IVORY OF THE TEETH. 105 



difference in consistence between the dental substance and the 

 pulp. There are therefore, at least, reasons enough to war- 

 rant our entering more particularly into the details of this 

 opinion. The pulp accords with all the other tissues of 

 the foetus, therefore with cartilage, in being composed of 

 cells : the difference between its consistence and that of the 

 cartilage of mammalia, depends on this, that the quantity of 

 cytoblastema (to which the latter owes its hardness) is very small, 

 for the cylindrical cells of the pulp lie quite close together, 

 at least such is the case on its surface. In this respect, the 

 pulp bears a closer analogy to certain cartilages of animals 

 lower in the scale, in which there is also only a small quantity 

 of cytoblastema present, and the consistence of the cartilage is 

 principally occasioned by thickening of the cell-walls. As I 

 have not actually observed the transition, I do not know 

 whether the filling up of the cavities also takes place by thick- 

 ening of the cell- walls, in this supposed conversion of the cells 

 of the pulp into the dental fibres. If such be really the case, 

 the cavities of the cells are in general so completely obliterated 

 by it, that no cartilage-corpuscles remain. From the observa- 

 tions of Ketzius, however, it might be supposed that some of 

 the cells retain their cavities, and even become transformed 

 into stellated cells ; for he saw true osseous corpuscles in the 

 dental substance. When the uppermost stratum of the pulp 

 consisting: of cylindrical cells has become converted into dental 

 substance by ossification, the round cells lying immediately 

 next beneath it in the parenchyma of the pulp, must first com- 

 mence their transformation into cylindrical cells, the vessels of 

 the stratum must become obliterated, and then this stratum 

 ossified, and so on. 



"What, then, are the dental tubes ? Retzius compares them 

 to the calcigerous canaliculi of bone which issue from the 

 osseous corpuscles, and I was myself at first of that opinion ; 

 for I regarded them as prolongations of cells, the bodies of 

 which lay in the pulp. For, when the pulp is drawn out 

 from the cavity of a pig's tooth, and its margins examined, 

 it will be seen that each of the cylindrical cells of the surface 



w 



of the pulp becomes elongated into a short minute fibre towards 

 the dental substance, and that these fibres are about as nume- 

 rous as the tubes projecting upon the surface of the pulp. 1 



