S18 



THE TEETH. 



afterwards elongates and becomes narrower, so as to serve as the basis 

 of the fang. Sooner or later, after the completion of the crown, this 

 ]iart of the tooth appears through the gum, whilst the growth of 

 dentine to complete the fang is continued at the surface of the 

 elongating pul}>, which gradually becomes encroached upon l)y suc- 

 cessive formations of hard substance, until only a small cavity is left 

 in the centre of the tooth, containing nothing but the reduced pulp, 

 supplied by slender threads of vessels and nerves, which enter by a 

 small aperture left at the point of the fang after the dentine is com- 

 pleted. In the case of teeth having complex crowns and more than 

 a single fang, the process is somewhat modified. On the surface of the 

 dental pulp of such a tooth, as many separate caps or shells of dental 

 substance are formed as there are eminences or jioints; these soon 

 coalesce, and the formation of the tooth proceeds as before as far as the 

 cervix. The ]3ulp then becomes divided into two or more portions, 

 corresponding with the future fangs, and the ossification advances in 

 each as it does in a single fang ; while, at the same time, a horizontal 

 projection or bridge of dentine is deposited across the l^ase of the pulp, 

 between the commencing fangs, so that if the tooth be removed at this 

 stage and examined on its under surface, its shell presents as many 

 apertures as there arc se]mrate fangs (fig. 210, 3 and 4). In all teeth, 

 the pulp originally adheres by its entire base to the bottom of the sac; 

 but, when more than one fang is to be developed, the pulp is, as it were, 

 separated from the sac in certain parts, so that it comes to adhere at 

 two or three insulated spots only, corresponding with the fangs, whilst 

 the dentine continues to be formed along the surrounding fi'ee surface 

 of the pulp. 



Formation of tlie hard tissues of the Teeth. — The account 



already given of the structure of 

 Fig. 220. the permanent pulp of a tooth will 



ap[)ly also to that of the papilla 

 or formative pulp of the growing 

 tooth, both before and after the 

 dentine has begun to be formed 

 from it. 



It may be added, however, that its 

 capillary vessels, which form a series 

 of loops a short distance beneath the 

 siu'face, are much more abundant op- 

 liosite the point or points where cal- 

 cification is about to commence. 



Fig. 220.— Part of Section of Dkvelop- 

 iNO Tooth of Young Rat, Showing the 

 Mode of Deposition of the Dentine. 

 Highly Magnified. 



The dentine is produced more 

 immediately by the elongated cells 

 (odontoblasts) already described as 

 forming the superficial stratum of 

 the puip (fig. 220, c). These cells 

 send out from their free extremi- 

 ties filamentous ])rocesses, as de- 

 scribed by Lent, and the intercel- 

 lular substance of the pulp tissue 

 between and around these pro- 

 cesses becomes changed into the solid matrix or intertubular substance 



a, outer layer of fully formed dentine ; 

 h, uncalcified matrix, with cue or two 

 nodules of calcareous nuitter near the cal- 

 ciiied part ; c, odontohla.sts sending pro- 

 cesses into tlie dentine ; d, iiulj). The sec- 

 tion is stained with carmine, which colours 

 the uncalcified matrix, but not the calciiied 

 part. 



