THE DENTINE. 



133 



Enamel 



The Dentine. The dentine or ivory, the substance which contributes 

 the bulk of the tooth, encloses the cavity containing the pulp and is itself 

 surrounded by the enamel and the cementum. In both its genesis and 

 chemical composition, dentine resembles bone, like the latter being a con- 

 nective tissue modified by impregnation with lime-salts. Dentine exceeds 

 bone in hardness and contains a larger proportion (72 per cent.) of earthy 

 matter and a smaller amount (28 per cent. ) of organic substance. After 

 decalcification with acids, the remaining animal material retains the previous 

 form of the dentine and yields gelatin on prolonged boiling in water. Den- 

 tine, like bone, is formed through the agency of specialized connective tissue 

 cells, the odontoblasts , but 

 differs from it in the small 

 number of these cells which 

 become imprisoned in the 

 intercellular matrix. When 

 this occurs, as it occasional- 

 ly does, the dentine-cells cor- 

 respond to the bone-cells, 

 both being connective tis- 

 sue elements within lymph- 

 spaces in the calcified inter- 

 cellular substance. 



Examined in dried sec- 

 tions under low magnifica- 

 tion, the dentine exhibits a 

 radial striation, composed 

 of fine dark- lines which 

 extend from the pulp-cavity 

 internally to the enamel ex- 

 ternally. These dark lines 

 are the dentinal tubules, 

 filled with air, which are 

 homologous with the lacunae 

 and canaliculi of bone and 

 contain the dentinal fibres, 

 as the processes of the odon- 

 toblasts are called. In the 

 crown, as seen in longitudi- 



! ^.u^ ~f 



nal Sections, the COUrse Of 



the dentinal tubules is radial 

 to the pulp-cavity; in the body and fang their course is approximately 

 horizontal and almost parallel. The canals, however, are not straight but 

 wavy, the first bend being directed towards the root and the second towards 

 the crown. In addition to these primary curves, especially marked in the 

 crown, the tubules exhibit numerous shorter secondary curves, the whole 

 arrangement imparting to the individual canals a spiral course. In conse- 

 quence of the uniformity of these curvatures, the tooth-ivory displays a 

 series of linear markings, Schreger 's lines, which parallel the outer surface 

 of the dentine. These markings, however, must not be confounded with 

 another set of striae, the contour lines of Owen, or the incremental lines of 

 Salter, which, best seen in the crown, run obliquely to the surface of the 

 dentine (Fig. 171) and depend probably upon variations in calcification 

 incident to the growth of the dentine. 



FIG. 173. Ground section of human tooth including adjoining 

 enamel and dentine. X 280. 



