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 feetus, 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 le 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, 1 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 Retzius, however, it might be supposed that some of 
the celis retain their cavities, and even become transformed 
into stellated cells; for he saw true csseous 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 lymg 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 calecigerous 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 
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. I 
