878 THE ORGANS OF DIGESTION. 



of the phosphate with the carbonate of calcium, with traces of fluoride of calcium, 

 phosphate of magnesia, and other salts. 



The cortical substance, or cementum (crusta 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 root, where it is usually very thick. In structure and chemical composition 

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

 characterize true bone; the lacunae placed near the surface have the canaliculi 

 radiating from the side of the lacunae toward the periodontal membrane, dental 

 periosteum, and those more deeply placed join with adjacent dentinal tubuli. In 

 the thicker portions of the crusta petrosa the lajnellse and Ha'versian canals pecu- 

 liar to bone are also occasionally found. 



As age advances the cement 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-dentine, Owen ; 

 secondary dentine, Tomes). It is formed by the odontoblasts, the dental pulp lessening in 

 volume. 



Development of the Teeth. 



The teeth are an evolution from the dermoid system, and not of the bony skele- 

 ton : they are developed from two of the blastodermic layers, the epiblast and 

 mesoblast. From the former the enamel is developed, from the latter the dental 

 pulp, dentine, cementum, and pericementum. It is customary to view the develop- 

 ment of the permanent and temporary teeth as separate studies. 



The earliest evidence of tooth-formation in the human embryo is observed in 

 about the seventh week. The mucous membrane covering the embryonic jaws is 

 seen to rise as a longitudinal ridge along the summit of each jaw. 1 A transverse 

 section through the jaws will show the elevation to be due to a linear and outlined 

 activity of the germinal epithelial layer : a corresponding epithelial growth is seen 

 to sink as a band into the mesoblastic tissue beneath. The local cell-activity con- 

 tinues, and in its descent the band appears to meet with a resistance which causes a 

 flattening of its extremity into a continuous lamina. From the inner (toward the 

 tongue) edge of the lamina epithelial cords are given off, ten in number, one for 

 each temporary tooth. 



The growth of each cord continues, and each expands into a flask -like form, the 

 walls covered by a layer of germinal cells, its interior by swollen mature cells. 

 The ingrowing bulb is now seen to flatten upon its lower surface, as though it had 

 met with an outlined resistance from the mesoblastic tissue beneath. The epithelial 

 ingrowth assumes the general form of the several teeth ; it is the enamel-organ of 

 the tooth (Fig. 478). At this period the mesoblastic tissue around each enamel- 

 organ is seen to become differentiated into fibrous tissue surrounding the enamel- 

 organs, but at some distance from them. Islets of bone are also seen to be forming 

 the beginning of the bony maxillae. 



The indentation of the base of the enamel-organ continues until it assumes 

 the form of the future teeth. The cells bounding the organ assume a cylin- 

 drical form ; the cells of the interior become much expanded, irregular in size 



-in r O 



and form. 



The mesoblastic tissue underlying the enamel-organ is much condensed ; evi- 

 dences of cellular differentiation and a vascular system appear. Bone continues to 

 develop until all of the tooth-follicles are embraced in a gutter of bone. From the 

 lingual side of the cords of the temporary teeth epithelial buds are given off, which 

 sink into the mesoblastic tissue and form the enamel-organs of the permanent teeth. 

 The condensation of fibrous tissue continues until each embryonic tooth is enveloped 

 in a sac, the dental sac ; this, together with all of its contents, is called the dental 

 follicle. 



The cells of the enamel-organ now undergo a series of differentiations : the 

 inner layer is arranged as columnar epithelium, and is called the ameloblastic or 

 1 The maxillary rampart of Kolliker Waldeyer. 



