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Rh difference in consistence between the dental substance and the pulp. There are therefore, at least, reasons enough to warrant 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 thickening 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 observations of Retzius, 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 commence 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 of the pulp becomes elongated into a short minute fibre towards the dental substance, and that these fibres are about as numerous as the tubes projecting upon the surface of the pulp. I