TEETH, T. A tooth is described by prof. Owen, the highest authority on this subject, as "a hard body attached to the mouth or commencement of the alimentary canal, partially exposed when developed. Calcified teeth are peculiar to the vertebrates, and may be defined as bodies primarily, if not permanently, distinct from the skeleton, consisting of a cellular and tubular basis of animal matter containing earthy particles, a. fluid, and a vascular pulp."—The Anatomy of Trertehrates, 1866, vol. i. p. 359. " They present," says the same writer, "many varieties as to number, size, form, structure, position, and mode of attachment, but are principally adapted for seizing, tearing, divid ing, pounding, or grinding the food.* In some species they are modified to serve as formidable weapons of offense and defense; in others, as aids in locomotion, means of anchorage, instruments for uprooting or cutting down trees, or for transport and work ing of building materials. They are characteristic of age and sex; and in man they have secondary relations, subservient to beauty and to speech. Teeth are always intimately related to the food and habits of the animal, and are therefore highly interesting to the physiologist; they form, for the same reason, important guides to the naturalist in the classification of animals."— Oircle of the Sciences; Organic Nature, vol. i. p. 264.
True teeth consist of one, two, or more tissues, differing in their chemical composi tion and in their microscopical appearances. "Dentine," which forms the body of the tooth, and "cement," which forms its outer crust, are always present;. the third tissue, the " enamel," when present, being situated between the dentine and cement. The dentine, which is divided by prof. Owen into hard or true dentine, vaso-dentine, and consists, according to that physiologist, of an organized animal basis, dis posed in the form of extremely minute tubes and cells, and of earthy particles; these earthy or calcareous particles being either blended with the animal matter of the inter spaces and walls of the tubes and cells, or contained in a minutely divided state in their cavities. The tubes and cells contain, besides the calcareous particles, a colorless fluid,. which is probably transuded blood plasma, or liquor sanguinis, and contributes to the nutrition of the dentine. In hard or true dentine, the dentinal tubes proceed from the hollow of the tooth known as the pulp cavity, in a slightly wavy course, nearly at right angles to the outer surface. " The hard substance of the tooth is thus arranged in hollow columns, perpendicular to the plane of pressure, and a certain elasticity results from these curves; they are upright where the grinding surface of the crown receives the appulse of the opposing tooth, and are horizontal where they have to resist the pressure of contigu ous teeth. The tubuli, besides fulfilling the mechanical ends above stated, receive the
plasma transuded from the remains of vascular pulp, which circulates by anastomosing branches of the tubuli through the dentine, maintaining a sufficient, though languid vitality of the system. The delicate nerve-branches on the pulp's surface, some minute production of which may penetrate the tubuli, convey sensations of impressions affecting the dentine —sensations of which every one has experienced the acuteness, when decay has affected the dentine, or when mechanical or chemical stimuli have "set the tooth on edge ;" but true dentine has no canals large enough to admit capillary vessels with the red particles of blood.'' When a part of the primitive vascular pulp from which the dentine is developed remains permanently uncalcified, red blood is carried by "vascular canals" into the substance of the tissue. Such dentine is called vaso-dentine, and is often com bined with true dentine 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. Another modification of the dentine is when the cellular basis is arranged in concentric layers around the vascular canals, and contains " radiated cells," like those of bone; this is termed esieo-dentine, and resembles true bone very closely. The cement always corresponds in texture with the osseus 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 canals, Moreover, when the osseous tissue contains minute radiated cells, precisely similar cells are likewise present in the canal, and constitute its most marked characteristic. The relative densities of dentine and cement vary according to the amount of earthy matter. In the complex grinders of the elephant and some other animals, the cement, which forms nearly half the mass of the tooth, wears down sooner than the dentine. 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 contains only per cent, and cement and ordinary bone 69 per cent of earthy matter. The earthy matter is contained in comparatively wide canals, composed of animal membrane of extreme tenuity.