THE BEAD AND KECK 



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taining from 05 to 97 per cent, of mineral substances 

 (chiefly phosphate, and some carbonate, of lime, with 

 traces of fluoride of calcium). Enamel is easily dis- 

 tinguished from dentine with the naked eye by its 

 clear, bluish-white, translucent appearance. 



4. The cement, or Grusta petrosa, is always the most 

 externally placed of the tissues of which teeth are com- 

 posed. It is often only found as a thin layer upon the 

 surface of the root ; but sometimes, as in the complex- 

 crowned molar teeth of the horse and elephant, it is a 

 structure which plays a very important part, covering 

 and filling in the interstices between the ridges of the 

 enamel. Its structure and chemical composition is 

 almost exactly that of ordinary bone. 



Succession of Teeth. — The dentition of all mammals 

 consists of a definite set of teeth, of constant and 

 determinate number, form, and situation, and, with few 

 exceptions, persisting in a functional condition through- 

 out the natural term of the animal's life. In many 

 species these are the only teeth which the animal 

 ever possesses — the set which is first formed being 

 permanent, or, if accidentally lost, or decaying in extreme 

 old age, not being replaced by others. But in the 

 horse, as in all other Ungulates, as well as in man, and, 

 in fact, the majority of the class, certain of the teeth are 

 preceded by others, of a smaller size, which occupy the 

 place of the permanent teeth during the growth and 



