DEVELOPMENT OF THE TEETH 259 



urn is firmly united to the granular layer of the dentine, the matrix 

 of the two tissues being continuous. 



The cementum is invested with a periosteal coat, the peri- 

 odontium, alveolar periosteum, or root membrane, of dense fibrous 

 tissue which, at the neck of the tooth, unites with the dense con- 

 nective tissue of the gum to form an annular thickening of very 

 be 



FIG. 217. FROM A SECTION OF A HUMAN TOOTH WHICH HAD BEEN GROUND TO EXTREME 



THINNESS. 

 a, dentine ; ft, granular layer of Thomes ; c, cementum. Photo, x 140. 



dense fibrous tissue which encircles the tooth and is known as the 

 circular dental ligament. The root membrane contains no elastic 

 fibres, but sends considerable numbers of slender white fibrous 

 bands (Sharpey's fibres) into the cementum. 



DEVELOPMENT OF THE TEETH 



The teeth arise partly from the epithelium of the oral cavity 

 and partly from the connective tissue of the alveolar processes. 

 In the seventh week of fetal life there appears upon the surface 

 of the maxillary ridges a thickening of the epithelium which 

 grows into the subjacent connective tissue in the form of a longi- 

 tudinal ridge or shelf, the dental ridge, whose position is indicated 

 by a dental groove which indents the epithelial surface. 



The dental ridge forms the earliest anlage of the enamel, and 

 at this early stage it shows no indication of the future subdivisions 

 which correspond to the several temporary teeth. On its inner 

 side is a similar ingrowth of epithelial cells which is destined to 

 form the vallum between the lips and the alveolar processes. 



At the beginning of the third month the dental ridge shows 

 upon its deep margin slight indentations, one for each of the tern- 



