PHYSIOLOGICAL ^63 



of the seventeenth century, the Dutchman Leeuwenhoek, who had 

 an extraordinary talent for minute observation, saw the two chief 

 kinds of fermenting organisms, namely, yeast-plants and bacteria. 

 But he did not realise the importance of what he saw. It was not 

 till about 1835 that the French physicist Cagniard de Latour recog- 

 nised that the yeast used in brewing was composed of living cells 

 that multiplied very rapidly by budding. He suggested in a somewhat 

 vague way that they probably acted on sugar "through some effect 

 of their vegetation". 



A firmer step was taken about twenty years later when Pasteur 

 proved beyond all possibility of doubt that alcoholic fermentation 

 was due to the activity of the yeast-plant, and that the lactic 

 fermentation of milk was similarly due to a "lactic acid bacterium". 

 This was an epoch-making step. 



The solid food that animals eat is digested in the food-canal, 

 becoming fluid and more readily diffusible — and this process of 

 digestion is in the main of the natiu-e of a fermentation. But for 

 many years a distinction was drawn between the non-living ferments 

 in the digestive juices, like the pepsin of the stomach, and the 

 ferments which are living organisms themselves, like yeast-plants 

 and some bacteria. Rennet, which one buys in a shop when one 

 wishes to make curds, is a preparation of the lining of the calf's 

 stomach. When it is put into milk it coagulates the cheesy material, 

 and we call the coagulation "curd". This rennet is almost, if not 

 quite, the same as pepsin, the common ferment made by the gland- 

 cells lining the stomach of backboned animals. No one regards 

 rennet or pepsin as in any way alive, as the yeast is; and thus a firm 

 line was drawn between the non-living and the living ferments, 

 "organised" and "unorganised" as they were called. 



But in 1897 Biichner subjected yeast to great pressure and heavy 

 crushing, and extracted a yellowish juice which showed considerable 

 power of fermenting sugar. This extracted yeast-ferment, which is 

 called zymase, cannot be obtained quite pure; it is accompanied by 

 another ferment which rather blunts its potency. But the point is 

 that a non-living juice can act in the same way as living yeast-plants 

 do when they change sugar into alcohol. They are able to do so, not 

 because they are alive, but because they contain a ferment. Thus 

 a somewhat artificial distinction was broken down. 



Extracts of a few fermenting bacteria have also been prepared, 

 and Biichner's important work was marked by the replacement of 

 the word "ferment" by the word "enzjmie", which is applicable to 

 the fermenting chemical substance whether it works inside living 

 cells, as in the case of yeasts, or outside living cells as in the digestive 

 juices. It should be clearly understood that ferments are common 

 in plants as well as in animals; thus a diastase ferment in every 

 green leaf changes starch into sugar every evening, and a ferment 



