A glance at the drawing will show you that here is a weak point in the foot. The head of the key-bone receives great weight from the leg, but is comparatively unsupported; and there is a considerable strain upon this part when the heel is being raised in walking. Moreover, a good deal of movement takes place between the key-bone (D) and the scaphoid bone (E), more than between any other two bones of the instep; and freedom in the range of movement is generally attended with some sacrifice of strength. The strong elastic ligament comes in therefore with peculiar advantage at this point; and it is underlaid, and additional support is afforded exactly when it is most required, by the tendon (b in fig. 12) of a strong muscle, the especial office of which is to assist in raising the heel and bending the instep, and which runs, from the back of the leg, behind the inner ankle, to the scaphoid bone. _Weak Ankle and Flat-foot._ In spite, however, of the thick elastic ligament and the strong tendon just mentioned, the joint between the astragalus or key-bone and the scaphoid bone still remains a weak point. The head of the key-bone, from being insufficiently supported or from being overweighted, is very apt to descend a little below its proper level; the consequence of which is that the plantar arch is lowered and the foot is flattened; and the more the foot is flattened the weaker it necessarily is, because the position of the bones then becomes less and less favourable for bearing weight, and an increasing strain is thus incurred by the ligaments and muscles. Hence the foot and ankle feel weak; and the weakness is especially felt when the person endeavours to raise the heel, so as to mount upon the balls of the toes, in walking. For the performance of that movement with ease and steadiness a well-formed plantar arch is essential; and the person, whose feet are defective in the manner we are considering, can never walk with a bold, firm step. The movement in him may be better described as a shuffling from one foot on to the other, than as a walk. To this I will recur again when I come to speak more of walking. The defect, when slight in degree, is commonly called “weak-ankle;” when more decided it is called “flat-foot,” because the sole is then nearly, or quite, flat. The head of the key-bone, under such circumstances, may even bulge downwards and inwards, and form a prominence on the inner side of the sole, so as to give more or less _convexity_ to the line on the inner side of the foot, which should be _concave_. [Illustration: Fig. 8. Flat-foot.]