THE SKELETON A1STD THE TEETH. 



33 



is, phosphate and carbonate of lime and other substances, 

 does not take place uniformly throughout the whole extent 

 of the pre-existing tissue, but only at certain points, called 

 centers of ossification, and from these points the ossification 

 or bone-making extends, so as, at length, to make complete 

 the whole bone. 



The mouth of the Vertebrates has two jaws ; and, except- 

 ing in the Birds and Turtles, these, with comparatively 

 few exceptions, are armed with teeth, which may be 

 noticed here, although teeth are not regarded as really a 

 part of the true skeleton (see p. 55). 



The teeth of vertebrate animals are bone-like in their 

 general appearance. But, although they are in part com- 



Fio. 32. FIG. 33. FIG. 34. 



Enamel Fibers of Human 

 tooth. Mag. 350 diam- 

 eters. 



Surface of the enamel, 

 with the ends of the 

 enamel fibers. Calf. 

 Mag. 350 diameters. 



Longitudinal Section of a Molar 



tooth of Man. Enlarged, 

 o, enamel; b, pulp-cavity ; c, ce- 

 ment; d, ivory or dentine. 



posed of bony material called cement, they are mainly 

 composed of substances called dentine and enamel, the 

 dentine being the chief constituent. 



Dentine or ivory differs from bone mainly in containing 

 less animal matter, in being denser, and in having no lacuna' 

 or canaliculi. Dentine has exceedingly fine tubes for the 

 passage of the plasma, that is, the colorless fluid of the 

 blood, through its substance ; but, unlike bone, it does not 

 2* 



