CHAP. XXIII.] 



DENTINE. 



169 



Fig. 149. 



pear to derive their peculiar characters from the mechanical exigences 

 of the case. The tooth having to sustain rude pressure on its crown, 

 chiefly in a vertical direction, great density 

 and compactness are requisite in its main 

 constituent, and its internal vascular surface, 

 which is small in proportion to the mass of 

 dentine, is centrally placed, and receives its 

 vessels and nerves at the deepest point, most 

 remote from injury. As the vascular surface 

 is small, and therefore at a great distance 

 from a large proportion of the tissue, the in- 

 terstitial channels of the dentine are capa- 

 cious, especially towards the vascular surface, 

 and comparatively direct in their course from 

 it; and, instead of commencing minute as the 

 canaliculi of bone do, and dilating at inter- 

 vals, after a tortuous and irregular route, into 

 hollow chambers, like the lacunae of that 

 texture, they are widest at their commence- 

 ment in the pulp-cavity, retain throughout 

 the simple tubular character, and are pro- 

 vided for the most part with proper Walls : SO . Vertical section of human in- 

 iiii -IT -iv. cisor ' shewin t he general ar- 



tnat each tubule may be regarded as a hollow rangement O f its constituent 



J parts. The dentine and pulp- 



rod, the stem of which is of a harder and com- CAVit y> the enamel on the crown, 



and the bone on the fang are 



pacter nature than the intertubular substance seen - Neck of the to th. 



. Magnified 3 diameters. 



through which it runs. 



The direction taken by the tubes is further interesting ; for not 

 only do they radiate on all sides from the vascular surface, as 

 being the conduits of nutrition to the dentine, but they thus confer 

 on every part of the tooth a greater power of resistance in an in- 

 ward direction from the surface towards the centre, a power in- 

 creased by the cylindrical shape of the pulp-cavity, and its tendency 

 to an arched figure towards the crown. But it would appear that 

 the beauty of the mechanical contrivance does not stop even here; 

 for it has been observed, that the tubes in many parts are doubly 

 waved, like the Italic/, and that within these primary curves are 

 comprised very numerous secondary meanderings ; from which, as 

 those of contiguous tubes have a lateral correspondence, a certain 

 elasticity, and a greater capacity of resisting external force, must 

 accrue. The tubuli of the dentine now described branch a few times 

 dichotomously, and the branches retain for some distance the diame- 

 ter of the trunk, this multiplication of their number enabling them 



