1562 



HUMAN ANATOMY. 



enamel increases in thickness by the addition of the last-formed increments at the 

 inner ends of the ameloblasts, the same cells sufficing for the deposit of the entire 

 mass. Owing to the expansion of the external surface of the crown, the diameter of 



Inlermediate layer of 

 eiiamel-organ 



P ^^jj" AmelohlaFit'; 



Young enamel with 

 Tomes 's processes 



Dentine 



mirir<"- 1 > 





-Last-formed dentine 





^> ^ ^^ \ 'C^w 



•'^JT^^ 



-Odontoblasts 



— ^ ejr -" ^ -^-^ JJ.-6 



:<^ 



fp--f^>: 



Embryonal pulp-tissue 



Section of developing tooth through junction of enamel and dentine. X 400. 



Fig 



the enamel-prisms augments towards their outer ends to compensate for the increased 

 area which they must fill, since no additional prisms are formed. 



The complex curvature of the enamel-prisms and the oppositely directed ranges 

 of the latter, producing the appearance of Schreger's stripes, result from changes in 

 the position of the enamel-cells incident to the growth of the crown, since the axes 

 of the newly formed prisms correspond with those of the ameloblasts, variations in the 

 direction of which affect the disposition of the enamel-columns. 



The earliest formed enamel lies in close apposition with the oldest dentine con- 

 stituting the membrana praeformativa ; the last devel- 

 oped immediately beneath the ameloblasts. The enamel, 

 therefore, is deposited from within outward, or in the 

 reversed direction followed by the growth of the dentine. 

 The oldest strata of both substances lie in contact ; the 

 youngest on the extreme outer and inner surfaces of the 

 tooth. 



After the requisite amount of enamel has been pro- 

 duced, differentiation into prisms ceases, in consequence 

 of which the last-formed enamel remains as a continu- 

 ous homogeneous layer investing the free surface of the 

 crown, known as the membrane of Nasmyth. 



The Tooth-Sac. — Coincidently with the develop- 

 ment of the enamel-organ and the growth of the dental 

 papilla, the surrounding mesoblast undergoes differen- 

 tiation into a connective-tissue envelope known as the 

 dental or tooth-sac. The latter not only closely invests the enamel-organ, but is 

 intimately related to the base of the dental papilla, with which it is continuous. In 

 contrast to the epithelial enamel-organ, which is entirely without blood-vessels, the 



Isolated ameloblasts from in- 

 cisor of new-born child, a, basal 

 plate; b, cuticular border; c, pro- 

 cesses of Tomes ; </, homogeneous 

 mass still covering process. X 4oo. 

 (Ebfter.) 



