32 NUTRITIONAL PHYSIOLOGY 



for the performance of each line of work. We do not know 

 the precise chemical nature of the active bodies, " diges- 

 tive principles " as they were formerly called, but we know 

 a great deal about the conditions of their working. 



When a digestive secretion has a single well-marked 

 effect upon one sort of food material we say that it contains 

 an enzyrrie.. We are thus naming a body which we know 

 chiefly by its power to promote a certain chemical reaction. 

 It is not many years since an able writer protested against 

 this confident reference to a substance where it is a property 

 rather than a compound which we are observing. It will 

 be admitted that it is doubtful whether anyone has ever 

 seen that which is an enzyme and nothing else. What 

 we see and handle are solutions possessing characteristic 

 powers or dry preparations capable of furnishing such 

 solutions. But it has seemed altogether reasonable to 

 connect the property with a substance and we shall con- 

 tinue to do so. Acting on this basis, we say of a_ juice 

 which hydrolyzes starch that it contains a diastatic enzyme, 

 and of one that acts in a parallel fashion on proteins that 

 it contains a proteolytic enzyme.. When the action is 

 upon fats we call the enzyme lipolytic. or, using the sub- 

 stantive, we call it a liyase. It is unfortunate that there 

 is much confusion at present in the use of such terms; 

 there are a great many more in use than there need be. 

 The simplest plan is, perhaps, to fall back upon our Saxon 

 and speak of enzymes as starch-splitting, protein-splitting, 

 sugar-splitting, and fat-splitting. We shall take pains, 

 however, in our detailed discussion to introduce various 

 equivalent terms. We shall find especially that enzymes 

 are often named with reference to their sources as well as 

 to their powers. 



Enzymes jare similar in many respects to the catalyzers 

 of inorganic chemistry. Their presence accelerates reac- 

 V^ tions which in their absence might not be appreciable. 

 We do not think of them as contributing either material 

 or energy to the process. They suggest the oil in a machine 

 which lessens the resistance of its parts to the driving force. 



