74 PRINCIPLES OF PHYSIOLOGY 



another fork, also Ut 4 , even if the two forks 

 are at a considerable distance from each 

 other As we know nothing of the chemical 

 constitution of enzymes, except that they 

 are proteins, this can only be a conjecture, 

 but it is to be borne in mind that proteins are 

 very unstable. Enzymes may act alone or 

 they may apparently be stimulated by other 

 enzymes or by the presence of other sub- 

 stances. In chemical reactions we have to 

 consider the element of time, or what we may 

 term the velocity of the reaction. It would 

 appear that all so-called catalytic actions, 

 including the action of enzymes, have the 

 effect of quickening the velocity of the chemi- 

 cal changes involved. 



33. As already pointed out, enzymes are 

 formed in cells. Cells may be frozen and then 

 pounded into a paste. Enzymes are thus set 

 free, and at the proper temperature they will 

 manifest their usual activities. This shows 

 that they do not depend on the life of the 

 cells. There can be no doubt that as almost 

 all cells contain enzymes, they take part in 

 nutritional processes, by exciting changes in 



