STRUCTURE OF THE ENAMEL. 



471 



parallel to the contour of the teeth in dentine, which in large teeth 

 can be easily seen with the naked eye, or with a low magnifying power. 

 In true dentine they present on section a characteristically decussating 

 course with small rhomboidal meshes between them. As Retzius (19) 

 and Owen (25) first correctly stated, the lines of Schreger are occa- 



Fig. 99. 



Fig. 99. Apex of a tooth from the lower jaw of the Pike (Esox 

 lucius). Magnified 80 diameters. The central portion consists of vaso- 

 dentine, which is covered with true dentine ; external to which again 

 is a thin layer of vitro-dentine. 



sioned by the corresponding primary curvatures of the dentinal tubes. 

 Owen (25) describes in addition a second system of parallel curved 

 lines in dentine, the contour lines occurring especially in the tusks of 

 the elephant, produced by regularly intercalated strata of small cells 

 (probably finely granular interglobular substance). Czermak and 

 Kolliker give similar illustrations, drawn from the teeth of man ; we 

 are not however justified from these appearances in concluding that 

 dentine possesses a lamellated structure. 



ENAMEL (Substantia Vitrea; Subst. Adamantina; Encaustum; 

 Adamas; Email). Enamel is the hardest substance met with 

 in the Vertebrata, being in this respect about equal to Apatite 

 (F. Hoppe-Seyler, 69). With its translucent mass and bluish 

 tint it forms a kind of cap of various thickness, investing the 



