THE INCISORS. 615 



D. Details of the Dental Table according to the Age. 



These details are dependent, in an intimate manner, upon the form 

 and the structure of the incisors. We will study them from the period 

 of the eruption of the teeth until very advanced age. 



1st. Primitively the free extremity is occupied by the external dental 

 cavity, circumscribed by the central enamel, almost conical in form, oj)lique 

 from before to behind, terminating in a cul-de-sac, and containing the 

 central cement, which, however, does not completely fill it. 



The border which limits it anteriorly is higher than the posterior. 



2d. At the end of a certain time the two borders of the central 

 enamel are placed on a level with each other from the effects of wear. 

 The external cavity, less deep, no longer occupies, as it did before, the 

 whole of the free extremity ; it appears more removed from the ante- 

 rior border on account of the direction of its great axis, which is oblique 

 backward. 



The layers of enamel, in consequence of this modification, present 

 a new disposition, for the enamel now forms two distinct parts (Fig. 

 287) : the one, E, which always circumscribes the external surface of 

 the tooth, receives the name peripheral enamel ; the other, E f , which 

 surrounds what remains of the external cavity, is called central enamel. 

 The latter is the less thick of the two ; it, as well as the peripheral 

 enamel, stands in relief above the dentine and in relation \vith the 

 cement which fills the external dental cavity. 



3d. When this cavity has entirely disappeared, there only remains 

 a small " island" of cement, limited by the central enamel. But the 

 configuration and location of this island do not remain invariable. It 

 diminishes little by little in area, tends to become round, and approaches 

 more and more the posterior border of the table, in consequence of the 

 disposition, the direction, and the length of this cone of enamel. 



A levelled incisor tooth is characterized by the disappearance of its 

 external cavity. 



All are agreed in this respect. However, after what has been said 

 concerning the structure, when the wear has reached this point, there 

 still exists a more or less considerable part of the external enamel cup, 

 whose bottom, filled with cement, does not present, as is believed, a solid 

 point prolonged far beyond into the thickness of the dentine. (See Fig. 

 285.) The presence of the central enamel, after the tooth is levelled, 

 furnishes an important indication for the determination of the age. 



About the same time a spot of a deeper yellow appears in the 

 middle of the dentine, between the anterior border of the table and the 



