360 



ANATOMY OF VERTEBRATES. 



236 



&C^ S 





colourless fluid, probably transuded 'plasma': they thus relate 

 not only to the mechanical conditions of the tooth, but to the 

 vitality and nutrition of the dentine. This tissue has few or no 

 canals large enough to admit capillary vessels with the red 



particles of blood, and it has been 

 therefore called ' unvascular dentine.' 

 ' Cement ' always closely corres- 

 ponds 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 

 ruminant, it is also traversed, like 

 bone, by vascular canals, fig. 236, c- 

 When the osseous tissue is excavated, 

 as in dentigerous Vertebrates above 

 fishes, by minute radiated cells, form- 

 ing with their contents the ' cor- 

 puscles of Purkinje,' fig. 15, 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 disgregated granules in 

 the cells, which are thus rendered 

 white and opaque, viewed by reflected 

 light. 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 masked 

 boar, and the capybara, the cement, which forms nearly half the 

 mass of the tooth, wears down sooner than the dentine. 



The ' enamel,' fig. 235, e, is the hardest constituent of a tooth, 

 and, consequently, the hardest of animal tissues ; but it consists, 

 like the other dental substances, of earthy matter arranged by 

 organic forces in an animal matrix. Here, however, the earth is 

 mainly contained in the canals of the animal membrane ; and, in 

 mammals and reptiles, completely fills those canals, which are com- 

 paratively wide, whilst their parietes are of extreme tenuity. The 



a 





Mas-nifieil section of incisor, Horse ; 

 c cement, d dentine, e enamel, v. 



