MICROSCOPIC STRUCTURE. 465 



compartments or cells, of which membranes scarcely a trace can 

 be detected in fully-formed teeth. The fibres are arranged closely 

 together, side by side, with occasional narrow angular fissures, or 

 interspaces, which are most common between the ends nearest the 

 dentine ; their general direction is perpendicular to the surface of the 

 dentine, where the ends of the prisms are fixed in shallow depres- 

 sions ; the opposite and larger ends form the exposed surface of 

 the enamel : the fibres proceeding to the horizontal masticating 

 surface are, therefore, vertical ; the greater number, which are 

 directed to the circumference of the crown, are horizontal or 

 nearly so ; every fibre, as a general rule, having, like the tubes 

 of the dentine, that direction which is best adapted for resisting 

 either the external force of mastication or the efi'ects of lateral 

 pressure. Besides the minute pits corresponding with the inner 

 ends of the enamel fibres, the outer surface of the dentine some- 

 times presents larger depressions, as shown at e*, fig. 1, pi. 123. 

 The enamel-fibres describe a flexuous course, the curves being 

 much stronger and shorter than the primary curves of the den- 

 tinal tubes(l). The parallelism of the fibres continues over a 

 much smaller extent of any part of the enamel than that of the 

 calcigerous tubes in the dentine : in some parts of the enamel 

 they curve in opposite directions to one another, like the vane of 

 a feather. Sometimes the fibres may be traced through the entire 

 thickness of the enamel ; where they fall short, and where the 

 larger fibres diverge from each other, shorter complemental fibres 

 fill up the interspaces. Each fibre is sith of an inch in thick- 

 ness, and is marked throughout its entire course by faint, close- 

 set, transverse striae (2). When a section of enamel includes several 

 fibres in its thickness, certain of the overlapping curves intercept 

 a portion of light, and occasion the appearance of dusky, brownish 

 waves. Another appearance, more immediately related to the for- 

 mation of the enamel, is produced by lines crossing the enamel- 

 fibres, parallel with the outer margin of the enamel, but not always 

 parallel with that attached to the dentine. These lines are not of equal 

 clearness, but are very nearly equi-distant, being about 2050th of an inch 



(1) PI. 122, a, fig. I & 2. (2) lb. fig. 3. PI. 123, fig. J, e. 



H H 



