TOOTH, one of the hard bodies in the mouth, attached to the skeleton, but not forming part of it, and developed from the dermas or true skin.
True teeth consist of one, two, or more tissues, differing in their chemical composition and in their microscopical appearances. Dentine, which forms the body of the tooth, and "cement," which forms its outer crust, are always pres ent, the third tissue, the "enamel," when present, being situated between the den tine and cement. The dentine, which is divided by Owen into hard or true den tine, vaso-dentine, and osteo-dentine, con sists of an organized animal basis, dis posed in the form of extremely minute tubes and cells, and of earthy particles. The tubes and cells contain, besides the calcareous particles, a colorless fluid, which is probably transuded blood plas ma, or liquor sanguinis, and contributes to the nutrition of the dentine. In hard or true dentine the dentinal tubes pro ceed from the hollow of the tooth known as the pulp cavity, in a slightly wavy course, nearly at right angles to the outer surface. When a part of the primi tive vascular pulp from which the den tine is developed remains permanently uncalcified, red blood is carried by "vas cular canals" into the substance of the tissue. Such dentine is called vaso-den tine, and is often combined with true den tine in the same tooth, as, for example, in the large incisors of certain rodents, the tusks of the elephant, and the molars of the extinct megatherium. When the cellular basis is arranged in concentric layers around the vascular canals and contains "radiated cells," like those of bone, this is termed osteo-dentine: resem bles true bone very closely. The cement always corresponds in texture with the osseous tissue of the same animal, and wherever it occurs in sufficient thickness, as on the teeth of the horse or ox, it is traversed like bone by vascular ca nals. The enamel is the hardest of all the animal tissues, and contains no less than 96.4 per cent. of earthy matter (mainly phosphate of lime), while dentine con tains only 72 per cent. and cement and ordinary bone only 69 per cent. of earthy
matter.
In a few fishes the teeth consist of a single tissue—a very hard kind of non vascular dentine. Teeth consisting of dentine and vaso-dentine are very corn mon in fishes. In all fishes the teeth are shed and renewed, not once only, as in mammals, but frequently during the whole course of their lives. Tortoises and turtles, toads, and certain extinct saurians are toothless. Frogs have teeth in the upper, but not in the lower jaw. Newts and salamanders have teeth in both jaws and upon the palate; and teeth are found on the palate as well as on the jaws of most serpents. In reptiles, as a general rule, the base of the tooth is anchylosed to the bone which sup ports it. The completion of a tooth is soon followed by preparation for its re moval and succession, the faculty of de veloping new tooth germs being appar ently unlimited in this class. The extinct Odontornithes are the only birds with teeth. Of mammals there are a few genera and species devoid of teeth. The true ant-eaters, the pangolins, and the echidna are strictly toothless. It is only in the mammals that we have a well marked division of the teeth into the four kinds of incisors, canines, premo lars, and molars.
The teeth are so admirably adapted for the special purposes which they are called on to fulfill that it is generally easy, from a careful examination of them, to say to what class of animals they belong, and to draw various conclusions regarding the habits4nd structure of the class gener ally. Thus, in carnivorous animals the molars are not grinding teeth, but pre sent sharp cutting edges, and those of the upper and lower jaw overlap each other, resembling a pair of scissors in their action. In insectivorous animals the molars have a tuberculater surface, with conical points and depressions, so arranged as to lock into each other. In frugivorous animals, living on soft fruits, these teeth are provided with rounded tubercles, while in herbivorous animals they have a broad, rough surface, re sembling a millstone.