50 MICROSCOPIC ANATOMY OF THE TEETH 



To the unaided eye enamel has a smooth, glistening, 

 almost crystalline appearance. It is seen under the micro- 

 scope to be built up of prisms or columns united by a densely 

 calcified intermediate substance, which in perfectly formed 

 enamel cannot easily be distinguished from the prisms. 

 Course The prisms pass from the outer surface of the dentine 



prisms ^ ^ e free margin of the enamel, but they do not follow 

 a straight or even course, being in many places spiral in 

 arrangement and disposed at various angles. Prisms in 



FIG. 13. Longitudinal section of enamel. (x400.) 



transverse section can often be seen passing more or less 

 at right angles to the others (fig. 15). Their usual arrange- 

 ment, however, is in radiating and undulating lines or 

 groups of prisms, maintaining a principal direction through- 

 out their course (figs. 13, 14). The prisms or columns, each 

 of which extends from the dentine to the surface in a more 

 or less undulating course, show a marked cross-striation 

 at regular intervals, and are separated from one another 

 by the intercolumnar or interprismatic substance, which, 

 like the prisms, is highly refractive and very fully calcified. 



In transverse section the prisms show oval, hexagonal, 

 or polygonal outlines (figs. 15, 16, 24), and in many places 



