Xlviii INTRODUCTION. 



sively decreases in size, fewer nuclei are develo|3ed in the cells, and 

 these do not acquire so large a size. The diminution in both 

 respects proceeds, however, unequally, in the cells of the same 

 stratum. Here and there the linear tract formed by the nuclear 

 matter in a part of a smaller calcifying cell, containing fewer 

 nuclei, may be observed to unite with the converging extremities 

 of two residuary tracts (arese of dentinal tubes) of a calcified cell 

 in advance (PL I. fig. I, g). It is thus that the bifurcation of the tubes 

 is produced, and a repetition of this confluence, which becomes more 

 frequent as the calcifying process approximates the centre and base 

 of the pulp, gives rise to the dichotomous divisions of the main 

 tubes. In some of the cells at and near the central and basal 

 part of the pulp, the nucleus has undergone no division, but has 

 become merely elongated and sometimes angular or radiated. In 

 others it has disappeared ; such cells occur not unfrequently close 

 to the field of calcification, when the process has made much 

 advance in its centripetal course. The altered mode of action or 

 change in the nuclei of the smaller central cells of the pulp is 

 the first and essential step in the modification of the dentinal tissue 

 which produces the substances which I have termed osteo-dentine 

 and vaso-dentine. In the former many of the cells retain their 

 nucleus undivided, and the hardening salts are impacted around it 

 in the interior of the cell, but enter only partially into the granular 

 substance of the nucleus, in the minutely disgregated form, which 

 produces the opacity and whiteness of the resulting corpuscle. In 

 the formation of vaso-dentine many of the cells lose their nucleus 

 which seems to have become dissolved. In both the latter modi- 

 fications of dental tissue the blood vessels remain, and establish 

 the wide tubular tracts in the calcified substance to which the 

 name of ' vascular canals' is given. In true, hard, or unvascular 



