36 



Basic Structure of Vertebrates 



stituents are the same as those of dentine. High magnification of thin 

 sections of a tooth shows that the enamel consists of minute hexagonal 

 solid prisms which rest on the surface of the dentine and radiate to the 

 outer surface of the crown or neck (Fig. 32). In its course from inner to 

 outer surface of the layer, a prism does not branch, nor does it increase 



) Crown. 



Dentine 



Fig. 30. Longitudinal ground section 

 of a human incisor tooth. (X 4.) (Cour- 

 tesy, Bremer: "A Text-Book of His- 

 tology," Philadelphia, The Blakiston 

 Company.) 



in its transverse dimensions. The uniform density and hardness of the 

 layer are maintained by occasional interpolation of additional prisms, 

 so that the number abutting on the outer surface of the layer is greater 

 than at the inner surface. The layer is thickest at the apex of the crown 

 and thins out toward the root, being thinnest in the neck region — a 

 reason why that part of the human tooth is especially vulnerable to 

 decay. 



