The exposed part of the teeth is covered by a dense substance of flinty hardness called enamel, which is developed from the epithelium, and consists of hexagonal prisms set on end, which are really modified epithelial cells, but only contain about 2 per cent, of animal matter (Fig. 47). The bulk of the tooth is made up of dentine, a substancelike bone in composition, pierced by numerous fine canals - dentine tubules - which radiate toward the surface, from the pu/p cavity, in the centre of the tooth. Filaments of protoplasm run in the dentine tubules from the tooth cells, which line the pulp cavity and preside over the nutrition of the tooth. The cavity contains vessels, nerves, etc., which enter at the root of the tooth, which is enclosed in a kind of modified bone tissue called crusta petrosa.
Section through a portion of the Fang of a Tooth. (a) Dentine tubules near the surface of the fang; (6) Granular layer; (c) Crusta petrosa.
The two rows of grinding teeth, composed of molars and premolars, of the lower jaw are made to rub against the corresponding teeth in the fixed upper jaw by the combined vertical and horizontal movements induced by the action of the powerful muscles of mastication, the temporal muscles, together with the mas-seters and internal pterygoids, all tending by their contraction to elevate the lower jaw and bring the teeth forcibly together. This action is opposed by the digastric, the genio- and mylo-hyoid muscles, which by their combined force depress the jaw and separate the teeth. The horizontal movements are in the main accomplished by the external pterygoid muscles, which, acting together, pull the lower jaw forward so as to make the lower teeth protrude beyond the upper. In this action they are opposed by the digastric and hyoid muscles. One external pterygoid on either side acting alone, advances that side of the lower jaw only, and thereby causes the lower teeth to incline toward the opposite side in a lateral direction. The two muscles acting alternately cause a horizontal motion from side to side. Thus, while the lower teeth are pressed firmly against the upper ones, they are at the same time made to glide over them, either from side to side or backward and forward. By these movements the bruised food is soon pushed from between the teeth, and passes toward either the tongue or cheek. The morsel is soon replaced between the teeth by the action of the tongue on the one hand and the buccinator muscle in the cheek on the other.
While the process of mastication is going on, the food becomes thoroughly moistened with the fluid secreted within the mouth.