THE MOUTH. 417 



replace the surfaces worn off by friction ; so that the crown is formed suc- 

 cessively by the various portions of the fang, each of which issues in its turn 

 from the alveolar cavity. 



Incisors.— These are so named because they serve, particularly in the 

 Herbivora, for the incision (incido, to cut) of the food. They are arranged 

 in the segment of a circle, at the extremity of the jaw, and are distinguished 

 by the names of middle or central, intermediate or lateral, and corner teeth. The 

 pincers are tlic two middle teeth, the inter juediatea the next, and the corners 

 occupy the extremities of the incisive semicircle. 



The general form of these teeth is that of a trifacial pyramid, presenting 

 a curve with its concavity towards the mouth. Tiie base of this pyramid, 

 formed by the crown, is flattened before and belli nd ; the summit, or ex- 

 tremity of the fang, is, on the contrary, depressed on both sides ; the shaft 

 of the pyramid offers, at different points of its height, a series of intermediate 

 conformations which are utilized as characteristics of age, the continual pushing 

 outwards of the teeth bringing each of them in succession to the frictional 

 surface of the crown (Fig. '2'.'>2, 1). 



Examined in a young tooth which has completed its evolution, the free 

 portion exhibits : an anterior face, indented by a slight longitudinal groove, 

 which is prolonged to the root ; a posterior face, rounded from side to side ; 

 two borders, of which the internal is always thicker than the external ; lastly, 

 the surface of friction (fable). The latter does not exist in the tooth which has 

 not been used ; but in its stead are found two sharp margins circumscribing a 

 cavity named the external dental cavity (or better, infundibidum). This cavity 

 terminates in a conical cal-de-sac, which descends more or less deeply into the 

 substance of the tooth. The margins are distinguished into anterior and pos- 

 terior ; the last, less elevated than the first, is cut by one or more notches which 

 are always deepest in the corner teeth. It is by the wear of these margins that 

 the surface of friction is formed, and in the centre of which the infundil)ulum 

 persists during a certain period of time (Fig. 232, 2). 



The fanii is perforated by a single aperture, through which the pulp of the 

 tooth penetrates into the internal cavity (Fig. 231, 3, c). 



In the composition of the incisor teeth is found the three fundamental sub- 

 stances of the dental organ. The dentine (Figs. 231, i ; '2d->, 3) envelops, as has 

 been shown, the pulp cavity. That which is deposited in this cavity after the com- 

 plete evolution of the tooth, to replace the atrophied pulp, has always a yellower 

 tint than the dentine of the first formation ; it forms on the table of the tooth 

 the mark designated by Girard the denial star (Fig. 232, 4, c). The enamel 

 covers the dentine, not only on its free portion, but also on the roots of the 

 incisors : it is not prolonged, however, to their extremities, though it is more on 

 the anterior than the posterior face. It is douljled in the external dental cavity, 

 lining it throughout (Figs. 231 ; 232, 4, a) ; and when the surface of friction is 

 established, there can be perceived a ring of enamel surrounding that surface, 

 and an internal ring circumscribing the infundibulum : the first circle forms 

 what is called the encircling enamel; the second, the central enamel (Fig. 232, 

 4, a, h). 



In the virgin tooth, the latter is continuous with the external enamel, and 

 passes over the border which circumscribes the entrance to the infundibulum. 

 The cement is applied over the enamel, like a protecting varnish ; but it does 

 not exhibit the same thickness everywhere : 0:1 the salient portions it is 



