IV INTRODUCTION. 



to the mechanical conditions of the tooth, but to the nutrition of the 

 dentine. 



Dentine, thus organized, is * unvascular': the teeth of most 

 mammals and reptiles, and of a few fishes present this modification 

 of their main constituent. But the dentine in the teeth of most 

 fishes, of a few mammals, and of still fewer reptiles, is traversed by 

 canals containing blood vessels or a vascular pulp ; the tooth- 

 substance, thus modified, I term ' vascular dentine.' Both the 

 ' vascular' and ' unvascular dentine' may be present in the same 

 tooth, as in those of the sloth, the walrus, and the cachalot: 

 the transition from the vascular dentine to true bone is gradual 

 and close. 



' Cement' always closely corresponds in texture with the 

 osseous tissue of the same animal, and wherever it occurs of sufficient 

 thickness, as upon the teeth of the horse, sloth or ruminants, 

 it is also traversed, like bone, by vascular canals. In reptiles and 

 mammals, in which the animal basis of the bones of the skeleton is 

 excavated by minute radiated cells, forming with their contents 

 the ' corpuscles of Purkinje', these are likewise present, of similar 

 size and form, in the ' cement', and are its chief characteristic 

 as a constituent of the tooth. The hardening material of the 

 cement is partly segregated and combined with the parietes of the 

 radiated cells and canals, and is partly contained in aggregated 

 grains in the cells, which are thus rendered opake. 



The relative density of the dentine and cement varies according 

 to the proportion of the earthy material, and chiefly of that part 

 which is combined with the animal matter in the walls of the cavities, 

 as compared with the size and number of the cavities themselves. 

 In the complex grinders of the elephant, the masqued boar and the 



