The saliva is primarily for the purpose of moistening the food, but besides this it is also a definite digestive juice, because it contains one of the digestive enzymes. This enzyme, to which is given the name _ptyalin_, acts upon starch, changing it to sugar, but not to the particular sugar of which the body makes use. Perhaps we should digress for a moment to say that there are chemically a number of sugars. These are in many respects alike, although they also vary a good deal among themselves. For example, some are much sweeter than others; glucose, which is a sugar that can be made from cornstalks and other plant products, is not nearly so sweet as cane or beet sugar, although one is fully equal to the other so far as nutritive value is concerned. Of all the various kinds of sugar that exist, only those in the glucose class can actually be used by the body, so that all other sugars have to be changed by digestion into a glucoselike sugar before passing on into the blood. The enzyme of saliva converts starch not into glucose but into a more complex sugar to which is given the name of maltose. It, therefore, starts digestion, but does not carry it through to completion. Starch digestion begins during the course of chewing and mixing the food with saliva; it goes on while the food is being swallowed, and for a short time after it enters the stomach, being stopped there sooner or later by means which will be described in a moment. No other real digestion occurs in the mouth, although the chewing and moistening are of very great importance in preparing the food for the digestion that is to come further along.