The _temporary_ set of teeth is completed at about two years of age and consists of twenty teeth. The teeth cannot grow as the jaw grows, and soon a larger and _permanent_ set starts to growing deeper in the jaw. At the age of twelve or thirteen years all the permanent set have appeared except the four wisdom teeth, which appear between the ages of seventeen and twenty-five. The second set not only replaces the twenty of the first set, but to fill the larger jaws twelve molars are added, three at the back in each half jaw, making thirty-two teeth in the second set (Exp. 7). The teeth in each quarter of the mouth, named in order from the front, are: two _incisors_, one _canine_, two _premolars_, three _molars_. [Illustration: FIG. 92.--THE PERMANENT TEETH in right half of lower jaw.] [Illustration: FIG. 93.--UPPER JAW WITH TEETH.] =Care of the Teeth.=--The best way to care for the teeth is _to keep the digestion perfect_. Perfect digestion tends to preserve the teeth, and sound teeth tend to keep the digestion perfect. The teeth should be _washed regularly_. Prepared chalk is the best dentifrice. Do not rub across, but from gums to teeth, to prevent rubbing the gums loose from the teeth. An unclean brush may harbor germs. _Toothpicks_ and dental floss are useful. If one eats only soft food, in which the mill and the cooking stove have left no work for the teeth, the teeth will decay; for it seems to be a law of nature that useless organs are removed. The pressure from _chewing hard food is an aid_ to the teeth by helping the circulation and nerves in the pulp. To take a _very hot or very cold_ drink into the mouth may cause the _enamel to crack_. If a tooth aches, or a small decayed place is found in it, a dentist should be consulted at once. A tooth is so valuable to the health that no tooth should be extracted when it can be saved. =The process of digestion= consists in liquefying the food that it may pass through the walls of the food tube into the blood, and through the walls of the blood vessels into the tissues. It is accomplished: (1) by _mechanical_ means, including the chewing muscles, the teeth, and three layers of muscles in the walls of the food tube; (2) by _chemical_ means, or the action of alkalies and acids upon the food; (3) by _organic_ agency, or the action of ferments. A _ferment_ (or _enzyme_) is a vegetable substance which has the power of producing a chemical change in large quantities of substance brought in contact with it, without being itself changed. There is one ferment secreted in the mouth, two in the stomach, and three in the small intestine.