ENAMEL 75 



ing as white bands and the intermediate substance 

 dark. 



These outcrops of the prisms in any case conform to the 

 imbrication lines above described, and are in all probability 

 evidences of a stratified deposition of the enamel. 



As stated by Pickerill, the incremental lines are well 

 marked in the teeth of native races, and he considers it incon- 

 ceivable, owing to their practically universal presence, that 

 they are due to any hypoplastic cause and that they must 

 be physiologically developed structures. He shows very 

 clearly the -correspondence of the striae of Retzius with the 

 imbrication lines on the outer side of the enamel. He says : 

 ' In the cervical portion, the angle of incidence ' (of these 

 lines with the surface) ' becomes progressively less and less 

 and the ridges longer and fewer until the striae become 

 parallel with the surface and the ridge ceases to be distin- 

 guishable as such, but forms part of the general contour 

 of the tooth. When the ridges cease to be apparent the striae 

 of Retzius also cease to be marked, although remaining 

 distinctly visible.' In those animals where the striae of 

 Retzius are absent, he points out that the imbrication lines 

 are also absent, and considers them absolutely dependent 

 on one another. Kolliker and Walkhoff believe that all 

 striation in enamel is due to the deposition of lime salts 

 in strata. Certainly intermittent deposit appears to be 

 indicated as the rule in the deposition of the hard tissues, 

 as bone cement, &c. (see Chapter III). 



Strong evidence that the striae of Retzius are due to the 

 outcrop of lines of prisms is afforded in floated specimens 

 of Nasmyth's membrane. The impressions of the prisms 

 on the clear layer of the membrane which is in immediate 

 contact with the enamel are seen to be arranged in parallel 

 lines (fig. 33), and these impressions show distinctly that the 

 prisms which made them were raised above the surface of 

 the rest of the enamel. There can be no question here of any 

 appearances produced by grinding, such as are referred to 

 by Von Ebner and Zsigmondy (22), as the specimen from 

 which this photograph was taken was floated off the 

 enamel in acid and must necessarily give an accurate 

 view of the undisturbed surface. Pickerill was the first 



