THE ALIMENTARY CANAL 187 



The importance of this standardization can be made 

 clear by an illustration. Take cane sugar as an example. 

 Here is a food which contains no waste matter and needs 

 no further refining. It is soluble and diffusible. Yet 

 it is not fit to be introduced into the circulation and if 

 the experiment is tried it will be excreted through the 

 kidneys, in other words, treated as useless material. 

 The trouble is that it is not a native compound. A 

 single change quickly accomplished in the intestine 

 transforms it into two other kinds of sugar which the 

 body can utilize. 



Digestive Secretions. The process of digestion is 

 carried on under the influence of the several digestive 

 juices. When one of these is found to have power to 

 advance the digestion of a certain kind of food we 

 naturally assume that an agent exists to bring about 

 the observed effect and we call the supposed agent an 

 enzyme. Enzymes are not known in an isolated or 

 pure condition; their existence is inferred from the be- 

 havior of mixtures of a very heterogeneous sort. We 

 say that saliva contains an enzyme capable of digesting 

 starch and we generally call the enzyme ptyalin but we 

 are speaking of something which is known to us only by 

 its action and not by its appearance. Since the water, 

 salts, and mucus of the saliva do not digest starch we are 

 warranted in saying that something else is there which 

 does have this property. 



Although we do not know what enzymes are in a strict 

 chemical sense we do know many of their qualities. 

 They are destroyed by heating their solutions to tem- 

 peratures short of boiling. They are restrained from 

 acting by cold but in this case they are not, as a rule, 

 prevented from resuming their action when warmed. 

 They are said to be specific, the idea being that one 

 enzyme has but one action. If a digestive juice affects 

 two distinct types of food it is considered to contain two 

 enzymes. We classify enzymes according to the com- 

 pounds on which they act: protein-splitting enzymes 



