DEVELOPMENT OF THE TEETH 157 



these cells are known as odonloblasts (Fig. 164). When the dentine 

 layer is developed, the odontoblast cells remain internal to it, but branched 

 processes from them (the dentinal fibers of Tomes) extend into the dentine 

 and occupy the dental canaliculi (Fig. 163). Internal to the odontoblast 

 laver, the mesenchymal cells differentiate into the dental pulp, popularly 



Dental sac 



Outer layer Inwr layer 



Dentine. 



Odontoblasts 



Dental papilla (future pulp) 



Blood vessel 



Outer enamel cells 



Enamel pulp 



Inner enamel cells 



(iinif/oblasts) 



I Enamel 



Epithelial sltealh 



Bony Irabecula of jaw 



FIG. 164. Longitudinal section of a decidual tooth of a newborn dog. X 42. Above the 

 enamel, on either side, are artificial shrinkage spaces. (Lewis and St&hr). 



known as the 'nerve' of the tooth. This is composed of a framework of 

 reticular tissue in which are found blood vessels, lymphatics, and nerve 

 fibers. The odontoblast layer persists throughout life and intermittently 

 lays down dentine, so that eventually the root canal may be obliterated. 

 The Dental Sac. The mesenchymal tissue surrounding the anlage of 



