122 THE STRUCTURE OF THE HORSE 



and the ridges not only more elevated, but more curved 

 and complex in arrangement. To give support to these 

 high ridges, and to save them from breaking in use, 

 the valleys and cavities between them became filled 

 up to the top with cement, which was also packed 

 round the whole outer surface of the enamelled crown, 

 and as the tooth wore down the result was an admir- 

 able grinding-surface, consisting of patches and islands 

 of the two softer substances — dentine and cement — 

 separated by variously-reduplicated and contorted lines 

 of intensely hard enamel, the greater resistance of which 

 to the attrition of the food caused it to project slightly 

 above the rest of the surface {see section of the tooth of 

 Hipparion, fig. 10, p. 66). To this lengthened form of 

 crown the term 'hypsodont' is applied. Instead of con- 

 tracting into a neck and forming roots, its sides continue 

 parallel for a considerable depth in the socket, and as 

 the surface wears away the whole tooth slowly pushes 

 up, and maintains the grinding-edge constantly at the 

 same level above the alveolar border, much as in the 

 perpetually-growing front teeth of many rodents, which 

 never contract at the base to form roots, but continue 

 throughout the life of the animal to grow from below to 

 the same extent as they are worn away at the outer, or 

 cutting-edge. But the horses have not quite attained 

 this condition. There is still a limit to the growth of 

 their teeth. After a length is attained which, under 



