258 THE DIGESTIVE SYSTEM 



careous cylinders, the enamel prisms. These radiate outward from 

 the dentine and are disposed after the manner of a mosaic. They 

 are firmly united to each other by a very thin layer of calcified 

 cement substance. Strangely enough the enamel 

 prisms are developed from epithelial cells, by 

 which they are apparently deposited as a calca- 

 reous cuticular formation, the calcification pro- 

 ceeding from within outward. The uncalcified 

 cytoplasm of the outer portion of these, cells 

 FIG. 2i 6. ENAMEL becomes highly cornified, so that the free sur- 

 PKISMS IN THAN- face of a recently erupted tooth is covered by a 

 thin horny cuticle, the cuticular membrane of 

 a c F a i? m !f ^(Af- ^ asm y tn - The cuticular covering which is thus 

 ter Kolliker.) formed by the horny uncalcified portions of the 



enamel cells is, however, removed by mechanical 

 violence in a relatively short time after the final eruption of the 

 tooth. 



The enamel prisms are grouped into bundles within which the 

 constituent prisms are parallel. The course of the prism bundles, 

 however, is variable, so that, though following a more or less radial 

 course through the enamel, the prism bundles frequently cross one 

 another at acute angles. In the thicker portions of the enamel 

 this peculiarity gives rise to an apparently laminated condition of 

 this tissue. 



Ground sections of dried tooth show brownish lines having a 

 general radial direction, but which are somewhat inclined toward 

 the apex of the tooth. These lines of Retzius are explained by 

 von Ebner as being the result of air-filled fissures in the dried 

 enamel. They are also said to be the result of the wavy direction 

 of the enamel prisms. 



Cementum. The dental cement, crusta petrosa, is a thin layer 

 of bony tissue which invests the root of the tooth. It forms a very 

 thin layer at the neck of the tooth, but gradually increases in 

 thickness as it approaches the tip of the fang. 



The cement um consists of parallel layers of bony lamellae be- 

 tween which many lacunae with their bone corpuscles are included. 

 Bone canaliculi radiate from the lacunae and frequently open into 

 the interglobular spaces of the granular layer. There are no 

 Haversian systems in the cementum, but the thicker portions are 

 frequently penetrated by vascular canals which, like Volkmann's 

 canals, are not accompanied by concentric lamellae. The cement- 



