THE GRIKDERS THEIR OWK WHETSTOKES. 63 



lower edges have ouly about half the liight of the up- 

 per, they do not require more than half the quantity 

 of enamel to strengthen them. Another use of this 

 unequal disposition of enamel is its tendency, by its 

 wear, to preserve the slant of the respective crown sur- 

 faces. Further, the dentine, which fills the interspaces 

 between the folds or ridges of enamel, being softer 

 than the enamel, wears out faster, thus keeping the 

 ridges sliarp.* The grinders are therefore, owing to 

 this "interblending of the dental tissues," their own 

 whetstones as well as the horse's millstones. 



Some writers, even of the present day, deny that the 

 enamel penetrates to the interior of the grinders; but 

 the fact that it does was established by John Hunter 

 over a century ago, and a cut of a section of a horse's 

 grinder (slightly magnified) showing the enamel folds, 



* Prof. R. Owen illustrates the above principle in tlie Intro- 

 duction to his " Odontoe;raph y," pao^e 26. He says : " It (the 

 enamel) sometimes forms only a partial itivestment of the crown, 

 as in the molar teeth of the iguanodon, the canine teeth of the 

 hog and hippopotamus, and the incisors of the Rodentia. In 

 these the enamel is placed only on the front of the tooth, but is 

 continued along a great part of tlie invested base, which is never 

 contracted int6 one or divided into more roots, so that the char- 

 acter of the crown of the tooth is maintained throughout its 

 extent as regards both its shape and structure. The partial 

 application of the enamel operates in maintaining a sharp edge 

 upon the exposed and worn end of the tooth precisely as the 

 hard steel keeps up the outer cutting edge of the chisel by being 

 welded against an inner plate of softer iron." 



Prof. C. S. Tomes, speaking of the grinder teeth of the horse, 

 says : " As each ridge and pillar of the tooth consists of dentine 

 bordered by enamel, and the arrangement of the ridges and pil- 

 lars is complex, and as, moreover, cementum fills up the inter- 

 spaces, it is obvious that an efficient rough grinding surface will 

 be preserved by the unequal wear of the several tissues." 



