5. MASTICATION.--As soon as solid food is taken into the mouth, it undergoes mastication, or chewing. It is caught between the opposite surfaces of the teeth, and by them is cut and crushed into very small fragments. In the movements of chewing, the lower jaw plays the chief part; the upper jaw, having almost no motion, acts simply as a point of resistance, to meet the action of the former. These movements of the lower jaw are of three sorts: a vertical or cutting, a lateral or grinding, and a _to-and-fro_ or gnawing motion. [Illustration: FIG. 17.--SECTION OF A TOOTH. _a_, Enamel; _b_, Cavity; _c c_, Roots; _d_, Body of the Tooth.] [Sidenote: 6. Composition of the teeth? Enamel of the teeth? Interior of teeth?] 6. The teeth are composed of a bone-like material, and are held in place by roots running deeply into the jaw. The exposed portion, or "crown," is protected by a thin layer of enamel (Fig. 17, _a_), the hardest substance in the body, and, like flint, is capable of striking fire with steel. In the interior of each tooth is a cavity, containing blood-vessels and a nerve, which enter it through a minute opening at the point of the root (Fig. 19). [Sidenote: 7. The milk teeth? The permanent teeth?] 7. There are two sets of teeth; first, those belonging to the earlier years of childhood, called the milk teeth, which are twenty in number and small. At six or eight years of age, when the jaw expands, and when the growing body requires a more powerful and numerous set, the roots of {83} the milk teeth are absorbed, and the latter are "shed," or fall out, one after another (Fig. 18), to make room for the permanent set. [Illustration: FIG. 18.--SECTION OF THE JAWS. 1' 2' 3' 4' 5', The Milk Teeth; 1" to 8", The Germs of the Permanent Set.] [Sidenote: 8, 9. Number of teeth? How distributed?] 8. There are thirty-two teeth in the permanent set, as many being in one jaw as the other. Each half-jaw has eight teeth, similarly shaped and arranged in the same order: thus, two incisors, one canine, two bicuspids, and three molars. The front teeth are small, sharp, and chisel-edged, and are well adapted for cutting purposes; hence their name incisors. The canines stand next, one on each side of the jaw; these receive their name from their resemblance to the long, pointed tusks of the dog (Fig. 19). {84} [Illustration: FIG. 19.--SECTION OF THE JAWS--RIGHT SIDE. V, A, N, Veins, Arteries, and Nerves of the Teeth. The root of one tooth in each jaw is cut vertically to show the cavity and the blood-vessels, etc., within it. 1 to 8, Permanent Teeth.]