74 MICROSCOPIC ANATOMY OF THE TEETH 



They would seem to be an indication of intermittent or 

 interrupted calcific deposit. 



Pickerill (14) speaks of the striae of Retzius or incremental 

 lines, and shows that they correspond with the outcrop on 

 the surface of the enamel of the imbrication lines which 

 he describes as giving rise to the furrows on the proximal 

 surfaces. Underwood (19) considers that the striae ' are due 

 to an excessive granularity of the prisms ; this granularity in 

 each prism corresponding with that of its neighbour, the 

 effect of a line is produced '. 



Von Ebner considers they are due to the entrance of air 

 between the rows of prisms, but they are evident in Weil 

 preparations and in others of teeth which have not been 

 allowed to dry. In his latest paper on enamel (6 a) Von 

 Ebner says, however, ' the typical Retzius lines, found only 

 in permanent teeth, the contour lines, are dependent, 

 as are also the contour bands i n milk teeth, on the arrest 

 of an early stage of development during enamel formation '. 

 He describes the sharp brown lines as well as the brownish 

 band-like striae in dry sections as due to the entrance of 

 air between the prisms, as first stated by Baume, but he 

 speaks also of two kinds of Retzius's striae, one in which 

 the prisms are seen to be broken across and others which 

 lie deeper in the enamel. He considers that both kinds 

 of striae are different appearances of the same structural 

 condition, and in dry sections contain clefts or fissures 

 enclosing air. Leon Williams says that the appearances 

 in his photographs show that these striae must be due to 

 a pigmentary deposit, but he scarcely conveys this idea 

 in the context, where he would appear to agree with Pickerill 

 and others that they are due to incremental deposit. An 

 appearance of pigmentation would appear to be only 

 a characteristic of these incremental lines in the finished 

 enamel. Pickerill considers that ' the only rational solution 

 of such appearances is, that imbrications and striations of 

 all varieties are to be regarded as evidence of checks in the 

 secretive functions of the ameloblasts ' (14). 



He does not consider that the striae of Retzius are due 

 to pigmentation, as they have a different appearance by 

 reflected and transmitted light : by reflected light appear- 



