76 MICROSCOPIC ANATOMY OF THE TEETH 



to describe these impressions on Nasmyth's membrane in 

 specimens floated off the tooth in acid after previous treat- 

 ment with silver nitrate. He speaks of the dark striations 

 visible in these preparations as corresponding ' to the fur- 

 rows between each imbrication ' and taking ' the stain more 

 deeply because the pellicle is there a little thicker '. In 

 the author's preparation, as shown in fig. 33, the ridges are 

 impressed upon the membrane from beneath and the lines 

 of the impressions of the prisms are seen to follow these 

 ridges, so that both methods of preparation show the im- 



FIG. 33. Floated preparation of Nasmyth's membrane showing 

 impressions of enamel prisms in parallel lines. ( x250.) 



press of the imbrication lines on the membrane. Both 

 Von Ebner and Zsigmondy in their study of the striae 

 of Retzius dwell chiefly on the different appearances in 

 ground sections of enamel. Many of these appearances 

 are produced by grinding the sections across the incremental 

 lines at different angles, and do not seem materially to affect 

 the question of the actual cause of the striae, which, there 

 can be little doubt, is the deposit of the enamel in these 

 alternating lines of growth. In hypoplastic teeth the 

 interruption of the deposit of the enamel is very clearly 

 shown. It can be seen in fig. 34 that the incremental line 

 at a is followed by one at b that stops short of the normal 



