188 HUMAN PHYSIOLOGY 



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 

 real cause of the arrest in such cases is the gathering 

 of the products of the change in the field of operations. 

 If the products can be removed the reaction will be 

 resumed. We must remember that the conditions in a 

 glass vessel must always be much less favorable to the 

 action of enzymes than those prevailing in the ali- 

 mentary canal. Nothing can escape from the flask or 

 the test-tube while there is the possibility of withdrawing 

 the digestive products from the canal and enabling the 

 change to proceed. 



There is a certain temptation to speak of enzymes as 

 though they were living. This is to be guarded against; 

 they are secreted by living cells but all that they do can 

 be explained without assuming that they have life. 

 Yeasts, moulds, and bacteria which are simple living 

 things may work upon solutions in which they are grow- 

 ing very much as enzymes in suitable variety would. 



