58 



HAND-BOOK OF PHYSIOLOGY. 



which are set on end on the surface of the dentine, and fit into corre- 

 sponding 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 toward the sides they tend to 

 the horizontal direction. Like the dentine tubules, 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 con- 

 nected 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 spacdte" 

 on the surface of the dentine. 



The enamel itself is coated on 

 the outside by a very thin calci- 

 fied membrane, sometimes termed 

 the cuticle of the enamel. 



III. Crusta Petrosa. 



The crusta petrosa, or cement 

 (Fig. 60, c, d], is composed of true 

 bone, and in it are lacunae (/ ) and 

 canaliculi (g~) which sometimes 

 communicate with the outer fine- 

 ly 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 in possessing Haver- 

 sian canals only in the thickest 

 part. 



DEVELOPMENT OF TEETH. 



FIG. 63. Section of the upper jaw of a f ratal sheep. 

 A. 1, common enamel-germ dipping down into the 

 mucous membrane ; 2, palatine process of jaw. B. 

 Section similar to A, but passing through one of the 

 special enamel-germs here becoming flask-shaped; c, 

 c', epithelium of mouth; /, neck; /', body of special 

 enamel-germ. C. A later stage ; c, outline of epithe- 

 lium of gum; f, neck of enamel-germ; /', enamel 

 organ; p, papilla; s, dental sac forming 

 enamel-germ of permanent tooth. (W 

 Kolliker.) Copied from Quain's Anatomy. 



ng; f p, the 

 aldeyer and 



Development of the Teeth. The 

 first step in the development of the 

 teeth consists in a downward growth (Fig. 63, A, 1) from the stratified 

 epithelium of the mucous membrane of the mouth, now thickened in the 

 neighborhood of the maxillae which are in the course of formation. This 

 process passes downward into a recess (enamel groove) of the imperfectly 

 developed tissue of which the chief part of the jaw consists. The down- 



