306 DEVELOPMENT OF TISSUES OF THE TOOTH. [sect. 141. 



Fig. 131. 



which membrane, so long as the dentine is not fully formed, is continued into 

 the mcmbrana prceformativa of the tooth-pulp, and thus gives rise to the 

 ajjpearance as if the enamel were developed beneath the mcmbrana praforma- 

 tiva. If we assume, for the moment, that this is correct, the following seem 

 to be the only admissible explanations of the formation of the enamel : 



1. The enamel-fibres are produced by a secretion of the cells of the enamel- 

 membrane, which penetrates the mcmbrana prceformativa in a fluid condition, 

 but hardens and ossifies beneath it. 



2. The enamel-fibres are formed by the dentine from an exsudation furnished 

 by the dentinal canals. Of these two possibilities, the second is of such a 

 nature, that I at least will not venture to enter further upon it, seeing that 

 according to this view, the formation of regular fibres in the enamel, its 

 growth in thickness by means of the apposition of new layers upon its outer 

 surface, are facts wholly unexplained ; at the same time the former assump- 

 tion, although the more probable, is not without many difficulties (see my 

 Handbook, second German Edition). 



As in the formation of the enamel, so in that of the 

 dentine, the whole of the pulp is not concerned, but only 

 the outermost epithelial-like cell-layer of it, and I deny 

 that the whole of the pulp is simply converted from 

 without inwards into dentinal cells and ossified, but think 

 it far more probable, that, like the spongy body of the 

 enamel-organ, it serves for the formation of the dentine, 

 only by supporting the vessels, which are necessary for the 

 growth of the dentinal cells. Its decrease is also very readily 

 conceivable without assuming an ossification of it from 

 without inwards, and, like the decrease of the contents 

 of the wide Haversian canals of foetal bones during the 

 formation of lamella? upon the walls of the canals, is 

 simply owing to a gradual absorption of its soft tissue ; 

 in which explanation there is no need to assume that its 

 vessels undergo a retrogressive process, in any consider- 

 able degree. 



With regard to the formation of the dentine from the 

 dentinal cells, it is certain, that no other tissue than the 

 cells contributes anything to the result, and Hvxley is 

 decidedly wrong in maintaining that no histological 

 element of the pulp is concerned in the formation of the 

 tooth. The manner in which the cells in question are 

 transformed, after remaining so long in doubt, appears, at 

 length, to have been really cleared up by the investi- 

 gations of Lent. Some years ago, I discovered upon the 

 human dentinal cells filamentous processes extending into the young dentine, 

 which I supposed to be dentinal canals ; still I was not at that time able to 

 raise this supposition to a certainty. This has been done by Lent, who has 

 succeeded in isolating the cells in question with complete dentinal canals 

 prolonged from them, in growing teeth which were macerated to disinte- 

 gration in hydrochloric acid. This once established, the formation of 

 dentine must, as Lent concludes, be conceived to take place in the following 

 manner : — 



h 



Isolated dentinal 

 cells with processes, 

 i.e., dentinal tubes. 

 a. of man ; b. of the 

 horse. After Lent. 



