4 io PHYSIOLOGY 
they have been simplified by cleavage into hexoses. Why this limitation 
exists, and why within this there are others even more specific, is not 
known. The organisms concerned are chiefly those known as yeasts 
(see Saccharomycetes, p. 70), but certain molds and bacteria also give 
rise to ethyl alcohol, though the lattT more commonly produce higher 
alcohols (propyl alcohol, butyl alcohol, etc.). In this connection it is 
to be remembered that even the higher plants produce ethyl alcohol in 
the. course of anaerobic respiration. 
The sugar is split up in large measure into CO 2 and ethyl alcohol, 
but there are other products, such as glycerin, succinic acid, etc., in 
smaller quantity. Fermentation proceeds very slowly when the yeasts 
are abundantly supplied with O 2 ; then, however, they grow and mul- 
tiply rapidly, and apparently use the sugar chiefly as food. But when 
the supply of O 2 is small, so that their vegetative processes are hindered, 
fermentative action is increased. Though alcohol is produced at all 
times, its quantity is in a sort of inverse ratio to the favorablencss of the 
conditions for life. When 12 per cent have accumulated in the liquid, 
the action is retarded, and by 14 per cent it is stopped. 
Fermentation by yeasts was long believed to be due to the direct action 
of their protoplasm on the sugar; now it has been proved that an extract, 
made by grinding the yeast with sand and filtering the juice under high 
pressure through porcelain, can produce the same effect. The active 
substance, known as zymase, is soon destroyed, unless protected from 
digestion by accompanying enzymes. Similar substances have been iso- 
lated in higher plants, which are believed to act upon carbohydrates in 
anaerobic respiration, 1 giving rise to alcohol and CO 2 in the same propor- 
tions as in fermentation. 
The economic uses of alcoholic fermentation are many. It plays a prominent 
role in the lightening of bread, in which, however, other organisms share with yeast 
the production of the gases that raise the dough; it is the source of commercial 
ethyl alcohol, which is distilled from fermented liquids, in which hexose sugars are 
first produced from corn and potato starch; it gives rise to the alcohol in a host of 
fermented liquids used as beverages : wine, beer, koumiss, pulque, sake, etc. 
Lactic fermentation. The lactic fermentation, giving rise to lactic 
acid, is best known in the souring of milk, and may be produced whenever 
lactose is present in a solution to which the lactic acid bacterium has 
1 The source of these carbohydrates is uncertain. They may be either the unassimi- 
lated carbohydrates of the food; or, equally well, a carbohydrate nucleus from the 
decomposition of the protoplasm. 
