TEETH. 317 



Some sections of dentine which I have examined have ex- 

 hibited numerous reticulated markings, the results of fracture 

 of the intertubular tissue, and occasioned probably by the 

 preparation of the section. Fracture of the dentine is capable 

 of reunion. 



Cementum. Of the cementum it will not be necessary to 

 say very much, it possessing the structure of bone and con- 

 taining both bone cells and Haversian canals; the latter, 

 however, but seldom. (See the figures.) 



The quantity of cementum differs in different teeth ; in 

 many cases it is very inconsiderable, but it usually increases 

 with age. 



In those cases in which there is but a slight development 

 of cementum, a layer of considerable thickness formed of 

 numerous, more or less hexagonal cells, and extending over 

 the whole of that portion of the dentine not covered by 

 enamel, may in most cases be clearly seen. 



This layer Mr. Tomes speaks of as a granular layer ; it is, 

 however, distinctly and regularly cellular. It is not easy 

 to decide whether it should be regarded as a distinct and 

 permanent structure of the tooth, or whether it merely forms 

 the basement substance in which the cementum is deve- 

 loped ; my own impressions incline to the former view.* 



A layer of granules, having the aspect of imperfectly 

 developed bone cells, is usually situated apparently between 

 the dentine and cementum, but really in the substance of 

 one or other of these ; this layer might well be called the 

 granular layer, and to it the description of Mr. Tomes seems 

 more applicable. 



Mr. Nasmyth f describes the cementum as passing over the 



* The following is Mr. Tomes's description of this layer : "In the in- 

 tertubular tissue hemispherical or elliptical cells are found, especially near 

 the surface of the dentine of the fang, where they form a layer joining 

 the cement. This in a paper read before the Royal Society I described 

 as the, granular layer ; on the coronal surface of the dentine they are not 

 numerous. With these cells the dentinal tubes communicate, as do 

 others coming from the cemental cells." 



f Memoir read before the Medico-Cli inimical Society by Alexander 

 Nasmyth, Jan. 22d, 1839. 



c c 4 



