118 THE STRUCTURE OF THE HOESE. 



young, growing tooth than afterwards, as, by the 

 calcification 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 com- 

 municates with the external surf ace 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 yel- 

 lowish tinge, and slightly translucent. Its chemical 

 composition is very like that of bone, but its micro- 

 scopical structure is altogether different. 



3. The enamel constitutes a thin investing layer, 

 complete or partial, of the exposed or working sur- 

 face of the dentine of the crown of the teeth of most 

 mammals. This is the hardest tissue met with in the 

 body, containing from 95 to 97 per cent, of mineral 

 substances (chiefly phosphate, and some carbonate of 



