Fig. 37.—Bones of left lower leg of Wolf, front side. T., tibia. F., fibula. sp. spine of tibia. cn. cnemial crest. i.m., internal malleolus. e.m., external malleolus.
Fig. 38.—Bones of left lower leg of Horse (much more reduced). cn. cnemial crest. F., lower end of fibula, coössified with tibia. Other letters as in [Fig. 37].
Fig. 39.—Bones of lower leg, left side, of Tapir. T., tibia. F., fibula. sp., spine of tibia. cn., cnemial crest. i.m., internal malleolus. e.m., external malleolus. N.B. This figure is on a much larger scale than [Fig. 38].
Fig. 40.—Left pes of Wolf, front side. Cal., calcaneum. As., astragalus. N., navicular. Ch., cuboid. Cn. 1, Cn. 2, Cn. 3, internal, middle and external cuneiforms. Mt. I, rudimentary first metatarsal. Mt. II-V, second to fifth metatarsals. Ph. 1, first phalanx. Ph. 2, second phalanx. Ung., ungual phalanx. I, rudimentary hallux. II-V, second to fifth digits.
Fig. 41.—Left pes of Man. Note the large size of Mt. I, the metatarsal of the first digit, or hallux. Letters as in [Fig. 40], except Cb., cuboid.
The hind foot, or pes, like the manus, is clearly divisible into three parts, the bones of which are called respectively the tarsus, metatarsus and phalanges, and the correspondence in structure between manus and pes is close and obvious. The tarsus consists typically of seven bones, which are tightly packed and rarely permit any movement between them. The upper row of the tarsus consists of two bones, which are peculiarly modified to form the ankle-joint and heel; on the inner side is the ankle-bone, or astragalus, the shape of which is highly characteristic of the various mammalian orders. The upper or posterior portion of the astragalus, according to the position of the foot, is a pulley which glides upon the lower end of the tibia and is held firmly in place by the internal and the external malleolus. Below the pulley-like surface the astragalus usually contracts to a narrow neck, which ends in a flat or convex head. The astragalus is supported behind (or beneath) by the heel-bone, or calcaneum, which is elongate and extends well above (or behind) the remainder of the tarsus; it frequently has a distinct articulation with the fibula, but more commonly is not in contact with that bone. The astragalus rests upon the navicular, which is moulded to fit its head and corresponds in position to the central of the carpus, but, unlike that carpal, it is a very important element and is never suppressed or lost in any land mammal. The navicular, in turn, rests upon three bones of the second row, which are called respectively the internal, middle and external cuneiform, which correspond to the trapezium, trapezoid and magnum of the carpus and to which are attached the three inner metatarsals, one to each. Finally, the cuboid, the external element of the second row, is a large bone, which supports the calcaneum and often part of the astragalus and to which the fourth and fifth metatarsals are attached; it is the equivalent of the unciform in the manus. The number of tarsals is more constant than that of the carpals, but some suppressions and coössifications do occur.