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 j 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 



