STRUCTURE OF TEETH. 153 



substances ; and their grinding surfaces present 

 ridges of enamel intermixed in a more irregular 

 manner with the ivory and crusta petrosa ; but 

 still giving the advantage of a very rough surface 

 for trituration. Fig. 278 represents the grinding 

 surface of the tooth of a horse, worn down by 

 long mastication, e is the enamel, marked by 

 transverse lines, showing the direction of its 

 fibres, and enclosing the osseous portion (o), 

 which is shaded by interrupted lines. An outer 

 coating of enamel {e) is also visible; and between 

 that and the inner coat, the substance called 

 crnsta petrosa (c), marked by waving lines, is 

 seen : on the outside of all there is a plate of 

 bone, which has been left white. In ruminants, 

 the plates of enamel form crescents, which are 

 convex outwardly in the lower, and inwardly in 

 the upper jaw ; thus providing for the crossing 

 of the ridges of the two surfaces ; an arrange- 

 ment similar to that which is practised in con- 

 structing those of mill-stones. The teeth of the 

 lower jaw fall within those of the upper jaw; so 

 that a lateral motion is required in order to bring 

 their surfaces opposite to each other alternately 

 on both sides. Fig. 2^7.9 shows the grinding sur- 

 face of the tooth of a Sheep^ where the layers of 

 bone are not apparent ; there being only two layers 

 of enamel (e), and one of crusta petrosa (c). 



These three component parts are seen to most 

 advantage in a vertical and longitudinal section 



