One characteristic feature of enzymes is the incompleteness of their action. Thus, the enzyme of saliva transforms starch by a series of progressive changes into soluble starch, two or more dextrins, and the sugar maltose as the chief end-product. A mixture of starch paste and saliva under ordinary conditions, however, never results in the formation of a hundred per cent of maltose, but there always remains a variable amount of dextrin which appears to resist further change. This is apparently due to what is known as the reversible action of enzymes. Thus, the chemical reactions involved here are reversible actions, _i. e._, they take place in opposite directions. The catalyzer not only accelerates or incites a reaction in the direction of breaking down the substance acted upon, but it also aids in the recomposition of the products so formed into the original or kindred substance. With reversible reactions of this sort the opposite changes sooner or later strike an equilibrium, which remains constant until some alteration in the conditions brings about an inequality and the reactions proceed until a new equilibrium is established. In the body, however, where the circulating blood and lymph provide facilities for the speedy removal by absorption of the soluble products formed, the reaction may proceed until the original substance undergoing change is completely transformed into the characteristic end-product. This reversible action of enzymes is an important feature, and helps explain certain nutritional changes to be referred to later. Whether all enzymes behave in this way is not as yet determined. Another peculiarity of digestive enzymes is their extreme sensitiveness to changes in their environment. Powerful in their ability to transform relatively large quantities of a given foodstuff into simple products better adapted for absorption and utilization by the body, they are, however, quickly checked in their action, and even destroyed, when the conditions surrounding them are slightly interfered with. They require for their best action a temperature closely akin to that of the healthy body, and any great deviation therefrom will result at once in an inhibition of their activity. Further, they demand a certain definite reaction of the fluid or mixture, if their working power is to be maintained at the maximum. Indeed, many enzymes, like the ptyalin of saliva, are quickly destroyed if the reaction is greatly changed. Enzymes are thus seen to be more or less unstable substances, endowed with great power as digestive agents, but sensitive to a high degree and working advantageously only under definite conditions. Many perversions of digestion and of nutrition are connected not only with a lack of the proper secretion of some one or more digestive enzyme, but also with the lack of proper surroundings for the manifestation of normal or maximum activity.