THE TEETH. 



53 



Fig. 191. 



The general course of the prisms of the enamel resembles that 

 of the dentinal canals of the crown of the tooth, but extensive 

 flexures are only to be met with towards the masticating sur- 

 face. Most of the prisms extend through 

 the whole thickness of the enamel, but 

 this is not the case with all. They also 

 decussate in a peculiar manner; thus, 

 in some transverse sections, we observe 

 annular layers of prisms, 0*08 — 012'" 

 thick, extending from the dentine to the 

 surface of the enamel, and correspond- 

 ing with fine circular lines upon its ex- 

 terior ; in each layer, all the prisms take a 

 similar direction, which is different from 

 that of the prisms of the contiguous 

 layers, so that perpendicular sections of 

 such enamel, especially when moistened 

 with hydrochloric acid, have a very sin- 

 gular striated appearance, arising from 

 the dark transverse, and clear longitudinal 

 sections of the prisms being alternately 

 presented to the eye. 



A similar decussation of the prisms 

 occurs constantly at the masticating sur- 

 face, and here the layers of enamel take a generally annular 

 arrangement, so that they describe circles, in the molars, 

 and ellipses in the incisor teeth; however, towards the 

 centre of the masticating surface, irregularities occur which we 

 are not yet in a condition to explain. Care must be taken 

 not to confound the colourless stria which indicate these pecu- 

 liar arrangements of the enamel fibres, with certain brownish 

 lines or coloured streaks which cross the prisms in various 

 directions and in perpendicular sections, appear like oblique 

 ascending lines or arches (fig. 187) ; in transverse sections, 

 like circles in the external layers of enamel, rarely extending 

 through its whole thickness. These I regard as the expression 

 of the lamellated development of the enamel. 



Fig. 191. Dentine and enamel, x 350 ; Man : a, cuticle of the enamel (Nasmyth's 

 membrane) ; b, enamel prisms with transverse markings and interposed clefts ; e, 

 larger cavities in the enamel ; d, dentine. 



