14 THE OX AND ITS KINDRED 
the latter the " hock " of the horse — to be raised far 
above the level of the ground. In primitive extinct 
ungulates, on the other hand, in some of which there 
were five toes to one or both pairs of limbs, the wrist 
(carpus) and ankle (tarsus) joints were much nearer 
the ground, and the cannon-bone was represented by 
four or five separate and comparatively short bones. 
The reason for the elongation of the upper, or, as 
it would be called in man, the hind, part of the 
skeleton of the feet, and the replacement of the 
original five comparatively short bones known in 
the fore-limb as the metacarpal, and in the hind- 
limb as the metatarsal bones by a single elongated 
cannon-bone — of dual origin in the ox, but formed of 
one element in the horse — is to permit of large and 
heavy animals running at a high rate of speed 
without danger of fracturing the bones of this part 
of the skeleton and at the same time to secure a 
slender but likewise a strong type of limb. Such a 
result could never have been attained by elongating 
the bones of a five-toed limb. 
As regards their foot-structure, the ox and the 
horse have independently attained the same goal, 
but by different routes ; the one by the enlargement 
of the third and fourth toes (accompanied by the 
diminution in the size of the lateral ones) and the 
elongation and fusion of their supporting metacarpal 
and metatarsal elements into a compound cannon- 
bone, and the other by the enlargement of the middle 
toe and the complete suppression of the lateral ones, 
accompanied by the lengthening and strengthening 
of the simple cannon-bone. 
It is frequently stated that in these respects the 
horse and its relatives are the most specialised of all 
