156 



ELEMENTS OF BIOLOGY 



enamel is the hardest substance in the body; it forms the cap on the 

 dentine and after its formation the enamel-producing organ ceases 

 to function, so that enamel of the human tooth once broken is 

 never repaired nor replaced. The dentine resembles bone in many 

 respects and is considerably softer than the enamel. It forms the 



ENAMEL 



DENTINE 



GUM 



BONE OF JAW 



NERVE AND BLOOD 

 VESSELS 



PULP 



PERIOSTIUM 



CEMENTVU 



Fig. 103. — Section of a human tooth in its socket. Compare with the structure of a 



placoid scale (Fig. 91). 



greater bulk of the tooth and its basal end is embedded in the bone 

 of the jaw. The centre of the tooth contains the pulp cavity; it is 

 filled with a form of connective tissue and contains minute blood 

 vessels and nerves. The socket in the jaw is lined with the perios- 

 teum of the bone; the tooth is attached to its socket by a substance 

 known as cementum. The enamel cap is discontinued at the level 

 of the gum and the cementum then forms the contact layer down 

 to the root of the tooth. 



