The Posterior limb.
This is as usual divisible into three portions, the thigh, the crus or shin, and the pes.
The thigh is formed by the femur (fig. 48, B, 12), a moderately long stout bone, not unlike the humerus; it articulates with the acetabulum by a fairly prominent rounded head. The distal end articulating with the tibia and fibula is also expanded, and is partially divided into equal parts by anterior and posterior grooves. The flexor surface bears a fairly prominent trochanteric ridge. Each end of the femur is formed by an epiphysis.
The crus or shin includes two bones, the tibia and fibula. Both are well developed, but the tibia is considerably the larger of the two.
The tibia (fig. 48, B, 13) is a strong bone with a flattened expanded proximal end articulating with almost the whole of the end of the femur, and a similarly expanded distal end articulating with a bone representing the fused astragalus and centrale.
The fibula (fig. 48, B, 14) is flattened proximally, and articulates with only quite a small part of the femur, while distally it is more expanded, and articulates with the fibulare (calcaneum) and with a facet on the side of the fused astragalus and centrale.
The Pes consists of the tarsus or ankle, and the foot.
The Tarsus. This, like the carpus, is much reduced and modified from the primitive condition. It consists of only four bones, arranged in two rows of two each. The two bones of the proximal row are much larger than are those of the distal row. The pre-axial of them (fig. 48, B, 15) representing the fused astragalus (tibiale and intermedium) and centrale, articulates proximally with the tibia and fibula, and distally with the first metatarsal, and a small bone representing the first three tarsalia. The postaxial bone, the calcaneum (fibulare) (fig. 48, B, 16), is drawn out into a prominent posterior process forming a heel such as is almost unknown elsewhere except in mammals. It articulates with the fibula, the tibiale-centrale, and distally with a bone representing the fourth and fifth tarsalia, and with the fifth metatarsal.
The two bones forming the distal row of tarsals are both small and rounded; one represents the first three tarsalia fused together, the other tarsalia 4 and 5.
The Foot. The foot has five digits, but the fifth is much reduced, consisting only of a short metatarsal. The first four metatarsals are all long bones, slightly expanded at each end, and terminated by small epiphyses. The first digit has two phalanges, the second three, the third four, and the fourth five. The terminal or ungual phalanx in each instance is grooved and pointed, and in the case of the first three digits bears a horny claw. The ungual phalanx progressively decreases in size from the first to the fourth. The fifth digit consists only of a small, somewhat square metatarsal (fig. 48, B, 21), attached to the bone representing the fused fourth and fifth tarsalia.