1552 



HUMAN ANATOMY. 



of fibrillae — more regular in dentine than in bone — is longitudinal and parallel to the 

 primary surfaces of the dentine. In addition to the fibres which extend lengthwise, 

 others run obliquely crosswise in the layers of dentine. The bundles of fibrillae 

 measure from .002-003 mm. in diameter, and appear in transverse sections as small 

 punctated fields. The fibrillae are knit together by the calcified organic matrix, in 

 which the lime salts are deposited in the form of spherules, the interstices between 

 which are later filled and calcification thus completed. When, as often happens, the 

 latter process is imperfect, irregular clefts, the interglobular spaces, remain, the con- 

 tours of which are formed by the spheres or deritinal globules of calcareous material. 

 The interglobular spaces are of irregular form and uncertain extent, being usually 

 largest in the crown. At the border between the dentine and the cementum there 

 exists normally a distinct zone, I'Vi^ granular layer of Tomes (Fig. 131 1), composed of 



Fig. 1310. 



Pulp 



p-tissue -5^3 



Granular layer of dentine^ 



Cementum 



Alveolar periosteum — ^^ <w^>, 



Transverse section of root of lower canin« tootii. X 30. 



closely placed interglobular spaces of small size ; under low magnification in ground- 

 sections the spaces appear as dark granules, hence the designation of the zone. Since 

 the existence of these spaces depends upon imperfect calcification of the intertubular 

 ground-substance, the dentinal tubules are unaffected and pass through the spaces on 

 their course to the surface of the dentine, several of the canals traversing the larger 

 spaces. The contour lines of Owen, or the incremental lines of Salter, appear as 

 linear markings, which usually run obliquely to the surface of the dentine (Fig. 1306). 

 They probably depend upon variations in calcification incident to the growth of the 

 dentine, and resemble the interglobular spaces in their origin. The contour lines 

 are best marked in the crown and are only exceptionally seen in the fang. As 

 pointed out by Walkhof?, the lines of Owen and those of Retzius in the enamel are 

 usually present at the same time, since both are expressions of imperfect calcification. 

 The Cementum. — The cement, or crusta petrosa of the older writers, forms 

 an investment of slightly modified osseous tissue from the neck of the tooth to its 



