190 DIGESTION MASTICATION, INSALIVATION, DEGLUTITION. 



the pulp-cavity toward the exterior. These are called the dentinal tubules or 

 canals. They are ^&loo to Tronr ^ an inch (1 to 2 /x) in diameter, with 

 walls of a thickness a little less than their caliber. Their course is slightly 

 wavy or spiral. Beginning at the pulp-cavity, into which these canals open, 

 they are found to branch and occasionally anastomose, their communications 

 and branches becoming more frequent as they approach the external surface 

 of the tooth. The canals of largest diameter are found next the pulp-cavity, 

 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 intertubular substance of the dentine ; but in 



some portions of the tooth, the tu- 

 bules are so abundant that their 

 walls touch each other, and there 

 is, therefore, no intertubular sub- 

 stance. Near the 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. Sometimes these cavities form 

 regular zones near the peripheral 

 termination of the tubules. The 

 dentine is sometimes marked by 

 concentric lines, indicating a lam- 

 ellated arrangement. In the nat- 

 ural condition, the dentinal tubules 

 are filled with a clear liquid, which 

 penetrates from the vascular struct- 

 ures 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 at the deeper 

 portions of the root, where it is 

 sometimes lamellated, and it be- 

 comes thinner near the neck. It 



3 



FIG. 52. Tooth of the cat, in situ (Waldeyer). 



finally becomes continuous with 



1, enamel ; 2, dentine ; 3, cement ; 4, periosteum of the the enamel of the Crown, SO that 

 alveolar cavity ; 5, lower jaw ; 6, pulp-cavity. , 



the dentine is everywhere com- 

 pletely covered. The cement is closely adherent to the dentine and to the 

 periosteum lining the alveolar cavities. 



Pulp-Cavity. In the interior of each tooth, extending from the apex of 

 the root or roots into the crown, is the pulp-cavity, which contains minute 



