DEVELOPMENT OF THE TEETH. 



1563 



inner part of the tooth-sac is richly provided with capillaries, and therefore is an 

 important source of nutrition to the developing dental germ. The part of the sac 

 opposite the root of the young tooth is at first prevented from coming into direct 

 contact with the dentine by the double layer interposed by the epithelial sheath. 

 This relation is maintained until the development of the cement begins, when the 

 vascular tissue of the dental sac breaks through the epithelial sheath to reach the 

 surface of the dentine, upon which the cementum is deposited by the mesoblast. In 

 consequence of this invasion, the epithelial sheath is disrupted into small groups or 

 nests of cells which persist for a long time as epithelial islands within the fibrous 

 tissue of the alveolar periosteum into which the dental sac is later converted. 



The formation of the cementum takes place through the agency of the 

 mesoblastic tissue in a manner almost identical with the development of subperiosteal 



FIG. 1323. 



Jaws of child of six years, showing all temporary teeth in place with permanent teeth in various stages of 



development. 



bone, the active cement-producing cells, or cementoblasts, corresponding to the osteo- 

 blasts which deposit the osseous matrix upon the osteogenetic fibres of the periosteum. 

 A conspicuous feature of cementum is the unusual number of transversely disposed 

 bundles of fibrillae, or Sharpey's fibres, among which many are imperfectly calcified. 

 The cementum appears first in the vicinity of the neck of the toqth, and progresses 

 towards the apex of the root as the dentine of the fang is deposited. After the tooth 

 is fully formed, the layer of cement continues to grow until thickest at the apex, which 

 it completely invests, with the exception of the canal leading to the entrance of the 

 pulp-cavity. The cement being deposited directly upon the homogeneous layer con- 

 stituting the external surface of the dentine, the firm connection between the two 

 portions of the teeth is one of adhesion rather than of union. Later secondary 

 changes may exceptionally bring the canaliculi of the cement into communication 

 with the terminations of the dentinal tubules. During the changes incident to the 



