TEETH 



The 



of the enamel is maintained throughout the crown of the tooth 

 mineral constituents of enamel are identical with those of dentine. 



Cement is a bone-like substance covering the root of the tooth as a 

 thin layer which becomes thickest at the apex. Like other bone, the 

 cement contains lacunae connected with one another by canalicuH. 

 The mineral constituents are identical with those of bone. Surrounding 



ODONTOBLASTS' 



Fig. 147. — Diagrams illustrating the difference in the secretion of dentine A and 

 of bone B. The functional polarization of the odontoblasts and osteoblasts is, how- 

 ever, similar. (Redrawn after Braus.) 



the cement, is a connective tissue dental sac or membrane continuous 

 with the periosteum of the alveolus and at the neck connected with the 

 covering of the gum, gingiva. 



Development of Teeth 



When the human embryo has attained a length of about 11 mm., 

 that is, by the end of the sixth week, the ectodermal epitheHum covering 



ERDERMIS 



DENTAL RIDGED 



DENTAL 

 RIDGE 



CORIUM 



BONY ALVEOLUi 



LINGUAL LAMINA 



ENAMEL ORGAN OF PERMANENT TOOTH 

 Fig. 148. — Diagrams of three stages in the development of a mammalian tooth as 

 seen in'sections of the jaw. The anlage of the permanent tooth lies on the lingual side 

 of that of the milk-tooth. (Redrawn after O. Hertwig and Arey.) 



the upper and lower jaws grows rapidly down into the underlying con- 

 nective tissue to form a horseshoe-shaped ridge or lamina extending 

 along the edge of the jaw. As growth continues, the lamina divides into 

 an outer labial lamina and an inner lingual lamina. The two ingrowths. 



