THE DENTAL TISSUES. 85 



in teeth of perpetual growth, such as the incisors of rodents, 

 the axial portion of the tooth is that latest calcined, and 

 consists of a dentine containing vascular canals, which are 

 not present in the other part of the tooth. When a change 

 thus occurs in the character of the tissue formed at a later 

 time than the rest of the dentine, the name "secondary 

 dentine " is applied to the resultant tissue. 



But secondary dentine may partake of several different 

 varieties of structure, so that the term must not be taken as 



FIG. 44 t 1 ). 



denoting anything more than the circumstances under which 

 it was formed. 



It is in the class of Fish, in which vaso-dentine is rather 

 common, that the most instructive examples of its nature 

 are to be found. 



The conical teeth of the common Flounder, and indeed of 

 most flat fish (Pleuronectidce) have their pointed tips formed 



( T ) Tooth of a Flounder, a, Dentinal tubes near apex of tooth ; 6, 

 Vascular canals ; c, Spear points of enamel. 



