DIGESTION 139 



addition to the changes that the food principles undergo in the alimentary 

 canal, the corresponding principles as well as many other compounds undergo 

 similar changes in the body tissues as the result of the action of ferments, 

 changes that underlie and condition many if not all the phenomena of the 

 nutritive process. In none of these instances, however, has the ferment been 

 satisfactorily isolated or its chemic or physical features determined. For 

 this reason these ferments have been termed unorganized ferments. 

 Investigations have demonstrated, however, that they are products of the 

 metabolism of the cells of plant and animal tissues. 



In recent years the distinction between organized and unorganized 

 ferments has become untenable owing to the fact that chemists have succeeded 

 in extracting from yeast cells as well as from bacterial cells, enzymes or 

 ferments that produce in sugar and protein the same reduction effects under 

 the same conditions as in the case of yeast cells and bacteria themselves. 

 It is therefore probable that these organized cells act not directly by virtue of 

 their own activities, but indirectly, by virtue of an unorganized ferment 

 which they secrete and discharge into the surrounding medium. All enzymes 

 that produce their effects after being discharged from cells are termed 

 extra-cellular enzymes, while those that produce their effects in the interior 

 of cells are termed intra-cellular enzymes. 



The Nature of Enzymes. An enzyme is in all probability organic in 

 character, though neither its chemic nature nor composition has been de- 

 termined. Some of them exhibit protein, others carbohydrate reactions, but 

 by reason of the difficulty in isolating enzymes and of freeing them absolutely 

 from all traces of protein and carbohydrates it is not possible to state 

 positively whether the reactions observed are due to the enzyme or its 

 associated organic matter. The purer the preparation, however, the less of 

 any chemic reaction is exhibited. 



From what is known of their action, of the effects produced and of the condi- 

 tions under which they act, ferments have a resemblance to various inorganic sub- 

 stances or agents that produce changes of composition and decomposition 

 apparently by their presence alone, for, as far as the evidence goes, they neither 

 enter into the end-products of the reaction nor are they destroyed. A chemic 

 change thus produced is termed catalysis and the agent causing it is termed a 

 catalyzer or catalyst. The substance on which the catalyst acts is termed the 

 substrate. In most, if not in all instances a catalyst acts not as an initiator, but 

 as an accelerator of a change that would spontaneously take place with extreme 

 slowness and in some instances with results so slight as to be inappreciable. 

 Oxygen and hydrogen, for example, spontaneously combine, there are reasons for 

 believing, at room temperatures though at such a slow rate that the formation of 

 water cannot be detected, but if a small quantity of finely divided platinum be 

 added the combination takes place almost immediately; carburetted hydrogen and 

 oxygen combine when they pass over platinum with the formation of carbon 

 dioxid and water; saccharose and water in the presence of hydrochloric acid will 

 combine and be reduced to equal quantities of levulose and dextrose; dilute 

 peroxid of hydrogen will slowly decompose spontaneously and yield up oxygen, 

 but if finely divided platinum or silver be added the decomposition is greatly ac- 

 celerated. In all these instances, to which many more might be added, the 

 catalyst, simply by its presence accelerates a change spontaneously taking place 

 without itself appearing in the end-products of the reaction. 



It has been experimentally demonstrated that the finer the catalyst is divided 



