In
addition, however, to this fully-developed toe, each foot in the
horse carries two rudimentary toes which are concealed beneath the
skin, and are known as the "splint-bones." These are respectively
the _second_ and _fourth_ toes, in an aborted condition; and the
first and fifth toes are wholly wanting. In _Hipparion_ (fig.
230, C), the foot is essentially like that of the modern Horses,
except that the second and fourth toes no longer are mere
"splint-bones," hidden beneath the skin; but have now little
hoofs, and hang freely, but uselessly, by the side of the great
middle toe, not being sufficiently developed to reach the ground.
In _Anchitherium_, again (fig. 230, B), the foot is three-toed,
like that of _Hipparion_; but the two lateral toes (the second
and fourth) are so far developed that they now reach the ground.
The _first_ digit (thumb or great toe) is still wanting; as also
is the _fifth_ digit (little finger or little toe). Lastly, the
Eocene rocks have yielded in North America the remains of a small
Equine quadruped, to which Marsh has given the name of _Orohippus_.
In this singular form--which was not larger than a fox--the foot
(fig. 230, A) carries _four_ toes, all of which are hoofed and touch
the ground, but of which the _third_ toe is still the largest. The
_first_ toe (thumb or great toe) is still wanting; but in this
ancient representative of the Horses, the _fifth_ or "little"
toe appears for the first time.
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