MATTER AND ENERGY 71 



various kinds of yeast cells or toruiae, and the 

 putrefaction of dead nitrogenous matter by 

 the activity of various bacteria. A recogni- 

 tion of these facts, and of the part played by 

 micro-organisms in many diseases, has led to 

 the evolution of the vast realm of knowledge 

 now known as bacteriology. Physiologists 

 have long been acquainted with the existence 

 in the body of ferments, such as the ptyalin 

 of the saliva and the pepsin of the gastric 

 juice, but it is only in recent years that there 

 has been an adequate recognition of the part 

 played by ferments in many physiological 

 processes. It is now known that all ferments, 

 or enzymes, as they are now called, are formed 

 in the interior of cells. For a long time it was 

 held that the plant known as the yeast cell 

 effected fermentation in the juice of the grape 

 or in a solution of sugar by its vital activity, 

 and this view was favoured by the fact that 

 durng fermentation there was a remarkable 

 multiplication of the cells of the yeast 

 It is now known, however, that even this 

 fermentation is caused by an enzyme (zymase) 

 formed in the interior of the yeast cell. 



