ON THE DEVELOPMENT OF THE TEETH 237 



and as they run somewhat obliquely it may very readily happen that, 

 unless the focusing of the microscope be very careful, one will run into 

 the other, and so produce the appearance of fibres described by 

 Raschkow. 



This young dentine is as transparent as glass. No trace of 

 " nuclei " can at any time be discovered in it ; the bodies which have 

 been described as such being, as I have said, simply lacuna; ; nor, if 

 strong acids be used so as to dissolve out the calcareous matter, are 

 any nuclei brought to light, though those which exist in the pulp 

 became much more distinct, and even coarse, in their outlines. Again, 

 if to a pulp thus treated, a weak solution of iodine be added, the 

 nitrogenous substance of the pulp is immediately coloured deep yellow, 

 the nuclei themselves becoming brown ; but the dentine remains pale 

 except that here and there a yellow process of the matrix of the pulp 

 may be seen stretching a little way into one of the canals of the 

 dentine. I have only observed this, however, once. I believe that 

 these facts afford sufficient demonstration that the pulp is not 

 converted directly into the dentine, and that the structure of the 

 latter does not depend upon the calcification of pre-existing elements. 



I am the more satisfied with this negative evidence, as in young- 

 bone it is easy to demonstrate the " nuclei " in the lacuna by the aid 

 of acids, &c. 



As to whether the perpendicularly crowded '' nuclei " of the pulp 

 under the dentine disappear, or whether they are merely pressed 

 inwards, I cannot pretend to offer a decisive opinion. The former 

 supposition, however, if we may judge by the analogy of bone, appears 

 more probable. Dentine, in fact, might be considered as a kind of 

 bone, in which the lacunae are not formed in consequence of the early 

 disappearance of the nuclei, whose persistence for a longer or shorter 

 period appears to be the sole cause of their existence in bone.^ 



Still less can the enamel be produced by any conversion of a 

 cellular structure. Between it and anything which can be called a 

 nucleated cell it has on the outer side Nasmyth's membrane ; on the 

 inner, the layer of dentine, which in Man is formed before it. The 

 fibres of which it is composed are structureless, and almost horny ; 



' I have here no space to enter into the discussion of the various hypotheses and assertions, 

 respecting the development of the dentine, made by the various authors whose names I have 

 cited. I trust it will not on that account be supposed that I have neglected to mal<e myself 

 acquainted with them. But there are two statements to which I must refer in confirmation 

 of my own view. The one is that by Dr. Sharpey already quoted : the other is the very just 

 declaration (in italics) by Professor Kolliker (Handbuch, p. 386), that "the most careful 

 investigation exhibits no trace of any elongation of nuclei" in the peripheral cells of 

 the pulp. 



