202 THE STRUCTURE OF THE HORSE. 



represents (as shown before) the large pad under the 

 middle of the foot. 



The hoofs of asses and zebras, though formed on 

 exactly the same general plan as those of the horse, 

 differ in being smaller, and especially narrower. The 

 different parts of the inferior surface, the wall and 

 bars, the sole and the frog, can be made out, though 

 they are less distinctly marked from each other than 

 in the horse. 



The mechanical arrangement of the under sur- 

 face of the horse's hoof in its natural state is admi- 

 rably adapted to the purpose it has to fulfill. The 

 different varieties of horny tissue of which it is com- 

 posed and their complex arrangement recall those of 

 the grinding-surf ace of the molar teeth. The wall 

 or crust, completely encircling the front and sides, 

 and reflected inwards and forwards almost to the 

 center, being composed of a harder and more resist- 

 ing material than the rest, like the enamel of the 

 teeth, always stands out as a ridge beyond the other 

 structures, and not only bears the principal weight, 

 but prevents -the tendency to slip which a uniformly 

 smooth surface would have. The sole is more or less 

 concave, being less dense and its surface exfoliating 

 more readily than the other parts, but it comes in 

 contact with the ground when this is of a soft 

 and yielding nature. The projections formed by the 



