THE TEETH OP THE HORSE. 355 



other, and witli tlieir concavities turned outwards, wticli 

 arise from the wear of the wall ; internal to these, of two 

 other crescents, partly transverse in direction, and con- 

 nected by their anterior ends with the wall, which arise 

 from the wear of the laminse ; and attached to the inner 

 surface of these, two hour-glass-shaped surfaces, produced 

 by the wear of the grooved pillars. 



In the mandible, the structure of the molars and the 

 resulting pattern are quite different. The outer wall pre- 

 sents two convex siu-faces separated by a longitudinal 

 depression, and thus reverses the conditions obsei-vable in 

 the upper molars. The result of the wear of this is, ne- 

 cessarily, two crescents, the concavities of which are turned 

 inwards. A vertical pillar, longitudinally grooved on its 

 inner face, is developed on the inner face of the tooth at 

 the junction of the anterior and posterior crescents, and 

 gives rise to a deeply bifurcated surface when worn. A 

 second smaller pillar appears in connection with the inner 

 face of the posterior end of the outer wall. 



Thus the grinding surface of the upper molars may be 

 represented by four crescents with two inner pillars ; and 

 that of the lower molars by two crescents with two inner 

 pillars. The upper crescents are concave outwards ; the 

 lower concave inwards ; and by this aiTangement, together 

 with the unequal wear of the dentine, enamel, and cement, 

 a permanently uneven triturating surface is secured. 



As is the general rale among Mammals, the first per- 

 manent molar is the fii'st permanent tooth which appears 

 (urdess the eruption of the inner incisor be contemporary 

 with it), and it comes into place and use long before the 

 deciduous molars are shed and replaced by the premolars. 

 Hence, when the last premolar comes into place as a fresh 

 and unworn tooth, the fii-st molar, which lies next to it, is 

 already considerably worn. This disparity of wear is main- 

 tained for a long time, and furnishes a very useful means 

 of distinguishing the last premolar from the first molar in 

 the adult, when, as in the Horse, the i^remolars and molars 

 are very similar. 



