194 



COMPARATIVE ANATOMY 



is organic material. At no place on the tooth does the dentine reach the 

 surface, since the crown and neck are covered with enamel, while the 

 rt)ot is surrounded Ijy a heavy cement. 



Enamel is the hardest substance in the human body, since it contains 

 only three and a half per cent of organic substance. It is thickest at the 



growth lines 



in enamel 



pulp chamber 



growth lines 



in dentine 



root canal 



oral epithelium 



osteoblasts 



of periosteum 

 of alveolus 



connective 

 tissue fibers 



cementoblasts 

 ( from 

 dental sac) 



bone of 

 mandible 



blood vessels 

 and nerves 



Fig. 146. — Schematic diagram showing the topography of a tooth and its relations 

 to the bone of the jaw. The numbered zones indicate empirically the sequence of 

 deposition of the dentine and enamel. The so-called growth lines in the dentine and 

 enamel follow the general contours indicated by the dotted lines in the figure but are 

 much more numerous. (From Patten's "Embryology of the Pig.") 



apex of the crown, and thins out towards the neck and root. High 

 magnification shows that the enamel consists of minute parallel hexagonal 

 prisms which rest on the dentine and extend to the outer surface of the 

 crown. Increase in the amount of enamel toward the outside of the 

 crown is effected by means of increase in the number of enamel prisms 

 and not by their enlargement or branching. In this way the soHdity 



