The long bones of the pes constitute the metatarsus, which is the counterpart of the metacarpus. There are never more than five metatarsals in any normal mammal, but there may be any number less than five, down to a single one. In form and size the metatarsals of any given mammal are usually so like the metacarpals, that it requires some experience to distinguish them, but when either manus or pes is especially adapted to some particular kind of work, there may be very decided differences between metatarsals and metacarpals. For example, the burrowing forefoot of the moles is very different from the hind foot, which has undergone but little modification, and even more striking is the difference between the wing of a bat and its foot. Many other instances of a less extreme divergence might be enumerated, but when manus and pes are used only for locomotion, as in nearly all hoofed animals and many other mammals, the metacarpals and metatarsals are very similar. When there is a difference in number, it is the general rule that there are fewer metatarsals; an instance of this is found in the tapirs, which have four toes in the front foot and three in the hind. Forms which have a cannon-bone in the manus have it also in the pes, and some, like the peccaries and the jumping rodents called jerboas, have it only in the pes. The first (or inner) metatarsal, that of the great toe, or hallux, is sometimes opposable to the others, as in the monkeys, apes and lemurs.
The word metapodial is a useful general term which includes both metacarpals and metatarsals. A metapodial with its phalanges is a digit, a term often employed because of the ambiguity which arises in the use of the words “fingers” and “toes,” and is applicable to both fore and hind feet.
Normally, the phalanges of the pes are so like those of the manus as to require no particular description; and only when the two pairs of extremities are specialized for entirely different functions, is there any notable divergence between the phalanges of manus and pes.
Fig. 42.—Left pes of Black Bear (Ursus americanus), showing the plantigrade gait. T., tibia. F., fibula. Cal., calcaneum. As., astragalus. N., navicular. Cn. 3, external cuneiform. Cb., cuboid. Mt. V., fifth metatarsal.
Fig. 43.—Left pes of Patagonian Deer (Hippocamelus bisulcus), showing the unguligrade gait. T., tibia. F., lower end of fibula (malleolar bone). Cal., calcaneum. As., astragalus. N.Cb., coössified cuboid and navicular. Mt. III, Mt. IV, cannon-bone, formed by the coössification of the third and fourth metatarsals. V., Rudimentary fifth digit.
Before leaving the subject of the skeleton, it will be well to explain the terms used in describing the gait and manner of using the feet. When the entire sole of the foot is in contact with the ground and weight is thrown upon the heel-bone, or calcaneum, the gait is said to be plantigrade and is exemplified in Man, bears, raccoons and many other mammals. The Dog is digitigrade, that is to say, the feet in the standing position are nearly erect and the wrist and heel are raised high above the ground; the weight is borne upon ball-like pads, one under the phalanges of each functional digit and one under the metapodials. The digitigrade gait is found not only in all the dogs and cats, but in many other Carnivora and in the camels and llamas, as well. Transitions between the plantigrade and digitigrade gait are so numerous and gradual, that such terms as semi-plantigrade and semi-digitigrade are sometimes necessary. An animal is said to be unguligrade when the weight is carried entirely upon the hoofs and is used only of hoofed animals; examples are the horses, pigs, deer, antelopes, oxen, etc. The so-called “knee” of a horse is really his wrist and the “hock” is the heel, so that the feet make nearly half the apparent length of the legs. Certain very large and massive animals, such as the rhinoceroses and elephants, are unguligrade in a modified sense; the foot is a heavy column, seemingly a part of the leg, and the weight is borne upon a great pad of elastic tissue, with the hoofs disposed around its periphery. A very peculiar mode of locomotion is exemplified by certain of the Edentata, in the forefoot of the existing Ant Bear (Myrmecophaga jubata) and in both extremities of some of the later representatives of the extinct †ground-sloths, or †Gravigrada. Here the weight is carried upon the outer edge of the foot, the palm and sole being turned inward. No term has been suggested for this very exceptional gait, which is a modified form of plantigradism.