236 ON THE DEVELOPMENT OF THE TEETH 



The dentinal substance, that is, is deposited within the pulp 

 beneath the membrana preformativa in definite masses (Raschkow 

 calls them fibres, to which, indeed, under a low power they have a 

 remarkable resemblance), the gaps between which eventually constitute 

 the dentinal tubules. This, if a name be wanted, might be called the 

 Deposition Theory, and is especially characterized by its asserting 

 that the histological elements of the pulp do not enter as such into 

 the dentine. The following description of the young dentine in the 

 human subject holds good for all the animals which I have examined ; 

 and if it be true, I think the incorrectness of the Conversion Theory 

 necessarily follows. 



To justify my own method of procedure, however, I am necessitated 

 to remark that I have been unable to verify the statement of Pro- 

 fessor Owen (/. c. Introduction, p. xxxix,), that the teeth of Man 

 " will not yield a view of the cap of new-formed ivory and the sub- 

 jacent pulp in undisturbed connexion by transmitted light with the 

 requisite magnifying power." On the contrary, I have found it 

 sufficiently easy, by cutting off the half-ossified cusp of a young 

 molar, or even by submitting an entire canine or incisor to slight 

 pressure, to obtain a most distinct view of the pulp in undisturbed 

 connexion with the dentine, and in a profile view. Indeed, had other 

 observers adopted this method, I do not think they would have been 

 led to consider the lacunae in young dentine, whose true nature was 

 demonstrated by Raschkow, as metamorphosed nuclei of the pulp. 



When the ossifying boundary of a tooth-pulp is examined in the 

 way which I have here pointed out, it is seen that where dentification 

 has not begun, the membrana preformativa is in immediate contact 

 with the substance of the pulp, composed of a homogeneous trans- 

 parent base, in which closely-arranged " nuclei " are embedded. These 

 are rounded or polygonal, apparently vascular ; contain one or more 

 granules, and are about i-2500th — i-3500th of an inch in diameter. 

 Passing to\\'ards the ossifying edge, we see in the profile view a 

 clear, more strongly refracting layer, gradually increasing in thickness, 

 which begins to separate the proper substance of the pulp from the 

 membrana preformativa. This is at first quite structureless to all 

 appearance, both in this view and in one perpendicular to its surface. 

 When it has attained a thickness of i -2500th of an inch, however, it 

 acquires a sort of mottled appearance in the profile view, while super- 

 ficially numerous very minute irregular cavities, about i -5000th of an 

 inch apart, present themselves (fig. 5). In a thick portion of the 

 dentine (3-5000ths) these cavities are very readily seen in the profile 

 view to be elongated into canals ; superficially they are rather larger ; 



