THE DENTAL TISSUES. 51 



enamel softened by prolonged maceration in a 1 per cent. 

 solution of chromic acid shows this well ; the central por- 

 tions of the fibre are dark, and are stained green by the 

 reduced chromium sesquioxide, while the clear interspaces 

 are colourless. Again, if dilute hydrochloric acid be applied 

 to a section of enamel, the axial parts of the fibres are first 

 attacked and are dissolved away, so that, if the section be 

 transverse, a fenestrated mass remains. During the forma- 

 tion of enamel the hardening salts are deposited first in the 

 periphery of the enamel cells, so that the youngest layer of 

 enamel is full of holes, each one of which corresponds to the 



FIG. 21 



centre of a fibre. Although calcification goes on to obliterate 

 the visible difference between the centre and the periphery 

 of the enamel fibre, yet the action of an acid reverses the 

 order of its formation and once more makes it fenestrated,, 

 indicating that there is not absolute identity of substance 

 in the inner part of the fibre. In imperfect enamel, indeed, 

 a central narrow canal has sometimes been observed in the 

 interior of an enamel fibre. 



In fractured enamel, the line of fracture is said to run. 

 through the centre of the fibres, and not, as might have been 

 expected, through their interspaces. 



There is also an appearance of striation upon a far larger 

 scale, consisting of brownish lines, which are never, or almost 

 never, quite parallel with the outer surface of the enamel, 

 but which nevertheless preserve some sort of parallelism 

 with it and the surface of the dentine. These are known 



(!) Transverse section of enamel, the axial portion of the prisms having 

 been removed by dilute hydrochloric acid. 



E 2 



