THE TEETH. 491 



only import of the pulp in the development of the dentine is to sup- 

 port the vessels which are necessary to enable the dentinal cells to grow 

 at all. 



The diminution of the pulp, therefore, is very readily intelligible 

 without supposing it to be ossified from without inwards ; it takes place, 

 like the diminution of the contents of the wide Haversian canals of 

 foetal bones when the lamellae are deposited upon their walls, by a 

 gradual resorption of its tissue, which, as in the latter case, is soft and 

 full of juices; and it is by no means necessary to suppose any extensive 

 retrogressive metamorphosis of its vessels. 



With regard to the formation of the dentine from the dentinal cells, 

 it is certain that no other tissue than these cells contributes anything to 

 its development and that they, like those of the enamel membrane, be- 

 come dentine by the gradual reception of calcareous salts. The dentinal 

 tubules are either the remains of the cavities of the dentinal cells, whose 

 walls, in the course of ossification, thicken and harden into them, but 

 do not quite close, or they are developed from the elongated and coalesced 

 nuclei of the dentinal cells whose cavity persists ; or finally, they are 

 the result of a process of resorption in the primarily homogeneous den- 

 tinal tissue, analogous to the formation of the Haversian canals, or of 

 those in the cement. Of these three hypotheses, the second would, at 

 first sight, appear the most probable, if we consider that the dentinal 

 tubules may be isolated, with distinct walls, that the dentinal cells are 

 abundantly provided with nuclei, and that certain filiform prolongations 

 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 ; but there 

 is one very remarkable fact, that no trace of any elongation of the nuclei 

 can be discovered ly the most careful investigation. The third hypo* 

 thesis is indeed conceivable, but in opposition to it, we find that pores 

 and canals exist even in the youngest and softest dentine, w r hen the de- 

 velopment of the tooth is at all advanced, and therefore, that they can 

 hardly be regarded as secondary formations. In favor of the first sup- 

 position, on the other hand, it may be said, that it would, if true, indi- 

 cate 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 lacunce. Certain objections may be urged, which are not, per- 

 haps, 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 deve- 

 loped out of peculiar vesicular structures, either nuclei or cells; and 

 secondly, that upon this supposition, the filamentous appendages to the 

 dentinal cells are not so readily interpreted. But as regards the former, 

 we have recently learnt that the osseous lacunce and canaliculi may also 



