76 



HANDBOOK OF PHYSIOLOGY. 



amels, between the prisms, are small lacuncB, or " interglobular spaces" 

 which have the processes or fibrils of the dentine tubes in connection with 

 them (fig. 77, c). 



Fig. 76. Enamel fibres. A, Fragments and single fibres of the transversely-striated enamel, 

 isolated by the action of hydrochloric acid. B, Surface of a small fragment of enamel, showing the 

 hexagonal ends of the fibres with darker cent-res, or not so highly calcified, x 350. (Kolliker.) 



III. Crust a Petrosa. 



The crusta petrosa, or cement (fig. 75, e, d), is composed of true bone, 

 and in it are lacunae (/) and canaliculi (g), which sometimes communi- 

 cate with the outer finely branched ends of the dentine tubules, and 

 generally with the interglobular spaces. Its lamina? are as it were bolted 

 together by perforating fibres like those of ordinary 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. 



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

 teeth consists in a downward growth (fig. 78, A, 1) from the Rete Mal- 

 pighi or the deeper layer of stratified epithelium of the mucous mem- 

 brane of the mouth, which first becomes thickened in the neighborhood 

 of the maxillae or jaws now in the course of formation. This process 

 passes downward into a recess 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 membrane of the jaw. The next step in the pro- 

 cess consists in the elongation downward of the enamel groove and of 



