THE TEETH. 67 



it is not the whole pulp which shares in the process, but only 

 its most external, epithelium-like layer of cells, which appears 

 to maintain a constant thickness by the elongation of the 

 original cells, accompanied by a continual multiplication of 

 their nuclei (Mikr. Anat. ii, 2, p. 103, et seq.) I by no means 

 intend to assert that one and the same cell suffices for the whole 

 duration of the development of the dentine, although this is 

 not at all inconceivable j indeed, I consider it possible that the 

 dentinal cells are from time to time replaced by others, which 

 are formed upon their inner surface ; but what I deny is, that 

 the whole pulp is simply changed progressively, from without 

 inwards, into dentinal cells and ossified, and I am of opinion 

 that, like the spongy tissue of the enamel organ, the only 

 import of the pulp in the development of the dentine is to 

 support the vessels which are necessary to enable the dentinal 

 cells to grow at all. 



The diminution of the pulp, therefore, is very readily intel- 

 ligible without supposing it to be ossified from without in- 

 wards; 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 den- 

 tinal 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, become 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 elon- 

 gated and coalesced nuclei of the dentinal cells whose cavity 

 persists ; or finally, they are the result of a process of resorp- 

 tion in the primarily homogeneous, dentinal 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, ap- 

 pear 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 pro- 



