The Evolution of the Horse 



each grinder is always as uneven as that of a 

 good millstone. 



I have said that the structure of the grinding 

 teeth is very complicated, the harder and the 

 softer parts being, as it were, interlaced with 

 one another. The result of this is that, as the 

 tooth wears, the crown presents a peculiar 

 pattern, the nature of which is not very easily 

 deciphered at first, but which it is important 

 we should understand clearly. Each grinding 

 tooth of the upper jaw has an outer wall so 

 shaped that, on the worn crown, it exhibits 

 the form of two crescents, one in front and one 

 behind, with their concave sides turned out- 

 wards. From the inner side of the front crescent, 

 a crescentic front ridge passes inwards and 

 backwards, and its inner fare enlarges into a 

 strong longitudinal fold or pillar. From the 

 front part of the hinder crescent, a back 

 takes a like direction, and also has its pillar. 



The deep interspaces or valleys between these 

 ridges and the outer wall are filled by bony 

 substance, which is called cement, and coats the 

 whole tooth. 



The pattern of the worn face of each grinding 

 tooth of the lower jaw is quite different. It 

 appears to be formed of two crescent-shaped 

 ridges, the convexities of which arc turned 

 outwards. The free extremity of each crescent 

 has a pillar, and there is a large double pillar 

 where the two crescents meet. The whole 

 structure is, as it were, imbedded in 

 107 



