THE TEETH OF THE HOUSE. 303 



eacb, surrounded by a band of enamel, and, outside this, by 

 the cement with which the interspaces are filled. The band 

 of enamel is simple and unplaited. The general pattern of 

 the worn surface may be described as consisting, externally, 

 of two longitudinal crescents, one behind the other, and with 

 their concavities turned outward which arise from the wear 

 of the wall; internal to these, of two other crescents, partly 

 transverse in direction, and connected 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 re- 

 sulting pattern are quite different. The outer wall presents 

 two convex surfaces separated by a longitudinal depression, 

 and thus reverses the conditions observable in the upper 

 molars. The result of the wear of this is, necessarily, two 

 crescents, the concavities of which are turned inward. A 

 vertical pillar, longitudinally grooved on its inner face, is de- 

 veloped 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 ap- 

 pears in connection with the inner face of the posterior end 

 of the outer wall. 



Thus the grinding surface of the upper molars may be rep- 

 resented 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 outward ; the lower concave 

 inward; and by this arrangement, together with the unequal 

 wear of the dentine, enamel, and cement, a permanently un- 

 even triturating surface is secured. 



As is the general rule among Mammals, the first permanent 

 molar is the first permanent tooth which appears (unless 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 

 first molar, which lies next to it, is already considerably worn. 

 This disparity of wear is maintained for a long time, and 

 furnishes a very useful means of distinguishing the last pre- 

 molar from the first molar in the adult, when, as in the Horse, 

 the premolars and molars are very similar. 



The first deciduous molar usually falls out when the first 

 premolar appears, and is not replaced ; but it is occasionally 

 retained. AH the other milk-teeth have successors, and there 



