From this relation between the food and the organs of mastication, naturalists can tell with certainty, by simply inspecting the teeth, on what kind of food the animal to which they belong is intended to live; and as the teeth of man partake of the characters of those of both herbivorous and carnivorous animals, there cannot be a doubt that his diet was intended to be of a mixed kind, not confined exclusively to either the vegetable or the animal kingdom. Hard and resisting as the teeth appear, they must still be regarded as living structures. Anatomically speaking, each tooth is divided into three parts: the _fang_ or _root_, implanted in the socket of the jaw-bone; the _neck_, or portion encircled by the gum; and the white _crown_, appearing above the gum, and covered with enamel. The root of each tooth is perforated longitudinally by a small canal, through which the bloodvessels and nerve are admitted to its central parts. From these bloodvessels the tooth derives its nourishment when growing; but they afterwards almost entirely disappear. From the nerve it derives that sensibility which makes us instantly aware of the contact of bodies either too hot or too cold with the teeth; and which, when the nerve is diseased, gives rise to the racking pain of toothach. So effectually is life maintained in the teeth by this provision of vessels and nerves, that a tooth newly extracted from the socket of a young animal, and implanted in the fleshy comb of a cock, has been found to adhere and retain its vitality; and, in like manner, if, in early life, a tooth extracted by accident be immediately replaced in its socket, it will generally adhere and live. The visible part or crown of the tooth is covered with a very hard white ivory-looking substance called _enamel_, which serves to prevent it from being worn down by friction, and into which neither bloodvessels nor nerves have been observed to penetrate. Owing to this structure, the tooth can be safely exposed without sustaining damage--a privilege on which most persons will be disposed to place a higher value after having experienced the pains consequent upon injury of the nerve from a portion of the enamel being broken off.