TEETH 



141 



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 canaliculi. 

 The mineral constituents are identical with those of bone. Surrounding 

 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 

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



ERDERMIS 



CORIUM 



BONY ALVEOLU 



LINGUAL LAMINA 



ENAMEL ORGAN OF PERMANENT TOOTH 



Fig. 129. — Diagrams of three stages in the development of a mammalian tooth as 

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

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



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, 

 however, soon separate, one growing in labially, the other lingually. 



The latter forms the dental ridge or lamina. As in development of a 

 hair, the dental ridge is formed by cell multiplication in the stratum 

 germinativum of the epidermis. 



Early in the development of the dental lamina, a series of bell-shaped 

 enlargements, ten in each jaw, appear along its labial border (Fig. 129). 

 These are known as enamel organs since they secrete the enamel covering 

 of the crowns of the teeth. Each of the twenty milk teeth has a separate 

 enamel organ, and all of them are present in a 2^^ months embryo. 

 Each enamel organ contains a mesenchymatous dental papilla, the outer 

 cells of which, the odontoblasts, secrete the dentine of the tooth. The 

 remaining cells of the papilla become the pulp of the tooth. As develop- 



