110 



THE STEUCTUKE OF THE HOUSE 



axis of the tooth, and affords the means by which its 

 vitality is preserved. This occupies a larger relative 

 space, and performs a more important purpose in the 

 young, growing tooth than afterwards, as, by the calcifi- 

 cation and conversion of its outer layers, the principal 

 hard constituent of the tooth, the dentine, is formed. 

 In teeth which have ceased to grow the pulp occupies 

 a comparatively small space, which in the dried tooth is 

 called the pulp-cavity. This communicates with the 

 external surface of the tooth by a small aperture at the 

 apex of the root, through which the branches of the 

 nutrient blood-vessels and sensitive nerves necessary 

 to maintain the vitality of the tooth pass in, to be 

 distributed to the pulp. In growing teeth the pulp- 

 cavity is widely open below, while in advanced age it 

 often becomes obliterated, and the pulp itself entirely 

 converted into bone-like material. 



2. The dentine, or ivory, forms the principal con- 

 stituent of the greater number of teeth. It is a very 

 hard but elastic substance, white, with a yellowish tinge, 

 and slightly translucent. Its chemical composition is 

 very like that of bone, but its microscopical structure is 

 altogether different. 



3. The enamel constitutes a thin investing layer, 

 complete or partial, of the exposed or working surface 

 of the dentine of the crown of the teeth of most mammals. 

 This is the hardest tissue met with in the body, con- 



