The two bones of the leg are placed, as we have previously seen (page 135), parallel to each other, the tibia on the inner side, the fibula on the outer side and somewhat behind (Fig. 49). The tibia can be felt beneath the skin in its whole length. Its shaft is triangular in section, and therefore presents three surfaces and three margins (1, Fig. 49): an internal surface covered only by the skin and appearing superficially as a long flat surface, broader above where it looks a little forward, and inclining directly inwards at its inferior part, where it becomes continuous with the prominence of the inner ankle or internal malleolus (Fig. 50, page 150). The external surface is slightly concave in order to lodge the antero-external muscles of the leg, of which the principal is the tibialis anticus; below, this surface inclines forwards, following the course of the tibialis muscle, which, from the antero-external region of the leg, is directed towards the inner border of the foot (Fig. 50, page 150). The posterior surface of the tibia is entirely covered by the strong and numerous muscles of the posterior region of the leg. Finally, of the three margins of the shaft of the tibia the anterior is particularly prominent, and is known by the name of the crest of the tibia or shin (6, Fig. 49).

The fibula appears superficially, as already seen, in the region of the knee, at its upper end. Its lower end is also obviously subcutaneous at the outer ankle, where it forms the external malleolus (9, Fig. 49). The shaft of the bone is long and slender, and is prismatic in form. It is curved from above downwards and forwards, and downwards and inwards. It serves the purpose of a strengthening bar for the tibia, at the same time that it increases the area for the attachment of the muscles of the leg. The shaft of the fibula is surrounded on all sides by these muscles.

The two bones are separated throughout their entire length by an interval called the interosseous space (Fig. 49), broader above than below, and filled up by a fibrous membrane (interosseous membrane), which, passing from one bone to the other, still further increases the area for attachment of the muscles of the leg. Above, the fibula articulates with the postero-external surface of the superior extremity of the tibia, and this superior tibio-fibular articulation possesses a very slight gliding movement, exercised chiefly through the action of the biceps muscle in external rotation at the knee-joint. Below, the fibula is attached to the corresponding part of the tibia by a strong interosseous ligament, forming a symphysis, or synarthrodial joint. This inferior tibio-fibular articulation has hardly any mobility: it only gives a certain amount of elasticity to the ankle-joint, into which the foot is received. We see, therefore, that there is, with regard to mobility, a great difference between the bones of the leg and those of the forearm; in the forearm the radius is moveable on the ulna, and can turn in such a manner as to cross the latter, and produce the movements of pronation and supination of the hand. Between the fibula and the tibia there is no movement of the kind; the foot is not capable of any movement which may be compared to that which takes place in the hand during pronation and supination. We may say that it is the same with monkeys, in the class called quadrumana; they have not the power of pronation and supination of the foot, which, from this point of view, and also in every other respect, is properly speaking a foot and not a posterior hand, as their ancient name of quadrumana would lead us to suppose.

Ankle-joint.—By their junction the inferior extremities of the tibia and fibula constitute an articular cavity, which forms the ankle-joint, by their articulation with a bone of the tarsus—the astragalus. This tibio-fibular cavity possesses three sides, of which two, the superior and internal, are formed by the tibia, and one only, the external, by the fibula; the two lateral walls correspond to the two osseous parts which form the prominences of the ankles and which are known by the name of malleoli (malleus, a hammer). They are distinguished as the internal or tibial and external or fibular malleoli (Fig. 49, page 147). As the internal ankle or malleolus (8) is of a form and situation very different to that of the external (11), it is important to note here the configuration of the bones by which the subcutaneous prominences are explained.

The malleoli differ in their level, in their situation, and finally in their form (Figs. 50, 51). First, we see at a glance that the external or fibular malleolus (11) descends much lower than the internal malleolus (8). Second, with regard to the transverse plane of the two malleoli, just as the shaft of the fibula is situated behind and to the outer side of the tibia, so the same position is maintained by the inferior extremities of the two bones, and the external malleolus is on a plane posterior to the internal malleolus. A transverse line which passes through the centre of the internal malleolus, passes outwards in front of the anterior border of the external malleolus; and, on the other hand, a transverse line, passing through the centre of the external malleolus, passes inwards behind the posterior border of the internal malleolus. Thirdly, with regard to the differences of form, these are the direct result of the shape of the osseous parts. The malleolar portion of the tibia, or internal malleolus, is square, presenting a horizontal inferior border, and two vertical borders—one anterior, the other posterior. On the contrary, the malleolar portion of the fibula, or external malleolus, is triangular in shape, or rather like the head of a serpent; it terminates below in a pointed extremity, formed by the convergence of the two oblique borders—one anterior, the other posterior—of which the anterior is the more sloping.

Before entering into a study of the articulation of the leg with the foot, or ankle-joint, we must glance at the bony structure of the foot as a whole, so as to understand properly the significance of the position of one of the bones (astragalus) in relation to this joint.

Just as the hand is composed of three sets of bones—the carpus, metacarpus, and fingers—so also the foot is composed of a similar series—the tarsus, metatarsus, and toes; but while in the hand, where the function is principally that of prehension, the fingers are long and the carpus very short, in the foot, which serves as a base of support, the toes are comparatively short, while the tarsus, which corresponds to the carpus, is of considerable size; it forms, in fact, one-half of the length of the foot. In order to understand the form of the foot and its mechanism it is necessary to make a particular study of the bones which compose the tarsus.

As the carpus in the hand is formed by two rows of bones, so also the tarsus is composed of two groups. In the hand the carpal bones are grouped in two more or less transverse rows. In the foot, on the other hand, the rows of tarsal bones are longitudinally arranged; and the inner row overlaps the outer row in relation to the back part of the tarsus. There are two bones in the outer row: the calcaneum or os calcis behind, which forms the prominence of the heel and rests on the ground below: and the cuboid, articulating with it in front and carrying anteriorly the two outer metatarsal bones and the two outer toes. The inner row consists of five bones: (1) the astragalus or talus behind, which articulates with the bones of the leg and helps to form the ankle-joint above, which rests below on the upper surface of the calcaneum, and which articulates in front with the navicular bone; (2) the navicular or scaphoid bone; and in front of this the three cuneiform bones (internal, middle, and external), which lie above and internal to the cuboid bone, and carry the three inner metatarsal bones and the three inner toes.

After this brief sketch of the tarsus, and before entering into the details of the configuration of its parts and the whole taken together, having seen the particular place occupied by the astragalus, we must study its articulation with the tibia and fibula. The part of the astragalus which is received into the cavity between the malleoli, is formed by the posterior three-fourths (1, Fig. 53) of the superior part of the bone, separated from the anterior fourth by a narrow portion called the neck (2, Fig. 53). This articular part is in the form of a pulley, with the antero-posterior groove hardly perceptible, but the lips are prolonged over the sides of the bone, and come in contact with the corresponding parts of the internal and external malleoli. It is readily seen (Fig. 53) that the articular surface of the astragalus is considerably wider in front than behind. The same is seen in the shape of the lower end of the tibia. The tibio-astragaloid articulation permits movement chiefly in the anterio-posterior plane, namely, movement forward (flexion of the foot) and backwards (extension). During flexion of the foot on the ankle the astragalus is received in the malleolar cavity as in a vice, and the result is that no lateral movement is possible. When, however, the foot is extended, and the toes are pointed, the narrower part of the articular surface of the astragalus comes into relation with the wider portion of the inter-malleolar articular surface, and the result is that in this position a certain amount of lateral movement of the foot at the ankle-joint is permitted, aided, it is true, by gliding movements taking place among the tarsal bones themselves. Of the movements of flexion and extension, that of extension is the most free, as it may be continued until the axis of the foot becomes continuous with that of the leg, and here it is arrested by the meeting of the posterior border of the joint with the projections on the posterior border of the astragalus; but the movement of flexion by which the dorsal surface of the foot is brought near the anterior surface of the leg is more limited, for it is impossible to cause the foot to make with the leg an angle less than forty-five degrees, opening upwards and forwards. This is accounted for by the shape of the articular surfaces. In proportion as flexion is produced, the larger part of the articular surface of the astragalus is wedged into the malleolar cavity; thus the movement of flexion is arrested, and the foot is fixed. We cannot carry flexion further without bursting asunder the tibio-fibular joint, just as we should split a piece of wood by driving violently into it a wedge larger than the cavity to be filled.