THE DEVELOPMENT OF THE TEETH. 161 



further calcification, by which their calibre becomes sensibly 

 diminished. Thus, my father states (speaking of the incisor 

 teeth of rodents), " the tubes which proceed from the pulp 

 cavity near the base of the tooth, are, in most cases, per- 

 ceptibly larger than those that are situated higher up ; 

 hence it follows that, as the latter were once near the base 

 of the tooth, the dentinal tubea undergo a diminution of 

 calibre after their original formation. In the teeth of the 

 Sciuridse I have found a difference of size amounting to a 

 third or half between the tubes near the base and those 

 near the surface in wear." 



And Dr. Lionel Beale calls attention to the fact that the 

 hollows of the canals are largest nearest to the pulp, and 

 smallest at the periphery of the tooth, in other words, at 

 the oldest pail ; also that calcification is still slowly going 

 on even in advanced life, so as often to lead to the oblite- 

 ration of the peripheral tubes. There is, too, the statement 

 of Robin and Magitot, that the teeth become more rich in 

 calcareous salts as age advances, so that analyses of human 

 teeth show great discrepancies. 



It is difficult to see how a dentinal tube once formed can 

 become contracted to a third or half of its diameter unless 

 we believe that that which was at first the soft tissue (den- 

 tinal fibril) occupying its canal may become at its periphery 

 metamorphosed into " dentinal sheath," while that which 

 was originally this latter has passed into the condition of 

 matrix. Other illustrations of this fact, observed by inde- 

 pendent writers, suggest themselves to me; the converti- 

 bility of the dentinal fibril into dentinal sheath and of the 

 latter into matrix, seems to be of necessity implied by the 

 narrowing of the calibre of a tube once formed, for the tubes 

 thus narrowed present no special character ; their walls do 

 not appear any thicker, nor do they in any way become dif- 

 ferent save in the one matter of diameter. The phenomena 

 of dental caries also appear to lend some support to this 



M 



