THE HEAD AND NECK. 119 



lime, with traces of fluoride of calcium). Enamel 

 is easily distinguished from dentine with the naked 

 eye by its clear, bluish- white, translucent appearance. 



4. The cement, or Crusta petrosa, is always the 

 most externally placed of the tissues of which teeth 

 are composed. 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 impor- 

 tant part, covering and filling in the interstices be- 

 tween the ridges of the enamel. Its structure and 

 chemical composition is almost exactly that of ordi- 

 nary bone. 



Succession of Teeth. — The dentition of all mam- 

 mals consists of a definite set of teeth, of constant 

 and determinate number, form, and situation, and, 

 with few exceptions, persisting in a functional con- 

 dition throughout 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 re- 

 placed by others. But in the horse, as in all other 

 Ungulates, as well as in man, and, in fact, the major- 

 ity 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 gradual 

 9 



