 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 Action of Gastric Juice. 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 disappears in cancer of the stomach and in 

 chronic gastric catarrh. It has been stated by a number of observers 

 that the properties of rennin are never 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 or precipitation of 

 caseinogen is merely an associated a.ction of the pepsin. Ferments 

 of the most varied origin will curdle milk. Pawlow has maintained 



