The individual whom we have just supposed placed in the attitude of a quadruped would be able to maintain this position by pressing on the ground more or less extensive portions of his hands; the whole palm of the hand may be applied to the ground ([Fig. 22]); or the fingers only—that is to say, the phalanges ([Fig. 23]); or the extremities of the fingers only—that is to say, the third phalanges ([Fig. 24]). This last position, which is certainly difficult to maintain, should here be regarded rather as theoretical.
Fig. 23.—The Human Hand resting on its Phalanges: Left Side, External Surface.
We shall meet with each of these modes of support in certain groups of animals. Thus, the bear, badger, and the majority of rodents, have the paws applied to the ground by the whole extent of the palmar surface of the hand, from the wrist to the tips of the fingers. They are therefore called plantigrade, from the analogy, in this case, of the palm of the hand to the plantar surface, or sole of the foot.
Fig. 24.—The Human Hand resting on the Tips of some of its Third Phalanges: Left Side, External View.
In others, such as the lion, tiger, panther, cat, wolf, and fox, the support is made no longer on the whole extent of the palmar surface, but on the corresponding surface of the fingers only—the metacarpus is turned back, and, consequently, the wrist—that is to say, the carpus—is removed from the ground. These are the digitigrades.
Lastly, the ruminants (sheep, oxen, deer, etc.), and also the pig, ass, and horse, rest on the third phalanx only. In them not only is the metacarpus turned back, but also the two first phalanges. The wrist is very far removed from the ground. In these animals, the third phalanx is enclosed in a case of horn, a nail (the hoof), and because the support of the limb is on that nail, the name of unguligrades has been given them. Nevertheless, as the point of support is on the third phalanx, which is also known by the name of phalangette, we are of opinion that, in order to specify definitely, although they walk on their fingers, as do the digitigrades, the support is provided not by the whole extent of those appendages, they might receive the name of phalangettigrades.