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 

 crusta 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. 279 shows the grinding sur- 

 face of the tooth of a SJteep, 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 



