DEVELOPMENT OF THE TEETH. C49 



boiling into gelatin. The earthy matter consists of phosphate of lime, carbonate 

 of lime, a trace of fluoride of calcium, and phosphate of magnesia, and other salts. 



The ENAMEL is the hardest and most compact part of a tooth, and forms a thin 

 crust over the exposed part of the crown, as far as the commencement of the fang. 

 It is thickest on the grinding surface of the crown, until worn away by attrition, 

 and becomes thinner towards the neck. It consists of a congeries of minute hexa- 

 gonal rods. They lie parallel with one another, resting by one extremity upon 

 the dentine, which presents a number of minute depressions for their reception ; 

 the outer extremity forming the free surface of the crown. These fibres are 

 directed vertically on the summit of the crown, horizontally at the sides ; they are 

 about the 5? V^ f an i ncn m diameter, and pursue a more or less wavy course, 

 which gives to the cut surface of the enamel a series of concentric lines. 



Numerous minute interstices intervene between the enamel-fibres near their 

 dentinal surface, a provision calculated to allow of the permeation of fluids from 

 the dentinal tubuli into the substance of the enamel. The enamel-rods consist of 

 solid hexagonal or four-sided prisms, connected by their surfaces and ends, and filled 

 with calcareous matter. If the latter is removed, by weak acid, from newly-formed 

 or growing enamel, it will be found to present a network of delicate prismatic cells 

 of animal matter. 



Chemical Composition. According to Bibra, enamel consists of 96.5 per cent. 

 of earthy matter, and 3.5 per cent, of animal matter. The earthy matter consists 

 of phosphate of lime, with traces of fluoride of calcium, carbonate of lime, phos- 

 phate of magnesia and other salts. 



The CORTICAL SUBSTANCE or cement (crmta petrosa) is disposed as a thin 

 layer 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 com- 

 position, it resembles bone. It contains, sparingly, the lacunas and canaliculi which 

 characterize true bone ; those 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 lamellae and Haversian canals peculiar to bone 

 are also found. As age advances, the cementum increases in thickness, and gives 

 rise to those bony growths or exostoses, 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-ckntine, Owen; secondary dentine, 

 Tomes). It appears to be formed by a slow conversion of the dental pulp, which 

 shrinks, or even disappears. 



DEVELOPMENT OF THE TEETH (figs. 324 to 329). 



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. 324), the mucous membrane covering the edge of 

 the upper jaw, presents a semicircular depression 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 germ of each tooth is formed by a conical 

 elevation or papilla of mucous membrane (fig. 325), which constitutes the rudimen- 

 tary 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 deciduous molar 

 of the upper jaw appears ; at the eighth week, that for the canine tooth is deve- 

 loped ; the two incisor papillae appear about the ninth week (the central preceding 

 the lateral); lastly, the second molar papilla is seen at the tenth week, behind the 

 anterior 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. 



