TEETH. 



73 



of eruption the outer enamel cells and the enamel pulp are broken through 

 and disappear. That portion of the inner cells which is applied to the 

 enamel prisms remains as an uncalcified but very resistant layer about i fj. 

 thick, the cuticula dentis [Nasmyth's membrane]. It may be detached by 

 acids which dissolve the enamel but have little effect upon the cuticula. 

 The latter, however, yields readily to mechanical erosion, and is soon 

 worn away. The enamel is the hardest portion of the tooth, surpassing 

 the dentine which is harder than bone. 



DENTAL PAPILLA AND DENTINE. 



The dental papilla has already been described as a dense mass of 

 mesenchyma enclosed and probably moulded by the enamel organ. Its 

 cells branch and anastomose, producing fibrils. The cells next to the inner 



FIG. 82. THE DEVELOPMENT OF DENTINE 



IN PIG EMBRYOS. (After v. Korff.) 

 d., Calcified dentine; e. c., inner enamel cells; 



f., fibrous ground substance of dentine; 



od., odontoblasts; p., mesenchymal pulp 



cells. 



FIG. 83. 



Six odontoblasts with dental fibers, f. p., pulp proc- 

 esses. From the pulp at birth. X 240. 



enamel layer become elongated as shown in Fig. 82, A, and soon constitute 

 a simple epithelioid layer as in B. Between them there are groups of fibrils 

 which spread beneath the enamel layer. Calcareous granules are de- 

 posited between the fibrils and produce the matrix of the dentine. The 

 elongated cells which are comparable with osteoblasts are called odonto- 

 blasts. Unlike the former they never become buried in the matrix, but 

 remain on its inner surface. Long processes extend from the odonto- 

 blasts radially through the dentine as seen in the isolated cells in Fig. 

 83. These processes are lodged in the dental canaliculi and are called 

 dental fibers [Tomes' fibers]. As in bone the canaliculi have an incom- 

 pletely calcified lining which resists acids. [The canaliculi of the teeth 

 have therefore been described as bounded by Neumann's membrane.] 

 They follow a wavy or spiral course from the outer to the inner surface of 

 the dentine, often being S-shaped as seen in median longitudinal sections. 



