68 SPECIAL HISTOLOGY. 



longations of the dentinal cells which I have noticed (figs. 9 and 

 197, and Mikr. Anat., II. 2, p. 105), might be regarded as 

 elongated nuclei j but there is one very remarkable fact, that 

 no trace of any elongation of the nuclei can be discovered by the 

 most careful investigation. The third hypothesis is indeed 

 conceivable, but in opposition to it, we find that pores and 

 canals exist even in the youngest and softest dentine, when 

 the development of the tooth is at all advanced and there- 

 fore, that they can hardly be regarded as secondary formations. 

 In favour of the first supposition, on the other hand, it may 

 be said, that it would, if true, indicate a close agreement 

 between dentine and osseous tissue, structures which are in 

 every case nearly allied, inasmuch as the dentinal tubules 

 would be homologous with long and narrow, simple or possibly 

 coalesced, osseous lacunae. Certain objections may be urged, 

 which are not, perhaps, so important as they at first appear. 

 These are, in the first place, that the dentinal canals have 

 special walls and may be isolated as tubes, which might be 

 regarded as demonstrative evidence that they are developed 

 out of peculiar vesicular structures, either nuclei or cells ; 

 and secondly, that upon this supposition, the filamentous ap- 

 pendages to the dentinal cells are not so readily interpreted. 

 But as regards the former, we have recently learnt that the 

 osseous lacuna and canaliculi may also be isolated, with special 

 walls which are not those of the original cells, and the same 

 is true of the Haversian canals, whence it would be con- 

 ceivable that the walls of the dentinal canals also, although 

 originally and genetically not special structures, might even- 

 tually become so. Since, again, the processes of the dentinal 

 cells may be nothing else than the still soft part of the cells in 

 which ossification is commencing, this first hypothesis may be 

 regarded as having a certain claim to consideration, the more 

 so as the osseous lacuna in the teeth, frequently assume forms, 

 resembling those of the dentinal canals, often communicate 

 with them, and, at least in animals, are interposed among 

 them. 



To sum up, it may be said, that in any case, the matrix of 

 the dentine proceeds from the cylindrical cells investing the 

 pulp of the tooth, which undergo a greater or less elongation, 

 coalesce, and ossify. The dentinal canals either arise from the 



