THE DENTAL TISSUES. 49 



believes that the enamel prisms are tubular, minute canals 

 running along their axes. 



On the whole the prisms are parallel, and run from the sur- 

 face of the dentine continuously to that of the enamel. Their 

 paths are not, however, either perfectly straight or perfectly 

 parallel, for alternate layers appear to be inclined in opposite 

 directions, while they are also wavy, forming several curves 

 in their length. The curvature of the enamel prisms is most 

 marked upon the masticating surface ; while the layers, 

 alternating in the direction of their inclination as just 

 described, are in planes transverse to the long axis of the 

 crown, and correspond to the fine striae on the surface of 

 the enamel, which appear to be caused by their outcrop. 

 The curvatures take place in more than one plane ; in other 

 words, the course of the individual prism is more or less of 

 a spiral. 



Although most prisms run through the whole thickness of 

 the enamel, yet inasmuch as the area of the outer is much 

 larger than that of the inner surface of the enamel, and the 

 individual prisms do not undergo any alteration in size as 

 they pass outwards, many supplemental fibres are present in 

 the outer portions of the enamel which do not penetrate far 

 inwards. 



The individual fibres are to all appearance structureless 

 in perfectly formed human enamel, but a faint transverse 

 striation, fainter, but otherwise not unlike that of voluntary 

 muscle, is so general that it cannot be regarded as patho- 

 logical, although it is most strongly developed in imperfect 

 brownish enamel. The striation in question may be seen 

 even in a single isolated fibre, and is not necessarily con- 

 tinuous over adjacent fibres, though it often is so ; it is 

 rendered more apparent by the slight action of diluted acids 

 upon the fibre. Very various interpretations of this appear- 

 ance have been given. It has been attributed to " an inter- 

 mittent calcification" of the enamel fibre (Hertz), but is 



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