DEVELOPMENT AND MORPHOLOGY OF THE TEETH. 



65 



formed before birth ; and that of the first permanent molar is 

 already partly deposited. 



-lower lip 



_gum ridge(epidermis) 



jdental groove 



■remnant of 

 \dental shelf. 



'epithelial remnants 



\r~ — tongue 



jj enamel bud perm, incisor 



- dental sac 

 ameloblasts 

 - papilla (pulp) 



~7ection of lower jaw 



Fig. 49. — Showing the stage of development in an incisor tooth of a foetus of 



six months. 



enamel of 

 milk incisor 



(2) Origin of the Dentine. — The dental papilla, formed from 

 the mesoblast, corresponds to a depressed skin (dermal) papilla, the 

 enamel cells representing its covering of epithelium. The dental 

 papilla determines the shape of the tooth. In its superficial 

 layers it contains numerous cells, odontoblasts, with branched 

 processes radiating towards the enamel epithelium. By the 

 agency of the odonto-blasts a substance is deposited which 

 becomes calcified into dentine or ivory. It is deposited round 

 the processes of the odonto-blasts. The cavities in which the 

 processes are enclosed form the tubules of the dentine. In 

 rodents especially, but also in all mammals, although only to a 

 slight extent in civilized races of mankind, the odonto-blasts 

 react to wear, add new layers of dentine to the wall of the 

 pulp cavity, and thus prevent the pulp from being exposed. 

 The dentine is deposited first in the crown of the tooth beneath 

 the enamel ; the neck is laid down next, and then the root, 

 the last point of all to be formed being the narrow canal at 

 the apex of the root by which the dental vessels and nerves 

 reach the pulp cavity. It is the formation of the root that 

 forces the crown of the tooth through the gum. The roots 



