The very long and slender feet are so raised from the ground that the animal walks upon the very tips of the toes, the wrist being what horsemen call the “knee” and the heel is the “hock,” and the gait is thoroughly unguligrade. Each foot has but a single functional toe, the third or middle one of the primitive five-toed foot; and, as this toe has to carry the whole weight supported by its leg, it is necessarily much larger than in animals which distribute the weight among several digits. The horses are therefore said to be _monodactyl_, or single-toed, but the term is not strictly accurate, for on each side of the functional digit is a rudimentary or vestigial one, the 2d and 4th of the original five. These rudimentary digits, which are not visible externally, have no phalanges and are merely “splint-bones,” metapodials (see p. 90) which have very slender shafts and end below in blunt points. The single functional metapodial has encircling its lower articular end a prominent ridge or keel, which fits into a corresponding groove on the upper end of the first phalanx and serves to prevent lateral dislocation. In most mammals this keel is merely a projection from the lower articular surface and is confined to the posterior side, so as not to be visible from the front. The terminal or ungual phalanx is much enlarged to carry the great weight which it supports and is enclosed in the characteristic hoof, unlike that of any other mammal. In brief, the whole structure of the horses is pre-eminently adapted to swift running; they are admirable “cursorial machines,” as they have been called, and every part of the skeleton has been modified and specialized to that end; the narrow, rigid hoofs fit them for walking on firm ground and they speedily are made helpless in quicksand or bog. Did we know nothing of their mode of life, we might confidently infer from their teeth that the horses were grazers, feeding principally upon grass. A long-legged, grazing animal must needs have a neck of sufficient length to enable the mouth to reach the ground easily, unless a long proboscis is developed; and so we shall find in the history of the horses that the elongation of the head and neck kept pace with the lengthening of the legs and feet.