72 ENZYMES OR SOLUBLE FERMENTS^ 



In this cise the enzyme appears to be closely associated with the 

 mucin and is presumably a secretory product of the mucous mem- 

 brane, for it is frequently obtained when there has been no opera- 

 tive use of surgical instruments which could account for the intro- 

 duction of micro-organisms from the exterior. 



In concluding this account of the more important enzymes of 

 the animal body it may not be out of place to say a few words on 

 the probable mode of action of the ferments and enzymes. 



The term fermentation was applied originally to the changes, 

 accompanied by characteristic frothing, foaming, and evolution of 

 gases, which saccharine solutions such as the expressed juice of 

 fruits or infusions of grain undergo on exposure to the air. The 

 chemical changes and products of the fermentation were studied 

 from the earliest times, and in 1680 Leuwenhcek described, with 

 the aid of the newly-invented microscope, the small, spherical par- 

 ticles which are now known as yeast-cells, to be the exciting cause 

 of the whole process. He did not however ascribe any organisa- 

 tion to these particles, and it was not until 1835 and 1837 that 

 Cagniard de Latour and Schwann respectively but independently 

 took up the investigation where Leuwenhoek had left it, and estab- 

 lished firmly and finally the organised and plant-like nature of the 

 yeast-cell and the absolute dependence of fermentation upon its 

 presence in the fermenting fluid. 1 The yeast-cell having thus been 

 definitely recognized as the cause of the fermentation, the interest- 

 ing question at once arose as to how the known cause produces 

 the observed effect, and to this question many answers have been 

 given, of which the following are the more important. 



Liebig regarded the ferments as substances in a state of pro- 

 gressing decomposition during which the equilibrium of their 

 constituents is upset and a rapid motion of their minuter parts 

 established. When brought into contact with other decomposable 

 substances the motion of the ferment's particles is communicated 

 to the former, whereupon it also undergoes a decomposition result- 

 ing in the formation of the simpler products which make their 

 appearance and are characteristic of the fermentation. According 

 to this view the organised nature of the yeast-cells is left out of 

 account and the phenomena attributed entirely to the purely 

 chemical decomposition of their constituent substance, set going 

 at the outset by oxygen. 2 Pasteur regarded alcoholic fermenta- 



1 Erxleben in 1818 had described and spoken of yeast as a vegetative organism, 

 as also in 1825 had Desmazieres, who ascribed to it an animal rather than vegetable 

 nature. 



2 Ann. d. Chem. u. Pharm. Bd. xxx. (1839), Sn. 250, 363. Stahl in 1734 had ex- 

 pressed practically identical views. 



