The incisors, or cutting-teeth, are situated in front, and possess a single conical root or fang, and a vertical crown beveled behind, so as to terminate in a sharp cutting edge. These teeth are specially fitted, as their name implies, for cutting the food. In j man, there are two of these incisors in each side of each jaw. In herbivorous animals, they crop the herbage; in rodents (the rabbit, hare, rat, beaver, etc.), these teeth are very much developed, and differ from any other teeth occuring in mammals in this respect, that their growth continues throughout life; and if their length does not con scantly increase, it is because their free extremity or edge is worn down by trituration as fast as they grow at the base from their roots.
The canines (so called from their prominence in the dog) come next to the incisors. Their crown is rather conical than wedge-shaped, and their fang sinks more deeply into the jaw than in the case of the incisors. In all carnivorous animals, they are largely developed, being obviously formed for tearing the flesh of their prey. In man, there is one canine tooth in each half-jaw; and there is never more than this number in any of the lower animals.
The premolars (known also as biscuspids and false molars) come next in order to the canines; they are smaller than the latter, and their crown presents two pyramidal emi nences. In man, there are two premolars in each half-jaw. Their function more nearly approaches to that of the true molars behind them, than to that of the canines.
The true molars (or multicuspids) are placed most posteriorly. They are remarka ble for their comparatively great size, the square form of the upper surface, on which are from three to five elevations or cusps, and for their short root, which is divided into from two to five branches, each of which is perforated at its extremity. In man there are three molars in each half-jaw, the posterior one being termed the wisdom-tooth from its being cut the latest; they are especially employed for grinding the food, under the action of the muscles of the lower jaw.
The teeth are so admirably adapted for the special purposes which they arecalled upon 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 habits and structure of the class generally. Thus, in carnivorous animals, the molars are not grind ing teeth, but present sharp cutting edges, and those of the upper and lower jaw over lap each other; resembling a pair of scissors in their action. In insectivorous animals, the molars have a tuberculated surface, with conical points and depressions, so arranged as to look 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, resembling a millstone.
There is also a close connection between the articulation or joint of the lower jaw and the nature of the food used by the animal. Thus, in purely carnivorous animals, in which the teeth simply tear and cut the food, no grinding motion is required, and the jaw is capable only of a simple hinge-motion in the vertical plane; while in herbivorous animals, the joint is so constructed as to allow of extensive sliding and lat eral motion of the lower molar teeth upon the upper. In man, both the form of this articulation and the general character of the teeth point to an intermediate position in relation to food, and form a good physiological argument for the mixed diet which gen eral custom has decided to be most favorable and natural to our species.---For further information on this subject, the reader is referred, not only to the three works of the professor from which quotations have been made in this article, but to his splendid Odon tography (1844),and to his article " Teeth," in The Cyclopcedia of Anatomy and Physiology; to F. Cuvier, Sur les Denies, etc., and to De Blainville's Osteographie.