DEVELOPMENT OF THE TEETH. 



715 



Development of the Teeth. 

 Fig. 389. 



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on the roots of the teeth, from the termination of the enamel, as far as the apex 

 of the fang, where it is usually very thick. In structure and chemical composi- 

 tion, it resembles bone. It contains, sparingly, the lacuna? and canaliculi which 

 characterize true bone : the lacuna placed near 

 the surface have the canaliculi radiating from 

 the side of the lacunae towards the periodontal 

 membrane ; and those more deeply placed, join 

 with the adjacent dental tubuli. In the thicker 

 portions of the crusta petrosa, the lamella? and 

 Ilaversian canals peculiar to bone are also found. 

 As age advances, the cement increases in thick- 

 ness, and gives rise to those bony growths, or ex- 

 ostoses, so common in the teeth of the aged ; the 

 pulp cavity becomes also partially filled up by a 

 hard substance, intermediate in structure between 

 dentine and bone (osteo-dentine, Owen ; secondary 

 dentine, Tomes). It appears to be formed by a 

 slow conversion of the dental pulp, which shrinks, 

 or even disappears. 



Fig. 390. 



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Fig. 391. 



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DEVELOPMENT OF THE TEETH. (Figs. 389 to 394.) 



According to the observations of Arnold and 

 Goodsir, the teeth are developed from the mucous 

 membrane covering the edges of the maxillary 

 arches. About the sixth week of foetal life (Fig. 

 389), the mucous membrane covering the edge 

 of the upper jaw presents a semicircular depres- 

 sion or groove: this is the primitive dental groove 

 (Goodsir), from the floor of which the germs of 

 the ten deciduous or milk-teeth are developed. 

 The gerrn of each tooth is formed by a conical 

 elevation or papilla of mucous membrane (Fig. 

 390) which constitutes the rudimentary pulp of 

 a milk-tooth. The germs of the milk-teeth make 

 their appearance in the following order : at the 

 seventh week, the germ of the first molar of the 

 upper jaw appears ; at the eighth week, that for 

 the canine tooth is developed; the two incisor 

 papilla? appear about the ninth week (the central 

 preceding the lateral); lastly, the second molar 

 papilla is seen at the tenth week, behind the an- 

 terior molar. The teeth of the lower jaw appear 

 rather later, the first molar papilla being only 

 just visible at the seventh week ; and the tenth 

 papilla not being developed before the eleventh 

 week. This completes the first or papillary stage 

 of their development. 



The dental groove now becomes contracted, 

 its margins thickened and prominent, and the 

 groove is converted into follicles for the recep- 

 tion of the papilla?, by the growth of membranous 

 septa, which pass across the groove between its 

 borders (Fig. 391). The follicles by this means 

 become the alveoli, lined by periosteum, from 

 the bottom of which the process of the mucous 

 membrane of the gum rises, which is the germ 



Fig. 392. 



Fig. 393. 



Fig. 394. 



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