152 THE VITAL FUNCTIONS. 



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 enamel on their anterior convex sur- 

 faces only ; 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 admirably calculated to 

 preserve these chisel teeth fit for use during the 

 whole life-time 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 struc- 

 ture 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 re- 

 ceived various names, but it is most commonly 

 known by that of the Crusta petrosa : it resem- 

 bles ivory both in its composition and its extreme 

 hardness ; but is generally more opaque and 

 yellow than that substance. 



Other herbivorous quadrupeds, as the horse, 

 and animals belonging to the ruminant tribe, 

 have also complex teeth composed of these three 



