STRUCTURE OF TEETH. 135 



the effect of that friction in wearing the enamel is 

 thus rendered the least possible. 



By the skilful application of the microscope, it 

 has recently been discovered that the ivory is com- 

 posed of extremely minute tubular fibres, passing 

 through a transparent and apparently homogeneous, 

 but in reality cellular, substance. The fibres are 

 closely compacted together, and disposed in radi- 

 ating, and slightly vuidulating lines, having direc- 

 tions perpendicular, in every part, to the surface of 

 the tooth.* The deceptive appearance of con- 

 centric layers, parallel to the surface, which is 

 represented at o, in Fig. 277, is produced by the 

 different refractions of light accompanying the 

 undulations of the fibres. Each of the main tubes, 

 in proceeding outwards, generally divides itself 

 into two branches, each of which is itself again 

 divided into two others, and so on successively. 

 The interior of the tubes is filled with calcareous 

 granules, and they have hence been termed by Mr. 

 Owen calcigerous tubes. 



In the teeth of some quadrupeds, as of the Rhi- 

 noceros, the Hippopotamus, and most of the Ro- 

 dentia, the enamel is intermixed with the ivory ; 

 and the two so disposed as to form jointly the sur- 

 face for mastication. In the progress of life, the 

 layers of enamel, being the hardest, are less worn 

 down by friction than those of the ivory, and 

 therefore form prominent ridges on the grinding 



* Leuwenhoeck liad long ago observed that these fibres were 

 tubes. (Phil. Trans. 1678, p. 1002.) But the form and direction 

 of the fibres were accurately determined by Purkinje : and their 

 tubular nature by Muller. The cells of the intertubular substance 

 were detected by Retzius. 



