THE MOUTH 



1119 



passing into a dental canaliculus, the inner being continuous with the processes of the connective- 

 tissue cells of the pulp matrix. 



The solid portion of the tooth consists of (1) the ivory or dentin, which forms the bulk of the 

 tooth; (2) the enamel, which covers the exposed part of the crown; and (3) a thin layer of bone, 

 the cement or crusta petrosa, which is disposed on the surface of the root. 



The dentin (substantia eburnea; ivory) (Fig. 1007) forms the principal mass of a tooth. It is 

 a modification of osseous tissue, from which it differs, however, in structure. On microscopic 

 examination it is seen to consist of a number of minute wavy and branching tubes, the dental 

 canaliculi, imbedded in a dense homogeneous substance, the matrix. 



Eoct 



FIG. 1006. Vertical section of a molar tooth. 



_Fic. 1005. Vertical section of a tooth in situ. X 15. 

 c is placed in the pulp cavity, opposite the neck of the 

 tooth; the part above it is the crown, that below is the 

 root. 1. Enamel with radial and concentric markings. 

 2. Dentin with tubules and incremental lines. 3. Cement 

 or crusta petrosa, with bone corpuscles. 4. Dental 

 periosteum. 5. Mandible. 



Crown 



Neck 



Boot 



FIG. 1007. Vertical section of a premolar tooth. 

 (Magnified.) 



The dental canaliculi (dentinal tubules) (Fig. 1008) are placed parallel with one another, and 

 open at their inner ends into the pulp cavity. In their course to the periphery they present two 

 or three curves, and are twisted on themselves in a spiral direction. These canaliculi vary in 

 direction: thus in a tooth of the mandible they are vertical in the upper portion of the crown, 

 becoming oblique and then horizontal in the neck and upper part of the root, while toward the 

 lower part of the root they are inclined downward. In their course they divide and subdivide 

 dichotomously, and, especially in the root, give off minute branches, which join together in 

 loops in the matrix, or end blindly. Near the periphery of the dentin, the finer ramifications 

 of the canaliculi terminate imperceptibly by free ends. The dental canaliculi have definite walls, 

 consisting of an elastic homogeneous membrane, the dentinal sheath of Neumann, which resists 

 the action of acids; they contain slender cylindrical prolongations of the odontoblasts, first 

 described by Tomes, and named Tomes' fibers or dentinal fibers. 



