40 ON DENTAL EXOSTOSIS. 



a well-defined line, and in some places a small chink 

 intervenes between them and the additional deposit. 

 Vascular canals frequently traverse its substance, 

 interfering with the regularity of the layers, and 

 sometimes complete concentric rings are found around 

 the canals, thus bestowing upon the cementum the 

 characteristics of bone. The lacunse are very variously 

 distributed in the different layers, some being quite 

 full of them, whilst in others they are entirely absent ; 

 and they are, moreover, usually larger, and furnished 

 with more numerous and larger canaliculi than 

 those found in the normal cementum. They do not 

 always occur in lines parallel with the layers, but 

 sometimes are to be found in little round clusters, 

 or scattered irregularly in the layer. That appear- 

 ance which has led to the belief tliat they are 

 situate in the centre of cells is frequently to be 

 observed in this cementum of abnormal growth. 

 The hyaline, or interlacunar substance, is by no 

 means homogeneous in appearance, and indeed is 

 much less so than in the natural layer of the cemen- 

 tum, being irregularly streaked across the layers. 

 After removal of the calcareous matter with dilute 

 hydrochloric acid this appearance is still preserved, 

 or indeed more forcibly shown, proving that this 

 . heterogeneous appearance is entirely due to the 

 structure of the cartilage with which the earthy 

 constituents are combined. I may remark that 

 cartilage thus obtained, when attempt is made to 

 tear it, splits up into the component laminae. 



