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HANDBOOK OF PHYSIOLOGY. 



only to about 2 or 3 per cent. It contains a larger proportion of inor- 

 ganic matter and is harder than any other tissue in the body. 



Structure. Examined under the microscope, enamel is found com- 

 posed of fine hexagonal fibres (Figs. 169, 170) 

 WOT ^ 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 den- 

 tine 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 tu- 

 bules, they are not straight, but disposed in wavy 

 and parallel curves. The fibres are marked by 

 transverse lines, and are mostly solid, but some of 

 them contain a very minute canal. 



The enamel-prisms are connected together by 

 a very minute quantity of hyaline cement-sub- 

 stance. In the deeper part of the enamel, between 

 the prisms, are small lacunce, which communicate 

 with the " interglobular spaces " on the surface of 

 the dentine. 



The enamel itself is coated on the outside by 

 a very thin calcified membrane, sometimes termed 

 the cuticle of the enamel. 



FIG. 169. Thin section 

 of the enamel and a part of 

 the dentine, a, cuticular 

 pellicle of the enamel; &, 

 enamel fibres, or columns 



III. Crust a Petrosa. 



cavities in the enamel, com- 

 municating with the extrem- 

 ities of some of the tubuli 

 (d). X350. (Kolliker.) 



The crusta petrosa, or cement (Fig. 168, c, d), 

 is composed of true bone, and in it are lacuna? 

 (f) and canaliculi (g), which sometimes com- 

 municate with the outer finely branched ends of 

 the dentine tubules. Its laminae are as it were 

 bolted together by perforating fibres like those of ordinary bone, but it 

 differs from ordinary bone in possessing Haversian canals only in the 

 thickest part. 



Development of the Teeth. 



Development of the Teeth. The first step in the development of the 

 teeth consists in a downward growth (Fig. 171, A, 1) from the stratified 

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

 thickened in the neighborhood of the jaws or maxillae which are in the 

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

 groove) of the imperfectly developed tissue of the embryonic jaw. The 

 downward epithelial growth forms the primary enamel organ or enamel 

 germ, and its position is indicated by a slight groove in the mucous 



