172 



ELEMENTS OF HISTOLOGY. [Chap. xxi. 



a vertical direction from the surface to the dentine. 

 When viewed in transverse section, the enamel prisms 

 appear of an hexagonal outline, and separated by a very 

 fine interstitial cement substance. The outline of the 

 enamel prisms is not straight, but wavy, so that the 

 prisms appear varicose. The prisms are aggregated 

 into bundles, which are not quite parallel, but more 

 or less slightly overlap one another. On a longitudinal 

 section through a tooth, the appearance of alternate 



Fig. 103. Enamel Prisms. 

 A, In longitudinal view ; B, in cross section. (Kollikcr.) 



light and dark stripes in the enamel is thus produced. 

 Besides this there are seen in the enamel dark hori- 

 zontal curved lines, the brown parallel stripes of 

 Retzius, probably due to inequalities in the density of 

 the enamel prisms produced by the successive forma- 

 tion of layers of the enamel. The enamel consists 

 of lime-salts : phosphate, carbonate, and fluoride of 

 calcium with corresponding magnesium salts. 



In young teeth the free surface of the enamel 

 is covered with a delicate cuticle (the cuticle of 

 Nasmyth), a single layer of non-nucleated scales. 

 In adult teeth this cuticle is wanting, having been 

 rubbed off. 



229. The dentine is the principal part of the hard 



