336 MICROSCOPIC ANATOMY OF THE TEETH 



It can be seen in fig. 223 that the outlines of the epithelial 

 cells are impressed upon the clear layer in areas where the 

 cells themselves are absent, and are distinctly visible on the 

 upper surface of the clear membrane, while the impressions 

 of the enamel prisms from beneath are seen at the margins. 

 These impressions of the prisms are very distinct, and in 

 many parts are arranged in parallel lines or striae. There 

 seems no doubt that the impressions so arranged indicate 

 the structure of the enamel surface which produces them, 



; 1 



FIG 223. Nasmyth's membrane allowing impressions of cells of cellular 

 layer on clear layer, c. Epithelial cells ; i. impressions of cells on clear 

 layer. (x250.) 



showing that the enamel is not smooth, but is arranged in 

 a series of ridges apparently produced by the outcrop of the 

 enamel prisms (fig. 224). The impressions would appear to 

 be produced by the incremental lines of Pickerill, which he 

 considers with several other authors to give rise to the 

 appearance of the striae of Ketzius in enamel. 



That such imbrication lines are present in enamel is 

 clearly shown by this moulding of the inner membrane upon 

 them. In some specimens, where the clear layer is seen 

 alone, having entirely separated from the cellular layer, 

 this stratification is very marked. It was shown by Pickerill 



