STRUCTURE OF TEETH. 113 



Other herbivorous quadrupeds, as the horse, and animals 

 belonging to the ruminant tribe, have also complex teeth com- 

 posed of these three 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 to the 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 cres- 

 cents, which are convex outwardly in the lower, and in- 

 wardly in the upper jaw; thus providing for the crossing of 

 the ridges of the two surfaces, an arrangement similar to 

 that which is practised in constructing 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 surface of the tooth of & 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 of the grinding tooth 

 of the elephant, in which they are more completely and 

 equally intermixed than in that of any other animal. Fig. 

 281 presents a vertical section of the grinding tooth of the 

 Asiatic elephant, in the early stage of its growth, and highly 

 polished, so as to exhibit more perfectly its three component 

 structures. The enamel, marked E, is formed of transverse 

 fibres; the osseous, or innermost structure is composed of 

 longitudinal plates. The general covering of crusta pe- 

 trosa, c, is less regularly deposited, p is the cavity which 

 had been occupied by the pulp. In this tooth, which is still 



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