SALIVA 51 



activity is stopped, but the ferments are not destroyed, by cold ; it is 

 stopped and the ferments killed by too great heat. A certain amount 

 of moisture and oxygen is also necessary ; there are, however, certain 

 mici'o-organisms that act without free oxygen ; these are called 

 anaerobic, in contradistinction to those which require oxygen, and 

 which are therefore called aerobic. 



The chemical nature of the enzymes is very difl&cult to investigate ; 

 they are substances that elude the grasp of the chemist to a great 

 extent. So far, however, research has taught us that they are either 

 proteid in nature or are substances closely alhed to the proteids. 



The distinction between organised ferments and enzymes is more 

 apparent than real ; for the micro-organisms exert their action by 

 enzymes that they secrete. By crushing yeast cells Buchner has 

 succeeded in obtaining from them an enzyme that produces alcohohc 

 fermentation. 



