DEFINITION OF DENTAL TISSUES. 233 



in the large incisors of certain rodents, the tusks of the 

 elephant, the molars of the extinct iguanodon. 



A second modification of the fundamental tissue of the 

 tooth is where the cellular basis is arranged in concen-' 

 trie layers around the vascular canals, and contains 

 "radiated cells" like those of the osseous tissue ; it is 

 called " osteo-dentine." The transition from dentine to 

 vaso-dentine, and from this to osteo-dentine, is gradual, 

 and the resemblance of osteo-dentine to true bone is very 

 close. 



" Cement" always closely corresponds in texture with 

 the osseous tissue of the same animal ; and wherever it 

 occurs of suf&cient thickness, as^upon the teeth of the 

 horse, sloth, or ruminant, 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, these are likewise present, of 

 similar size and form, in the cement, and are its chief 

 characteristic as a constituent of the tooth. 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 num= 

 ber 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" is the hardest constituent of a tooth, 

 and consequently the hardest of animal tissues; but it 

 consists, like the other dental substances, of earthy mat- 

 ter, 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 



20* 



