CH. V.] 



ENAMEL 



71 



dentine and bone. According to Tomes, it contains no animal matter 

 at all and only 2 or 3 per cent, of water. Gelatin is a characteristic 

 product of connective tissue, and enamel is not a connective tissue, 

 but is epithelial in origin. 



Examined under the microscope, enamel is found composed of six- 

 sided prisms (figs. 96, 97) ^oVy of an inch in diameter, which are set 

 on end on the surface of the dentine, and fit 

 into corresponding depressions in the same. 



They radiate in such a manner from the 

 dentine that at the top of the tooth they are 

 more or less vertical, while towards the sides 

 they tend to the horizontal direction. Like 

 the dentine tubules, they are not straight, but 

 disposed in wavy and parallel curves. The 

 prisms are marked by transverse lines and are 

 solid. 



The enamel prisms are connected together 

 by a very minute quantity of hyaline cement 

 substance. In the deeper part of the enamel, 

 between the prisms, are often small lacunce, 

 which have the processes or fibrils lying in 

 the dentinal tubes in connection with them 

 (fig. 97, c). 



Crusta Petrosa. 



The crust a petrosa or cement (fig. 95, e, d) is 

 composed of true bone, and in it are lacunae (/ ) 

 and canaliculi (y), which sometimes communi- 

 cate 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. 



FIG. 97. Thin section of the 

 enamel, and a part of the 

 dentine. a, Cuticular 

 pellicle of the enamel 

 (Nasmyth's membrane) ; 

 ft, 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). 

 x 350. (Kolliker.) 



Development of the Teeth. 



The first step in the development of the teeth consists in a down- 

 ward growth (fig. 98, A, 1) from the deeper layer of stratified epi- 

 thelium of the mucous membrane of the mouth, which first becomes 

 thickened in the neighbourhood of the maxillae or jaws now in the 

 course of formation. This process passes downward into a recess of 



