274 



MANUAL OF HISTOLOGY. 



Fig. 262. Transverse 

 section of human 

 enamel prisms. 



their direction roughly taken corresponds to that of the canaliculi of the 



tooth. 



If a transverse section of the enamel layer be made, the cut prisms 

 appear like a delicate tesselated pavement of four or 

 six sided plates, reminding one of epithelium (fig. 

 262). 



Finally, the enamel is coated and protected by an 

 extremely hard and resistent homogeneous membrane 

 discovered by Nasmyth (fig. 261, a). This is the 

 so-called " enamel cuticle" or cuticula dentis (Koel- 

 liker). Its thickness is about '00 1-0 '00 13 mm. 



157. 



Nearer inspection discloses to us many peculiari- 

 ties in the texture of enamel. 



Owing to the fact that certain groups of the prisms project deeper 

 into the surface of the dentine than others, the latter becomes rough and 

 uneven. Further, a question arises whether the prisms do not increase in 

 breadth externally, since the internal surface of the layer appears to be 

 less extensive than superficial, and since no considerable interstitial sub- 

 stance can be detected; or whether a certain number of the prisms, 

 shorter than the others, may not terminate at some distance from the 

 surface of the subjacent dentine. The occurrence of such short pillars 

 has been supposed by many, although it is hardly possible to decide the 

 question owing to their unsteady course. Czermak 

 states, however, that he has often observed a widen- 

 ing of the columns externally. 



The latter (fig. 263) display as a rule, in vary- 

 ing clearness and distance, a transverse linear mark- 

 ing, which may be partly dependent perhaps upon 

 the progressive laminar calcification of the struc- 

 ture (Hannover, Hertz}. 



Finally, as to the direction of the individual 

 prisms, we find it very variable; owing to their 

 undulations and different bends, whole groups of 

 them may intersect others. Thus in longitu- 

 dinal sections, the prisms are cut through, in part 

 longitudinally, in part transversely and obliquely, 

 and so communicate a streaky appearance to the surface. 



Enamel possesses no special nutritive canals. But a system of acci- 

 dental cavities is met with in it (fig. 261, c), which vary greatly in magni- 

 tude, and are sometimes simple, sometimes branched, mostly elongated 

 in a direction parallel to that of the prisms : they may, however, run 

 obliquely also. They are usually situated in that portion of the enamel 

 tissue nearest to the cement. But rents and cracks resulting from the 

 grinding of sections may give rise to the same appearances. Finally, it 

 seems probable that some of Tomes' fibres penetrate with their canaliculi 

 from the dentine into the substance of the enamel, as already mentioned, 

 and run here for a short distance between the prisms, either sinking into 

 the cavities or coming to an end among the prisms. 



REMARK. Comp. Tomes' work (Phil. Transact.), p. 522. 



Fig. 263. Pieces of human 

 enamel prisms. 



