Physiology. 15 



undergo during this process has been vastly augmented 

 during the latter half of this century, but even yet, the 

 problem has not been completely solved. The results of 

 fermentation have been made use of from time immemo- 

 rial, but no adequate conception of the changes involved 

 was ever recognized until this century. The earlier the- 

 ories of this action were mainly chemical, and it was not 

 until the important studies of Pasteur were made that 

 the relation of these changes to the action of living or- 

 ganisms was fully proven. He showed that fermentation 

 was closely connected with the growth and multiplication 

 of minute forms of organic life, and that the process was 

 usually inaugurated by vital forces rather than by purely^ 

 chemical activity. 



Previous to his work the action of rennet upon milk 

 had been known, and the difference between this fermen- 

 tation and other types had been recognized by Schroeder 

 and Dusch. They showed that the rennet ferment was 

 unaffected by alcohol and other chemicals, while other 

 fermentations due to organisms were checked in these 

 fluids. Pasteur emphasized the distinction between these 

 two sets of fermentative action and soon was able to clas- 

 sify these changes into two distinct types. 



1. Organized or living ferments those in which the 

 fermentative change takes place as a result of the activity 

 of a living organism. 



2. Unorganized or chemical ferments those in which 

 the change is caused by a chemical substance, devoid of 

 vitality, that is itself unchanged in the fermenting pro- 

 cess. These unorganized, non- vital ferments are known 

 as enzymes. Among the better known of these are ren- 

 net, that has the power of coagulating milk; diastase, 

 the enzyme that converts starch into sugar; pepsin and 

 trypsin, the digestive ferments of the animal body. In 



