THE TWO RmGS OF ENAMEL. 59 



scribing a cavity named the external dental cavity, or, 

 better, infundibulum. This cavity terminates by a 

 conical cul-de-sac, which descends more or less deeply 

 into the substance of the tooth. The margins are 

 designated the anterior and posterior. The latter, less 

 elevated than the former, 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 center of which the in- 

 fundibulum persists during a certain period of time. 



" The root is perforated by a single aperture, through 

 which the pulp of the tooth penetrates into the inter- 

 nal cavity. 



"In the composition of the incisor teeth are found 

 the three fundamental substances of the dental organ. 

 The dentine envelops the pulp cavity. Dentine is de- 

 posited in this cavity after the complete evolution of 

 the tooth to replace the atrophied pulp, the yellow tint 

 of which distinguishes it from the dentine of the first 

 formation. It forms on the table of the tooth the 

 mark designated by Girard the dentinal star. 



" The enamel covers the dentine, not only on its free 

 portion, but also on the roots; it does not extend, 

 however, to their extremities. It is doubled into the 

 external dental cavity, lining it throughout; and when 

 the surface of friction is established, a ring of enamel 

 may be seen surrounding it, and an internal ring cir- 

 cumscribing the infundibulum. The first circle is 

 called the encircling enamel, the second the central 

 enamel. In the virgin tooth the central enamel 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 pro- 



