TISSUES OF THE BODY. 



271 



group. This is converted into dentine by a diffuse calcification similar 

 to that of the so nearly allied osseous tissue ; while the walls of the 

 canaliculi of the teeth are 

 formed by the bounding / j 



laminae of the mass sur- 

 rounding the fibres of 

 Tomes. 



The following sketch 

 may be accepted as toler 

 ably faithful as far as we 

 are acquainted with the 

 course of development, so 

 difficult to follow. 



Young dentine cells (fig. 

 259, b; fig. 260) present 

 themselves as membrane- 

 less nucleated structures 

 closely crowded together, 

 and of irregular jagged 

 form, and united one 

 with another by means 

 of short processes. Exter- 

 nally, they send off single 

 or multiple prolongations, 

 which form interlacements 

 by means of side branches 

 with the processes of adja- 

 cent cells. The odonto- 

 blasts eventually become 

 longer and narrower, and the more peripheral portions of their processes 

 attain a considerable length. Thus are formed the soft fibres of Tomes. 



The calcification already mentioned commences at the apex of the 

 dental germ, in the tissue just described, in the form of a single, or 

 frequently, at first, of several separate thin plates. This plate has been 

 named the "dental cup" (Zahnscherbchen) (fig. 259, c). On the further 

 superficial extension of this structure the calcified layer spreads down 

 over the sides of the dental germ, in which, with the commencement 

 of calcification, the vascular network has already reached its fullest 

 development. 



But, owing to the continued production of the fibres of Tomes, of 

 the canaliculi and the ground-substance, through the agency of the 

 still soft ivory cells below the dental cup, and the progressive calcifica- 

 tion of the ground-substance, the thickness of the dental germ decreases 

 more and more, although it has grown considerably in length. 



This increase in length leads eventually to the formation of the root, 

 which is changed into ivory exactly in the same way as the crown, and 

 becomes calcified externally. 



The production of the cement commences before the passage of the 

 teeth through the gums, as soon as the root is developed. But the bony 

 mass in this case arises, it is supposed, from a growth of the inferior 

 portion of the dental sac, the latter becoming converted into osteogenic 

 substance, as in the growth of the periosteum, and undergoing diffuse 

 calcification. Osteoblasts and bundles of connective-tissue are also to be 



itf. 260. Dentine cells after Lent. At a and 6, simple filiform 

 processes, which become converted into canaliculi: c, d, 

 specimens of the latter with branches; e, fusiform cell; and 

 /, one of the latter undergoing division. 



