134 



NORMAL HISTOLOGY. 



The dentinal tubules are minute canals (from 1-2 /* in diameter), 

 which begin at the pulp-cavity, where they are largest, and extend to the 

 outer surface of the dentine, to end beneath the enamel or the cementum. 

 Each spirally coursing canal undergoes branching of two kinds, a dichoto- 

 mous division at an acute angle in the vicinity of the pulp-cavity, resulting 

 in two canaliculi of equal diameter, and a lateral branching during the outer 

 third of their course, whereby numerous tubuli of diminishing size are given 

 off. The dentinal tubules are occupied by the dentinal fibres, the processes 

 of the odontoblasts within the pulp, which in the young tooth are proto- 



Pulp-tissue ./ 



Granular layer of 

 dentine 



Cementum 



Alveolar periosteum 5fS 



^. V* 



FIG. 174. Transverse section of root of lower canine tooth. X 30. 



plasmic threads ; later they lose this character and become harder and stiffer. 

 The dentinal tubules differ in their mode of ending in the crown and the 

 root. In the former situation, the outer surface of the dentine is indented 

 with small crescentic depressions, filled with enamel, in which the tubules 

 abruptly end, as if cut off. On the root, where the surface of the dentine is 

 smooth and covered with cementum, the tubules end in curves or loops 

 beneath the cementum, only in exceptional cases communicating with the 

 canaliculi of the cementum. The immediate walls of the dentinal tubules 

 are formed by delicate membranes, the dentinal sheaths of Neumann, which 

 are specialized parts of the intertubular matrix of greater density and less 

 complete calcification. After softening the decalcified dentine by alkalies, 

 the sheaths may be isolated, since they resist the action of reagents which 

 attack the surrounding substance. 



