EQUINE CHARACTERISTICS. The horse (includ ing under his name the asses and zebras, as well as the true horses—see EQuiox) is unique among modern animals in the fact that it walks on the extreme tip of the central digit of the foot, cor responding to the middle finger-nail of man. and that all the remaining toes have completely disappeared. The feet are greatly elongated, so as to equal the other segments of the limbs in length, and to raise the animal mueh higher above the ground than if he walked. as does a man or a bear, on the sole of the foot. In each foot the first and fifth digits (thumb and little finger) have completely disappeared and left no trace. while of the second and fourth digits only a small slender rudiment exists which represents the metapodial. or bone of the palm. and is called a These two splint-hones lie closely against the cannon-bone, or metapodin1 of the central digit, and are not indicated on the sur face of the foot. The modern horse is therefore one -t oed.
The teeth are equally peculiar. There are six grinding teeth in a closely set row on each side of each jaw; the crowns of these are very much elongated, so that they can be pushed up in their sockets as fast as they wear away at the grinding surface. The grinding surface displays a emn ilicated pattern caused by infoldings of the enamel of the tooth, the enamel edges being sup ported on one side by the dentine of the tooth, on the other by a similar substance called cement, deposited on the outer surface of the unworn enamel before the teeth are extruded from the gums. This arrangement. secures at all stages
of wear a series of hard enamel ridges projecting a little above the surface of the softer dentine and cement, and makes a remarkably efficient grinder for the hard. dry grasses which are the natural food of the horse (q.v.). In the series of animals which lead up to the modern horse, we can trace every step in the evolution of these marked peculiarities of teeth and feet, from an ancestor so little suggesting the horse that when first found it was named by Richard Owen, the greatest comparative anatomist of his time, Ilyracotherium, or 'coney-like beast.' its rela tion to the horse was not at all suspected, and was recognized by Huxley and Marsh only when the series of intermediate stages between Ilyra cothetium and the modern horse was discovered. This first ancestor of the horse line is very much more like the contemporary ancestors of other lines of descent, and indicates how all the mod ern quadrupeds have diverged from a single type. each becoming adapted to its especial mode of life.