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STRUCTURE OF TEETH. 



Ill 



The teeth, though composed of the same chemical ingre- 

 dients as the ordinary hones, differ from them by having a 

 greater density and compactness of texture, whence they 

 derive that extraordinary degree of hardness which they re- 

 quire for the performance of their peculiar office. The sub- 

 stances of which they are composed 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 ge- 

 nerally given to it. Its earthy ingredient oonsists almost 

 entirely of phosphate of lime, the proportion of the carbo- 

 nate of that earth entering into its composition being very 

 small; and the animal portion is albumen, with a small quan- 

 tity of gelatin. 



A layer of a still harder substance, termed the enamel, 

 usually covers the ivory, and, in teeth of the simplest struc- 

 ture, forms the whole of their outer surface: this is the 



case with the teeth of man and of carnivorous quadrupeds. 

 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 atrace of gelatin; 



