112 THE VITAL FUNCTIONS. 



it is the hardest of all animal substances, and is capable of 

 striking fire with steel. It exhibits a fibrous structure, ap- 

 proaching to a crystalline arrangement, and the direction of 

 its fibres, as shown by the form of its fragments when bro- 

 ken, is every where perpendicular to the surface of the ivory 

 to which it is applied. The ends of the fibres are thus alone 

 exposed to the friction of the substances on which the teeth 

 are made to act; and the effect of that friction in wearing the 

 enamel is thus rendered the least possible. 



In the teeth of some quadrupeds, as of the Rhinoceros, 

 the Hippopotamus, and most of the Rodentia, the enamel is 

 intermixed with the ivory, and the two so disposed as to 

 form jointly the surface for mastication. In the progress of 

 life, the layers of enamel, being the hardest, are less worn 

 down by friction than those of the ivory, and therefore 

 form prominent ridges on the grinding surface, preserving 

 it always in that rough condition, which best adapts it for 

 the bruising and comminuting of hard substances. 



The incisors of the rodentia are guarded by a plate of en- 

 amel only on their anterior convex surfaces, so that by the 

 wearing down of the ivory behind this plate, a wedge-like 

 form, of which the enamel constitutes the fine cutting edge, 

 is soon given to the tooth, and is constantly retained as long 

 as the tooth lasts (Fig. 280.) This mode of growth is admi- 

 rably calculated to preserve these chisel teeth fit for use 

 during the whole lifetime of the animal, an object of greater 

 consequence in this description of teeth than in others, which 

 continue to grow only during a limited period. The same 

 arrangement, attended with similar advantages, is adopted 

 in the structure of the tusks of the Hippopotamus. 



In teeth of a more complex structure, a third substance is 

 found, uniting the vertical plates of ivory and enamel, and 

 performing the office of an external cement. This substance 

 has received various names, but it is most commonly known 

 by that of the Crusta petrosa: it resembles ivory both in 

 its composition and its extreme hardness; but is generally 

 more opaque and yellow than that substance. 



