1548 



HUMAN ANATOMY. 



TOOTH-STRUCTURE. 



In principle, and among the lower vertebrates in fact as well, teeth may be 

 regarded as hardened papilLx of the oral mucous membrane ; they consist, therefore, 

 of two chief parts, — the connective-tissue core and the epithelial capping. Of the 

 three constituents present in typical mammalian teeth, the enamel is the derivative of 

 the ectoblastic epithelium, the dentine, with the pulp, and the cementum being con- 

 tributions of the embryonal connective tissue. 



The Enamel. — This, the hardest tissue of the body, covers the crown, being 

 thickest on the cutting edge or grinding surface of the tooth. It gradually thins away 



Fig. 1306. 



Stripes of Retzius (longitudinal) 



Contour lines 

 Schreger's lines 



Neck 



Prism-stripes of Schreger (light and 

 dark) 



u. Cementum 

 Alveolar periosteum 



Osseous tissue of jaw 



^" Root-canal 



Sagittal section of canine tooth in situ. Semi-diagrammatic. 



towards the neck, around which its terminal border appears as a more or less distinct 

 and often serrated edge. The external surface of the enamel, especially in young 

 teeth, often exhibits a fine striation composed of horizontally disposed lines. Under 

 a hand-glass these lines are seen to be minute elevations, the enamel-ridges, which 

 encircle the crown. The remarkable hardness of this tissue is due to the large amount 

 (97 per cent.) of earthy material and the small proportion of organic matter, which 

 latter in adult enamel averages only about 3 per cent. ; in infantile enamel the amount 

 of animal material is from five to six times greater (Hoppe-Seyler). 



