Fig. 158. 



166 v///; i>n;w !\-i-: APPARATUS /.\ 



two borders, of which the internal is always thicker than the cxte: 

 lastly, the surface of friction (table). The latter d t in the 



\\hich has not been used; but in its stead is found two sharp margins 

 circumscribing a cavity named the external <li-ittl i-nrihj (or better, in- 

 fundibulum). This cavity terminates by a conical <///-,/,-.,,, which 



descends more or less deeply into the sub- 

 stance of the tooth. The margins are dis 

 tinguishcd into anterior and posterior; the 

 last, loss elevated than the first, is cut by on 

 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 infundibulum 

 persists during a certain period of time 

 (Fig. 159, 2). 



The fang is perforated by a single aper- 

 ture, through which the pulp of the tooth 

 penetrates into the internal cavity (Fig. 

 159, 3, c). 



In the composition of the incisor teeth is 

 found the three fundamental substances of 

 the dental organ. The dentine (Fig. 159, 3, 6) 

 envelops, as has been shown, the pulp cavity. 

 That which is deposited in this cavity after 

 the complete evolution of the tooth, to replace 

 the atrophied pulp, has always a yellower 

 tint than the dentine of the first forma- 

 tion ; it forms on the table of the tooth the 

 mark designated by Girard the dftitul xt<n- 

 (Fig. 159, 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. It 

 is doubled into the external dental cavity, 

 lining it throughout (Fig. 159, 3, 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 fn<-ir<-l!it<i 

 enamel; the second, the cent ml nunm-l (Fig. 

 159, 4, a, I). 



In the virgin tooth, the latter is con- 

 tinuous with the external enamel, and : 

 over the border which circumscribes the 

 entrance to the infundibuluni. The <-i /< nl 



n..,sult ii.,n"-ji"fr"theiu.|,titi.,nf >'s applied over the enamel like sl protecting 

 the upper jaw. varnish ; but it docs not exhibit the same 



thickness every where : on the salient portions 



it is extremely thin, and does not even exist when the tooth 1ms been 

 submitted for some time to the friction arising from the contact of 

 the aliment, the lips, and the tongue. It is more abundant in d' pi 

 situations, as in the longitudinal groove on the anterior face, and particularly 



TIII: DiATinoN 01 Tin: IXFKRIOR 



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