70 SPECIAL HISTOLOGY. 



The cement, therefore, is not formed by the ossification of the 

 sac itself. I met with the first traces of it in newly-born in- 

 fants, 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 exhibited distinct osseous 

 lacunae and a faint yellow tinge, but were quite soft and trans- 

 parent, passing at their edges into a clear cellular blastema ; 

 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 

 membrane 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 ossification 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 differ- 

 ent structures in the teeth and their mutual relations, we 

 perceive that although they agree in certain respects, yet 

 they cannot be brought under one class. Dentine and cement 

 are much more closely allied to one another, than to enamel, 

 and should it prove to be correct that the dentinal canals are 

 formed by the coalescence of the cavities of thickened, elongated 

 cells, the dentine will correspond with an osseous tissue, whose 

 matrix is constituted only by the thickened walls of the original 

 cells, and whose lacunae are all directly connected. Cement, 

 or bone, and dentine, often have a very close external resem- 

 blance 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 lacunae; and, 

 on the other hand, when, in bone the lacunse are either greatly 

 elongated, with numerous canaliculi, vascular canals also ex- 



