The enzymes are a class of bodies capable of instituting chemic changes without apparently entering into the reaction or forming a part of the end-products. Their activity is very persistent, but not unlimited. They are unstable bodies, and are nearly all destroyed at a temperature of about 60° C. (1400 F.). Examples are: invertase, which transforms cane-sugar into fructose and glucose; lactase, which changes sugar-of-milk into glucose and galactose; maltase, which converts maltose into glucose; emulsin and myrosin, of whose reactions with certain glucosides we have spoken, and pepsin, trypsin, and the other enzymes of the digestive tract. A number of enzymes have a reversible action, i.e., can, under certain circumstances, bring about changes just the reverse of the usual.

It is not improbable that a great many of the metabolic changes going on in the animal body are brought about by enzymes. The oxidases, for example, are concerned in the oxidation processes of the tissues.