The act of chewing is simple and yet important, for if hurriedly or imperfectly done, the food is in a condition to cause disturbance in the digestive process. Thorough mastication is a necessary introduction to the more complicated changes which occur in the later digestion. 133. The Teeth. The teeth are attached to the upper and lower maxillary bones by roots which sink into the sockets of the jaws. Each tooth consists of a _crown_, the visible part, and one or more fangs, buried in the sockets. There are in adults 32 teeth, 16 in each jaw. Teeth differ in name according to their form and the uses to which they are specially adapted. Thus, at the front of the jaws, the incisors, or cutting teeth, number eight, two on each side. They have a single root and the crown is beveled behind, presenting a chisel-like edge. The incisors divide the food, and are well developed in rodents, as squirrels, rats, and beavers. Next come the canine teeth, or cuspids, two in each jaw, so called from their resemblance to the teeth of dogs and other flesh-eating animals. These teeth have single roots, but their crowns are more pointed than in the incisors. The upper two are often called eye teeth, and the lower two, stomach teeth. Next behind the canines follow, on each side, two bicuspids. Their crowns are broad, and they have two roots. The three hindmost teeth in each jaw are the molars, or grinders. These are broad teeth with four or five points on each, and usually each molar has three roots. The last molars are known as the wisdom teeth, as they do not usually appear until the person has reached the “years of discretion.” All animals that live on grass, hay, corn, and the cereals generally, have large grinding teeth, as the horse, ox, sheep, and elephant. The following table shows the teeth in their order: Mo. Bi. Ca. In. In. Ca. Bi. Mo. Upper 3 2 1 2 | 2 1 2 3 = 16 | } = 32 Lower 3 2 1 2 | 2 1 2 3 = 16 The vertical line indicates the middle of the jaw, and shows that on each side of each jaw there are eight teeth. 134. Development of the Teeth. The teeth just described are the permanent set, which succeeds the temporary or milk teeth. The latter are twenty in number, ten in each jaw, of which the four in the middle are incisors. The tooth beyond on each side is an eye tooth, and the next two on each side are bicuspids, or premolars. The milk teeth appear during the first and second years, and last until about the sixth or seventh year, from which time until the twelfth or thirteenth year, they are gradually pushed out, one by one, by the permanent teeth. The roots of the milk teeth are much smaller than those of the second set. Illustration: Fig. 48.—Temporary and Permanent Teeth together. _Temporary teeth:_ A, central incisors; B lateral incisors; C, canines; D, anterior molars; E, posterior molars