THE TEETH. 



479 



ties of various kinds mny be not nufrequcntly found in the enamel. Such, 

 for instance, are — 1. The continuations of the dentinal canals into the 

 enamel, to which reference has been made above, with the elongated 

 cavities at the border of the dentine which arise from their expansion 

 (Fig. 191, c); and 2. The cleft like gaps in the middle and external 

 portions of the enamel (Fig. 191), which are not in communication with 

 the preceding, are never entirely absent in any enamel, and often occur 

 in very gteat numbers, as narrower or wider spaces which, however, 

 never contain air. 



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 surface. 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, 0-08-0-12 of a line thick, 

 extending from the dentine to the surface of the 

 enamel, and corresponding Avith fine circular lines 

 upon its exterior ; 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, espe- 

 cially when moistened with hydrochloric acid, 

 have a very singular 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 surface, 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 colorless strice which indicate 

 these peculiar arrangements of the enamel fibres, with certain hroivnish 

 lines or colored 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 



Fig. 191. — Dentine and enamel, magnified 350 diameters; Man : a, cnticle of the enamel 

 (Nasmytli's membrane) ; b, enamel prisms with transverse markings and interposed clefts ; 

 c, larger cavities in the enamel ; d, dentine. 



