The great incisors, which are characteristic of the Rodents, exhibit the following peculiarities:--They possess no roots, but spring from a permanent pulp, so that they continue growing during the whole life of the animal; and their form, and that of the cavity which constitutes their socket, is always that of a segment of a circle,[49] in consequence of which, they always protrude from the front of the jaws in the same direction, and meet at the same angle. By this means, as the teeth are worn away at their summits by use in gnawing, a fresh supply of tooth is continually being pushed forward to take the place of the portion thus removed, and, in fact, so intimately are the two functions of use and growth correlated in the teeth of these animals, that if by chance one of the incisors should get broken, or the natural opposition of these teeth should be disturbed in consequence of injury to the jaw, the teeth, thus deprived of their natural check, continue growing, and, following the curve of their sockets, gradually form circular tusks, which must always be greatly in the way of the animal when feeding, and sometimes, by actually penetrating again into the mouth, cause its death by absolute starvation. The teeth themselves are composed of dentine, coated along the front surface with a layer of hard enamel, which substance is wanting on the other surfaces of the teeth, except in the Hares, Rabbits, and other forms with additional rudimentary incisors in the upper jaw, in which, as further evidence of their relationship to the other Mammalia, the whole surface of the incisors is encased in enamel, although this coat is excessively thin except on the front or outer face. The purpose of this structure of the incisors is easily understood. In the action of gnawing, the dentine, which forms the greater part of the tooth, is more easily abraded than the harder enamel, which is thus left as a sharp front edge, to which the mass of dentine behind it, being worn away into a bevelled surface, gives the necessary firmness and support, the whole forming a chisel-like instrument, constructed precisely on the principle of those tools in which a thin plate of hard steel forms the cutting edge, and is stiffened by a thicker bevelled plate of softer iron.