TEETH. 323 



hexagonal depressions, into which the enamel fibres are re- 

 ceived. 



Formation of the Enamel. Reference has been made to a 

 blastemic matter consisting of nucleated cells embedded in 

 a granular matrix, and situated between the dentine pulp and 

 the inner surface of the sac of the tooth ; this is the enamel 

 pulp. 



The cells of which this is formed are larger than those of 

 the dentinal pulp, more transparent, and with nuclei which 

 are less distinct ; they adhere to the inner surface of this sac, 

 which is formed of a process of the mucous membrane of the 

 mouth itself, and from which indeed they are evolved. 



This membrane, like all mucous membranes, consists of two 

 layers, an outer basement or fibrous and vascular layer, and an 

 inner colourless and blastemic layer ; and it is from this last 

 that the enamel cells proceed. It is marked with depressions 

 similar to those existing on the surface of the membrane of 

 the dentinal pulp, and into which the terminations of the 

 fibres of the enamel are received. 



It is out of the cells just described that the enamel fibres 

 are formed, and this in a manner almost similar to that in 

 which the tubes of the dentine are themselves developed ; 

 thus the cells are first arranged in vertical lines ; these com- 

 mence in the depressions on the inner surface of the tooth sac, 

 and proceed from without inwards. The cells next elongate 

 until they touch each other by either short or oblique sur- 

 faces ; some of them coalesce by their extremities, and thus 

 form fibres in which earthy matter is deposited, and which 

 at length terminate in the hexagonal depressions situated on 

 the outer surface of the membrane of the pulp of the dentine. 



The nuclei elongate with the cells, and either disappear 

 altogether or else remain as minute cavities running down 

 the centre. 



At first the union between the fibres is but slight, so that 

 in newly-formed enamel they may be easily separated from 

 each other when placed in water, to which they will impart 

 a whitish appearance, in consequence of their separation and 

 diffusion through the fluid. 



