362 



ANATOMY OF VERTEBRATES. 



239 



Section of tooth of Cachalot, half natural 

 size. v. 



manently carry red blood into the 

 substance of the tissue. These 

 so-called ' medullary ' or ' vascu- 

 lar' canals present various dis- 

 positions in the dentine which, 

 they modify, and which is called 

 e vaso-dentine.' It is often com- 

 bined with true dentine in the 

 same tooth ; e.g. in the scalpri- 

 form incisors of certain Rodents, 1 

 the tusks of the Elephant, 2 the 

 molars of the extinct Megathe- 

 rium, fig. 238, v. 



A third kind of dentine is 

 where the cellular basis is ar- 

 ranged in concentric layers around 

 the vascular canals, and contains 

 ' radiated cells ' like those of the 

 osseous tissue : it is called ' osteo- 

 dentine,' fig. 239, o. The transi- 

 tion 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. 



The chemical composition of 

 teeth is exemplified in the sub- 

 joined analyses of those organs 

 and their tissues from species of 

 the different vertebrate classes : — 



V. p. 405. 



p. 643. 



