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DIGESTIVE ORGANS. 



[sect. 137. 



a 



Fisr. 120. 



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appears translucent or transparent. In the dry state it is white, 

 with a satiny or silky lustre from the entrance of air into a special 

 system of canals, which pervades the substance. The dentine is 

 considerably harder and more brittle than the bones and the 

 cement, but much less so than the enamel. It alone bounds the 

 pulp cavity, with the exception of a small place at the root, and 

 is never actually seen outwardly in uninjured teeth, for even at the 

 neck of the tooth it is covered by enamel, although in a thin layer, 

 and where the latter ceases, by cement. 



The dentine consists of a matrix, 

 and of numerous tubules running in it, 

 the dentinal tubules, dental canals, canali- 

 culi dentium. The matrix, even in the 

 finest section, is quite homogeneous, 

 without presenting a trace of being 

 constructed of cells, fibres, or other 

 elements. The dentinal canals (fig. 

 120), are microscopical tubes o'ooo6'" 

 to o'ooi"', some of them 0-002'" (at 

 the root) in width, which begin with 

 free openings on the wall of the pulp- 

 cavity, and run through the entire 

 thickness of the dentine as far as the 

 enamel and the cement. Each canal 

 has a special wall, which is less in thick- 

 ness than the diameter of the channel, 

 and can be recognised in transversely 

 cut canals, although not always even 

 here, as a small yellowish ring around the cavity ; in a longitu- 

 dinal section, it is almost entirely withdrawn from view. During 

 life, the canals contain clear, perfectly fluid contents, and are, 

 accordingly, not so readily visible in fresh preparations; it is 

 different in dry sections, when they are filled with air, and the 

 individual tubes appear as black lines when viewed by transmitted, 

 and .as shining filaments by reflected light. In consequence of 

 the very large number of the canals, which in many places is so 

 considerable, that their walls are almost in contact, dry sections 

 appear milk-white, and, if they are not very thin, are quite unfit for 

 microscopical examination, except when the air has been driven 

 out of the canals by the addition of some pellucid and not too 

 viscid fluid. 



Transverse section of dentinal canals, 

 as they are usually seen. a. Ordinary 

 distance apart ; b. more crowded ; c. 

 another view. Human molar. Mag- 

 nified 400 times 



