50 A MANUAL OF DENTAL ANATOMY. 



with more probability referred to varicosities in the indi- 

 vidual fibres (Kolliker, Waldeyer) ( 1 ). It is very marked in 

 the enamel of the common rat, which shares with that of 

 other muridse the peculiarity of having the individual fibres 

 almost serrated, those of adjacent crossing layers being fitted 

 to one another with great exactness. In human enamel the 

 adjacent fibres if united without any intermediate cement- 

 ing medium, and pursuing courses slightly different, must 

 of necessity be of slightly irregular form, or else interspaces 

 would be left, which is not found to be the case. Thus the 

 "decussation of the fibres " is a plausible explanation of this 

 appearance of striation ; indeed isolated fibres do present an 

 appearance of slight varicosities, repeated at regular in- 

 tervals. The penetration, at regular intervals, of the prisms 

 by the " thorns " of cement substance (see page 48), affords 

 another explanation. 



Although the perfect enamel fibre appears to be entirely 



FIG. 20 (*). 



homogeneous, it is not really so, for acids act with far greater 

 rapidity upon the central or axial portion of the fibre than 

 upon its periphery. The accompanying figure, taken from 



(*) The striation of voluntary muscle has been very recently proved to 

 be due to this same cause (Dr. Haycraft, " Proceedings of Royal Society," 

 Feb. 1881). 



( 2 ) From human enamel, softened in chromic acid, until it could be 

 -readily cut with a knife. 



