THE ENAMEL. 



THE TEETH. 



In principle the teeth may be regarded as hardened papillae of the oral 

 mucous membrane; they consist, therefore, of two fundamental parts, the 

 connective tissue body or core and the epithelial capping. The primary 

 tissues become greatly modi- 

 fied and give rise to the three 

 constituents of the typical 

 mammalian tooth, of which 

 the enamel is derived from 

 the ectodermic epithelium, 

 and the dentine, with the pulp, 

 and the cementum are pro- 

 duced by the mesoderm. 



The Enamel. This, 

 the hardest tissue of the body, 

 covers the crown, the part of 



Schreger's.. 

 lines 



Stripes of Retzius 

 (longitudinal) 



Contour 

 lines 



tooth projecting beyond 

 gum, and is thickest on 



Neck 



Prism-stripes of 

 I Schreger (light 

 and dark) 



Gum 



Pulp-tissue 



Dentine 



the 



the gum, ana is 

 the cutting edge or grinding 

 surface. It gradually thins 

 off towards the neck, around 

 which its terminal border ap- 

 pears as a wavy or serrated 

 line. The remarkable hard- 

 ness of the enamel is due to 

 the excessive amount (97 per 

 cent. ) of earthy material and 

 the small proportion (3 per 

 cent. ) of organic matter which 

 it contains. The enamel 

 the product of epithelial cells, 

 the ameloblasts consists of 

 an aggregation of five- or 

 six-sided columnar elements, 

 the enamel-prisms, which 

 measure from 3-5 mm. in 

 length and from 35 /*. in 

 width. Their general dispo- 

 sition is at right angles to 

 both the surface of the den- 

 tine, upon which they rest, 

 and the exterior surface of 

 the crown. Since the prisms 

 usually extend the entire 

 thickness of the enamel, they are of slightly larger diameter at the surface 

 of the tooth than next the dentine. They run for a short distance almost 

 at right angles to the surface of the dentine, then bend laterally for a con- 

 siderable part of their course, but reassume a vertical path on approaching 

 the external surface. In addition to these general curves, the ranges of 

 enamel-prisms have a spiral arrangement, in consequence of which the 

 parallelism of the prisms is disturbed and the bundles appear in sections 



'- Root-canal 



FIG. 171. Sagittal section of canine tooth in situ. 

 grammatic. 



Semidla- 



