STRUCTURE OF THE TEETH. 



1551 



Fig. 



1309. 



Enamel 



Crack 

 in enamel 



Junction of 

 enamel and 

 dentine 



impart to the individual canaliculi a spiral course. The cause of the latter Kollmann 

 refers to the more rapid growth of the dentinal fibres than of the slowly forming 

 dentinal matrix. In consequence of the correspondence of the curvature of the den- 

 tinal tubules, the tooth-ivory exhibits a series of linear markings, Schreger' s lineSy 

 which run parallel to the inner surface of the dentine. These markings must not be 

 confounded with the contour lines of Owen (page 1552), also within the dentine, or 

 with Schreger' s prism-stripes within the enamel (Fig. 1306). ,' 



The dentinal tiibides are minute canals, from .001 3-. 002 mm. in diameter, which 

 begin at the pulp-cavity with the largest lumen and extend to the outer surface of 

 the dentine, to end beneath the enamel or cementum. Each spirally coursing canal 

 undergoes branching of two kinds, a dichotomous 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 twigs are given off 

 with a corresponding dimi- 

 nution in the size of the cana- 

 liculi ; the terminal tubes, 

 often reduced in diameter to 

 mere lines, frequently anas- 

 tomose with one another or 

 form loops. The dentinal 

 tubules are occupied by the 

 delicate dentiyial fibres, the 

 processes of the odonto- 

 blasts, which in the young 

 tooth constitute a net-work of 

 protoplastic threads through- 

 out the dentine of importance 

 for the nutrition of the tis- 

 sue. The relation of the den- 

 tinal tubules on the external 

 surface of the dentine varies 

 on the crown and root. In 

 the former situation the free 

 surface of the dentine pre- 

 sents crescentic depressions, 

 filled by the enamel, in which 

 the tubules appear as ab- 

 ruptly terminating or cut off ; 

 on the root, on the contrary, 

 where the dentinal surface is 

 smooth, the tubules stop in 

 curved ends or loops beneath 

 the cementum, only in very 

 exceptional cases communi- 

 cating with the canaliculi of 

 the latter. 



The immediate wall of the dentinal tubules is formed by a delicate membrane, 

 the sheath of Neumann, which in appropriate transverse sections appears as a con- 

 centric ring. On softening the decalcified dentine by acids or alkalies, the sheaths 

 may be isolated, since they resist the action of the reagents which attack the sur- 

 rounding intertubular substance. The sheaths of Neumann are formed through the 

 agency and at the expense of the dentinal fibres, the latter being smaller in old than 

 in young dentine. The sheaths, therefore, may be regarded as specialized parts of 

 the intertubular matrix, distinguished by less complete calcification and greater 

 density. 



The intertubular groioid- substance of dentine resembles that of bone in being 

 composed of bundles of extremely delicate fibrillae of fibrous connective tissue. The 

 latter, best seen in decalcified tissue, swell on treatment with water containing acids 

 or alkalies, and yield gelatin after prolonged boiling. The disposition of the bundles 



Dentina 

 tubules 



Ground-section of dried tooth including adjacent enamel and dentine. 



X 30O- 



