150 THE VITAL FUNCTIONS. 



chemical ingredients as the ordinary bones, 

 differ from them by having a greater density 

 and compactness of texture ; whence they derive 

 that extraordinary degree of hardness which 

 they require for the performance of their peculiar 

 office. The substances of which they are com- 

 posed are of three different kinds ; the first, 

 which is the basis of the rest, constituting the 

 solid nucleus of the tooth, has been considered 

 as the part most analogous in its nature to bone ; 

 but from its much greater density, and from its 

 differing from bone in the mode of its formation, 

 the name of ivory has been generally given to it. 

 Its earthy ingredient consists almost entirely of 

 phosphate of lime ; the proportion of the car- 

 bonate of that earth entering into its composition 

 being very small ; and the animal portion is 

 albumen, with a small quantity of gelatin. 



A layer of a still harder substance, termed the 

 etmmel, usually covers the ivory, and, in teeth of 

 the simplest structure, forms the whole of their 

 outer surface : this is the case with the teeth of 

 man and of carnivorous quadimpeds. These two 

 substances, and the direction of their layers, are 

 seen in Fig. 277, which is the section of a simple 

 tooth. E is the outer case of enamel, o the 

 osseous portion, and p the cavity where the 

 vascular pulp which formed it was lodged. The 

 enamel is composed almost wholly of phosphate 

 of lime, containing no albumen, and scarcely a 



