THE TEETH. 493 



and the enamel organ, and commences, even before the eruption of the 

 teeth, contemporaneously with the formation of their fangs. About 

 this time the dental sac elongates inferiorly, applies itself to the grow- 

 ing fang, yields, from its abundant vascular network, a soft blastema, 

 in which nucleated cells are developed, and then ossification takes place. 

 The cement, therefore, is not formed by the ossification of the sac itself. 

 I met with the first traces of it in newly-born infants, in the form of 

 isolated, elongated, or rounded scales, which were firmly attached to 

 the dentine of the, as yet, very short fang, and looked exactly like the 

 developing osseous substance in the cranial bones ; the smallest exhi- 

 bited distinct osseous lacuna', and a faint yellow tinge, but were quite 

 soft and transparent, passing at their edges into a clear cellular blas- 

 tema ; in the larger ones, the margins were similar, but the centre was 

 darker and firmer, and in this way every stage of transition to actual 

 bone was presented, without any granular deposit of calcareous matter. 

 With the elongation of the fang, new osseous scales of this kind were 

 formed and gradually coalesced from above downwards into a single 

 layer, to which continual additions were made from without, until the 

 whole thickness of the cement was produced. 



I am unacquainted with the manner in which the Nasmyth's mem- 

 brane is produced. No structureless layer exists upon the enamel 

 organ, by the ossification of which it might be supposed to be formed, 

 and therefore I should be inclined to regard it as a calcified, amorphous 

 exudation secreted from the enamel organ immediately after the ossifi- 

 cation of the last enamel cells, which glues together and protects the 

 ends of the prisms of the enamel. 



If we now, in conclusion, take a general view of the different struc- 

 tures in the teeth and their mutual relations, we perceive that althougli 

 they agree in certain respects, yet they cannot be brought under one 

 class. Dentine and cement are much more closely allied to one ano- 

 ther, than to enamel, and should it prove to be correct that the den- 

 tinal canals are formed by the coalescence of the cavities of thickened, 

 elongated cells, the dentine will correspond Avith an osseous tissue, 

 whose matrix is constituted only by the thickened walls of the original 

 cells, and whose lacunce are all directly connected. Cement, or bone 

 and dentine, often have a very close external resemblance to one another, 

 particularly, on the one hand, when the latter is traversed by numerous 

 Haversian canals, and, as Retzius believes he has observed, contains 

 osseous lacunar ; and, on the other hand, when, in bone, the lacuna? are 

 either greatly elongated, with numerous canaliculi, vascular canals also 

 existing; or when with few lacunce, i\\Q canalicuU are numerous and 

 parallel, like dentinal canals. This much is certain, that the two sub- 

 stances never become exactly alike, and it is probable that their develop- 

 ment is always to a certain extent different. 



