470 



HUMAN DENTITION. 



to operate upon the food in preparing it for deglutition, is the 

 hardest, the most sohd, the least organized. We have seen that 

 every fibre of the enamel is so disposed, by the preliminary dis- 

 position of the cells in which it was moulded, as to give it 

 the utmost strength and power of resistance of which such a 

 tissue could be capable. The polished surface, the pearl-white 

 colour of this dense and brittle substance, adds ornament to use. 

 In the second and principal substance of the tooth, the dentine, 

 we have traced an equally beautiful arrangement of the earthy 

 salts in directions which best resist both vertical and lateral pres- 

 sure, but with the additional economy of the substitution of the 

 hollow column for the solid prism. The saving of material is 

 however, the least of the benefits gained by this tubular structure 

 of the dentine : the vitality of the tissue, which Hunter recognized 

 so forcibly, but which, being equally convinced of the non-vascu- 

 larity of the tissue, he was unable to explain, — ^willing rather to 

 enunciate an apparent paradox, or be taunted with dilemma, than 

 yield one iota of either of his convictions, (1) — is explicable by the 

 possible and highly probable fact of a circulation of the colourless 

 plasma of the blood through the dentinal tubes. That some ele- 

 mentary prolongations of nerve may also be continued into these 

 tubes, who may confidently deny ? Whoever has felt the pang 

 produced by contact of a probe with the recently exposed surface 

 of the dentine, must at least allow the tubes to be most efficient 

 conductors of the impression to the sentient pulp. Nature has 

 suffered no part of the dentine to be exposed : its organization 

 and vital powers are adequate to its own support in health, and 

 to control that stimulus which, when the dentine is dead and 

 truly ' an extraneous body', operates detrimentally upon the sur- 

 rounding more highly organized parts : but the vitaUty of the 

 dentine is insufficient for the reproduction of its tissue, or for 

 the arrest or repair of decay. The third substance which is 

 chiefly developed around the implanted part of the tooth is more 

 highly organized than the dentine, more analogous to bone, and 

 accordingly better adapted for vital connexion with the vascular 



(1) See Prof. Bell's preface to his Edition of ' Hunter on the Teeth,' p. xiii. 



