STRUCTURE OF TEETH. 137 



fish, a fourth substance is added to the structure 

 by the coarser ossification of the pulp, after its 

 periphral portion has been converted into the 

 dense ivory. The teeth, thus consisting of dentine, 

 enamel, cement, and coarse bone, are the most 

 complicated as regards their substance, that have 

 yet been discovered.* 



Other herbivorous quadrupeds, as the horse, and 

 animals belonging to the ruminant tribe, have also 

 complex teeth composed of the three first of these 

 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 sur- 

 face 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 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 



* Owen. Ibid. p. 9. 



