40 



SPECIAL HISTOLOGY. 



the lower half of the crown or the neck of the tooth. Secondly, 

 the periosteum of the alveolus, which unites the tooth very 

 closely with the alveolus. Finally, the pulp, a soft, vascular and 

 nervous mass, which occupies the cavity of the tooth, and is 



Fig. 183. 



connected with the periosteum 

 of the alveolus, through the 

 aperture in the fang. 



The proper tooth (fig. 183) 

 consists of three distinct struc- 

 tures. 1. The dentine, which 

 constitutes its principal mass, 

 and determines its general form. 

 2. The enamel, which forms a tolerably thick investment to 

 the crown; and 3. The cement, which covers the fang ex- 

 ternally. 



§ 138. 



The dentine or ivory, (fig. 183, d) is yellowish- white and 

 translucent or transparent, in thin sections of a recent tooth ; 

 when dry it has a silky or satiny aspect, in consequence of the 

 reception of air into a special system of canals. It is consi- 

 derably harder and more brittle than either the cement or 

 bone, but, on the other hand, yields in these qualities to the 

 enamel. With the exception of a very small spot in the root, 

 the dentine forms the sole boundary of the pulp cavity, and in 

 an uninjured tooth it is never exposed, inasmuch as it is covered, 



Fig. 183. Molar tooth (human), x about 5 : 1, longitudinal ; 2, transverse sec- 

 tion : a, enamel ; 6, pulp cavity ; c, cement ; d, dentine, with its canals. 



