DIGESTIVE ORGANS AND THEIR ANATOMY. 



269 



dentinoblasts lie next to the dentine in the pulp cavity and 

 are probably not essentially different, except in position, 

 from the osteoblasts in bone. The reason for the absence 

 of lacunae and Haversian canals in the ivory is of course 

 apparent. Such a system of holes and spaces would materi- 

 ally interfere with the hardness and solidity of dentine and 

 so make it much less serviceable in the crushing of foods. 



In the fang this dentine is covered over by a thin 

 layer of cement by means of which it is bound to the 

 jaw-bone. This cement is nothing but ordinary bone, 

 and as such contains lacunae and not infrequently Haversian 

 canals. 



On the crown the dentine is covered over with a coat- 

 ing of an exceedingly hard substance known as enamel. 

 This is totally different from the dentine both in structure 

 and in origin. The dentine is essentially bone, but the 

 enamel is derived from the skin, like the nails of the fin- 

 gers. There has, however, been such a mineral deposition 



Fig. 102. ENAMEL PRISMS. (After Kb'lliker.) 



A, fragments and single columns of enamel; B, surface view, showing the hexagonal 

 ends of the prisms. 



in these epidermal cells as to transform them into the hard- 

 est substance in the body. In structure the enamel con- 

 sists of more or less hexagonal prisms arranged vertically 



