158 TEETH. 



dentine is invariably the internal substance ; it needs to occupy such a 

 position, as within it the sensation resides. The crusta petrosa and the 

 enamel may be tampered with without perception being aroused; but' 

 the dentine is capable of communicating the acutest agony; and it is 

 upon the dentine that rogues operate, when they "bishop" an old 

 horse. 



To convince the reader that nature has not needlessly sacrificed the 

 whiteness of the horse's tooth, the author will dilate fully upon the many 

 services afforded by the dark-colored crusta petrosa. To render the 

 explanation more intelligible, reference will be here made to a common 

 tool seen every day in the hands of an ordinary mechanic. The brick- 

 layer's trowel appears to be nothing more than a thick layer of metal ; 

 but it is hourly put to uses for which iron would be too soft, and steel 

 would be too brittle. Therefore, the blade is composed of a thin layer of 

 steel, inclosed within two comparatively thick layers of iron. By the 

 combination of opposite qualities, perfect utility is produced ; and this 

 trowel, it seems hard to believe, was not suggested by that arrangement 

 which is conspicuous in the horse's tooth. 



The enamel, hard, brittle, and readily fractured, but presenting a fine 

 or a cutting edge, is developed as a thin layer, convoluted upon the sides 

 of the dentine, and securely covered by crusta petrosa. That the incisive 

 substance may fulfill its oflSce, may sever or comminute the tough and 

 fibrous herbs upon which the equine race subsist, it is inclosed between 

 two elastic bodies, the whole being held together by the vessels which 

 pass from the exterior to the interior of the organ, though these vessels 

 do no more than travel through the enamel without nourishing or sup- 

 porting it ; the latter structure being of a crystalline nature, or strictly 

 inorganic, therefore not fitted to appropriate nutriment. 



The crusta petrosa is, however, of further use than has been already 

 stated. The horse's grindei-s are generally supposed to be gifted with a 

 power of growth whereby they are enabled to repair that perpetual loss 

 of substance to which their employment must subject them. The teeth, 

 certainly, are not perfected when the crowns first appear in the mouth ; 

 so far the opinion is capable of being upheld. But when once completed, 

 the dentine is not endowed with any innate ability to renew its loss of 

 substance. The wear consequent upon continual use is provided for by 

 the length of fang which characterizes the permanent molar of the quad- 

 ruped. As the surface gradually decreases, so are the lower parts of the 

 teeth, by the contraction of the jaw-bones, forced into the mouth, while 

 the outward investing substance — the crusta petrosa — being gifted with 

 a hmited power of increase, is enabled thereby to firmly retain the 

 protruded fang in its new position; although the contraction of the 



