56 



THE CONNECTIVE TISSUES 



[CH. V. 



Crusta Petrosa. 



The crusta petrosa or cement (fig. 66, e,d) is composed of true bone, 

 and in it are lacunse (/) and canaliculi (g), which sometimes com- 

 municate with the outer finely branched ends 

 of the dentinal tubules, and generally with the 

 interglobular spaces. Its laminae are bolted to- 

 gether by perforating fibres like those of ordi- 

 nary bone (Sharpey's fibres). Cement differs 

 from ordinary bone in possessing no Haversian 

 canals, or, if at all, only in the thickest part. 

 Such canals are more, often met with in teeth 

 with the cement hypertrophied than in the 

 normal tooth. 



Development of the Teeth. 



The first step in the development of the 

 teeth consists in a downward growth (fig. 69, 

 1) from the deeper layer of stratified epi- 

 thelium of the mucous membrane of the mouth, 

 which becomes thickened in the neighbour- 

 hood of the maxillae or jaws now in the 

 course of formation. This process passes down- 

 ward into a recess of the imperfectly developed 

 tissue of the embryonic jaw. The downward 

 epithelial growth forms the common enamel or 

 dental germ, and its position is indicated by a 

 slight groove in the mucous membrane of the 

 jaw. After this there is an increased develop- 

 ment at certain points corresponding to the 

 situations of the future milk-teeth. The com- 

 mon enamel germ thus becomes extended by 

 further growth into a number of special 

 enamel germs (fig. 70) corresponding to 

 each of the milk-teeth, and connected to the common germ by a 

 narrow neck (/). Each tooth is thus placed in its own special 

 recess in the embryonic jaw. 



As these changes proceed, there grows up from the underlying 

 connective tissue into each enamel germ (fig. 71, p), a distinct 

 vascular papilla (dental papilla), and upon it the enamel germ 

 becomes moulded, and presents the appearance of a cap of two 

 layers of epithelium separated by an interval (fig. 71, /). Whilst 

 part of the subepithelial tissue is elevated to form the dental 

 papilla, the part which bounds the embryonic teeth forms the dental 



FIG. 68. Thin section of the 

 enamel, and a part of the 

 dentine. a, Cuticular 

 pellicle of the enamel 

 (Nasmyth's membrane) ; 

 6, enamel columns with 

 fissures between them 

 and cross striae ; c, larger 

 cavities in the enamel, 

 communicating with the 

 extremities of some of 

 the dentinal tubules (d). 

 X350. (Kolliker.) 



