32 PHYSIOLOGY CHAP. 



highly probable that both classes of substances act in virtue of the 

 same principle. 



To Ostwald is due the special distinction of having effectively 

 contributed to our knowledge of the mode of action of catalysers. 

 According to him, every catalytic process consists essentially of a 

 change of velocity in a chemical process, which occurs spontaneously. 

 " A catalyser is a body which, without appearing in the end product 

 of a chemical reaction, alters its velocity by accelerating or by 

 retarding it." 



This theory is especially applicable to the example cited of 

 hydrogen peroxide and platinum black : we know, in fact, that 

 the hydrogen peroxide slowly decomposes by itself into water and 

 oxygen, to such an extent that after a few days there is no longer 

 any trace of the hydrogen peroxide in an open vessel containing 

 it. The platinum black merely accelerates the spontaneous 

 process of scission. The same thing must occur in the case of 

 enzymes and the substances which they split up. 



This is not the place for discussion of the various theories 

 put forward to explain the action of catalysers and of fer- 

 ments : it need only be said that nowadays everything points to 

 the conclusion that this action is effected not directly, but by 

 the formation of intermediate products (which do not, however, 

 appear in the end products of cleavage), and that according to 

 Euler enzymes and catalysers act as collectors of ions. 



X. In the present state of physiology the only possible basis 

 for a classification of the different enzymes is the changes which 

 they effect. 



According to Hammarsten, the enzymes which have more 

 especially been made the subject of experimental research may be 

 subdivided into two great classes, i.e. hydrolytic and oxidative. 



The class of hydrolytic ferments, i.e. those which split up 

 complex chemical aggregates into simpler molecules by the 

 absorption of molecules of water, comprises all the several digestive 

 ferments, which, as we shall see, fulfil the office of disintegrating 

 complex proteins, polysaccharides and alimentary fats into simpler 

 compounds. The latter are better adapted for absorption by the 

 intestinal epithelium, where they are either finally split up, or 

 elaborated into new and more complex chemical compounds by the 

 metabolic activity of the tissues. They are : 



(a) Proteolytic or proteodastic enzymes, which split up 

 proteins, and of which we have already spoken. In the animal 

 body there are two (according to some authors, three) different 

 types of proteolytic ferments pepsin, trypsin, and to these, 

 according to some modern workers (0. Cohnheim), erepsin must.be 

 added. We shall deal fully with these enzymes in the chapter on 

 Digestion. Vegetable proteolytic ferments (e.g. papain) are also 

 known. 



