686 PHYSIOLOGY OF DIGESTION AND SECRETION. 



are formed from living matter, and the solutions as usually prepared 

 give protein reactions. Increased study, however, has made this be- 

 lief uncertain. The enzymes cling to the proteins when precipitated, 

 and it seems possible that the protein reactions of their solutions may 

 be due, therefore, to an incomplete purification. In fact, it is stated 

 that solutions of some of the enzymes may be prepared (pepsin, 

 invertase, thrombin) which show ferment activity, but give no 

 protein reactions. Much of the older work upon the composition of 

 supposedly purified preparations of enzymes is not accepted to-day, 

 on the ground that the evidence for the purity of the preparations is 

 insufficient. In spite, however, of the very great amount of atten- 

 tion that has been paid to these substances in recent years, there is at 

 present no agreement as to their chemical structure. Some au- 

 thorities (Arthus) have gone so far as to suggest that the enzymes 

 or, more properly, enzyme actions, are not due to definite material 

 substances, but are to be classified as forms of energy like heat, 

 electricity, etc. The suggestion is not very helpful, but it indicates 

 forcibly the present uncertainty regarding the real nature of these 

 bodies. 



