THE ALIMENTARY CANAL 185 



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 

 digest proteins, fat-splitting enzymes digest fats, starch- 

 splitting enzymes starches, etc. 



When we compare enzymes with other chemical agents 

 a most striking fact is recognized, namely, that enzymes 

 are not used up in direct proportion to the work they 

 do. If we are pouring hydrochloric acid upon iron 

 filings to make hydrogen gas we know that we must 

 keep adding the acid if we are to continue to evolve the 

 hydrogen. But if we are turning starch to sugar by the 

 action of saliva the amount of sugar formed depends 

 more on the time than on the quantity of the saliva. 

 A very small amount of a digestive secretion, containing 

 a much smaller amount of the actual enzyme, can act 

 continuously under favorable conditions and suffer 

 only the most gradual loss of virtue in the process. 



The power of an enzyme to carry on a transformation 

 without itself being destroyed might fail to be evident 

 if the trial were not carefully regulated. If the test 

 were made in a flask the reaction would be found to 

 lag more and more until finally arrested. It might be 

 thought that the enzyme had been exhausted. But the 



