It is particularly important to understand the natural growth and proper care of the teeth because there are few organs in the body for which we are able to do so much by direct personal attention. Our stomachs, our livers, and our kidneys, for instance, are entirely out of sight, and more or less out of reach; but our teeth are both easily got at and in full view; and, to a large degree, upon the care that we give them while they are young, will depend not only their regularity and whiteness, but also the length of their life and the vigor and comfort of our digestion all our lives. [Illustration: TEETH--A QUESTION OF CARE] The first thing to be remembered about the teeth is that, hard and shiny and different from almost everything else in the body as they look, they are simply a part of the skin lining the mouth, hardened and shaped for their special work of biting and chewing. Much of the care needed to prevent decay should be given, not to the teeth themselves directly, but to the gums and the mucous membrane of the whole mouth. The gums and the mouth literally _grew_ the teeth in the first place; and when they become diseased, they secrete acids which slowly eat away the crowns and roots of the teeth. Their diseases come chiefly from irritation by decaying scraps of food, or from the blocking of the nose so that air is breathed in through the mouth, drying and cracking the soft mucous membrane. After the acids from the diseased gums have attacked the teeth, the poisons of the germs that breed in the warmth and moisture of the mouth cause the teeth to decay. Eight times out of ten, if you take care of the gums the teeth will take care of themselves. Structure of the Teeth. The upper half of the tooth, which pushes through and stands up above the jaw and the gum, we call the _crown_; and this is the portion that is covered with _enamel_, or "living glass." The body of the tooth under the enamel is formed of a hard kind of bone called _dentine_. The lower half of the tooth, which still is buried in the jaw, we call the _root_. Wrenching the lower or root part of the tooth loose from its socket in the jaw is what hurts so when a tooth is pulled. The crown of the tooth is hollow, and this hollow is filled with a soft, sensitive pulp, in which we feel toothache. Tiny blood vessels and nerve-twigs run up from the jaw to supply this pulp through canals in the roots of the tooth. [Illustration: A TOOTH (Lengthwise section.) _E_, enamel; _D_, dentine; _P_, pulp cavity; _C_, cement; _B_, blood vessels; _N_, nerve.]