DESCRIPTIVE ANATOMY OF THE TEETH. 141 



cavity, into which these canals open by innumerable little 

 orifices, they are found to branch and occasionally anasto- 

 mose, their communications and branches becoming more 

 numerous as they approach the external surface of the tooth. 

 The canals of largest diameter are found next the pulp-cav- 

 ity, and they become smaller as they branch. The structure 

 which forms the walls of these tubules is somewhat denser than 

 the intermediate portion, which is sometimes called the inter- 

 tubular substance of the dentine ; but in some portions of the 

 tooth, the tubules are so numerous that their walls touch 

 each other, and there is, therefore, no inter-tubular substance. 

 Near their origin and near the peripheral terminations of the 

 dentinal tubules, are sometimes found solid globular masses 

 of dentine, called dentine-globules, which irregularly bound 

 triangular or stellate cavities of very variable size. These 

 cavities have been considered as lacunae, like the lacunae of 

 true bone ; but this view is not held by the best and most 

 recent observers. Sometimes these cavities are very numer- 

 ous, and form regular zones near the peripheral termination 

 of the tubules. The dentine is sometimes marked by con- 

 centric lines, indicating a lamellated arrangement. In the 

 natural condition, the dentinal tubules are filled with a clear 

 fluid, which penetrates from the vascular structures in the 

 pulp-cavity. 



Cement. Covering the dentine of the root, is a thin layer 

 of true bony structure, called the cement, or crusta -petrosa. 

 This is thickest at the summit and the deeper portions of the 

 root, where it is sometimes lamellated, and becomes thinner 

 near the neck. It finally becomes continuous with the en- 

 amel of the crown, so that the dentine is everywhere com- 

 pletely covered. The cement contains true bone-lacunae and 

 canaliculi, and in very old teeth, a few Haversian canals, 

 except near the neck, where the layer is very thin. It is 

 closely adherent to the dentine and the periosteum lining 

 the alveolar cavities. 



