I have stated the movements that take place in the three joints of the foot with the leg in a simple manner, for the sake of avoiding confusion. In reality, however, they are not so simple, but very difficult to analyse and make out correctly. The difficulty is due, partly, to the close proximity of the joints to one another, which renders it no easy matter to distinguish the movements of one from those of the others, and, partly, to the fact that the movements in each joint are a little oblique.
In the latter respect the foot-joints resemble most of the others in the body; and it is this obliquity in the movements of the joints, added to the curves and twists in the shape of the bones, that constitutes one of the chief difficulties in investigating and clearly understanding the mechanism of the human frame. It has been said that “Nature abhors a vacuum:” it may with equal truth be said that she abhors a straight line. In the Human Skeleton, at any rate, all the bones are bent and twisted, some in two or three directions; and the surfaces by which any bone is jointed to the adjacent bones, are invariably oblique with regard to each other.
Fig. [9].
Take, for instance, the tibia, or large bone of the leg, of which a front view and an inner side view are given in the drawings. The tibia is a column transmitting weight from the thigh to the foot; and in any machine of man’s construction a column fulfilling similar purposes would be made straight and of uniform diameter throughout. The bone, on the contrary, does not present the same thickness at any two parts of its length. It has a distinct bend, forwards, in nearly its whole length (fig. 10): there are lateral curves, alternating like those in the letter S, seen along its front (fig. 9): and the articular surface at the lower end is placed obliquely with regard to that at its upper end, in consequence of a twist in the shaft, in such a manner that when the hinder surface of the upper end of the bone rests upon a board, the lower end touches the board only by its outer corner (fig. 10). This disposition of the lower end, I may remark, assists to give the foot a slant outwards from the heel to the toe, so that when we stand, with the heels together, the great toes of the two feet diverge a little from one another.
Fig. [10].
Moreover, the surfaces by which the tibia is jointed with the thigh-bone at the knee are arranged with a varying degree of obliquity, so that the relation of the leg to the thigh varies somewhat in different positions of the limb. For instance, when we stand upright, the thigh slants inwards from the pelvis, and the leg descends in a vertical direction to the ground. While, however, the knee is being bent the leg is carried, not in a vertical plane, but a little obliquely, so that the lower part soon begins to slant outwards; and when the knee is fully bent the obliquity of the leg and that of the thigh correspond, and the leg is, as it were, folded up against the thigh. The heel is thus brought up, not to the middle line of the body, but to the hip, and we are enabled to sit with the hips upon the heels, as the Japanese are represented doing, or with one hip upon one heel—a position in which our riflemen are trained to take aim, and in which their predecessors with the arrow were wont to shoot, as is shown by the accompanying sketch of a bowman (fig. 11), taken from one of the Æginetan marbles in the Glyptothek at Munich.
Fig. [11].