The length of a man's foot is about one-sixth, of a woman's about one-eighth, the length of the body. Its widest part is about the same width as the widest part of the hand. A comparison of the structure of the foot with that of the hand is the best way of understanding it. It is built on the same principle as the hand, but it is placed at right angles to the shin instead of being a continuation of it; and the big toe, which corresponds to the thumb, does not stand in opposition to the other toes, so that the toes can only be used for grasping, not holding or manipulating. The foot has become exclusively adapted for walking and standing.
In this connection the foot has developed as a very efficient shock absorber. If one suffers a loss of this function - from tob much motor driving, for example - considerable damage can result to the vertebrae and inner organs. The structure of the skeleton of the foot shows this function. Both lengthways and across, the bones are like conical vaulting stones, joined together into arches; but being organic, they are more flexible than stones and more like strongly bent springs.
The sole of the foot is further cushioned with flexing muscles, fat, and skin tissue. This enlarges the tread area of the foot, but the double arch is still discernible in the characteristic print of the naked foot. These arches are higher if the foot is turned inwards, and sink if the foot turns outwards or after long periods of standing.
The ankle, like the wrist, can rotate, being worked partly by the muscles from the fibula and tibia, and more strongly by the largest extensor of the foot, the threefold peroneal muscle which works through the Achilles' tendon on the heel bone. Without this tendon it is impossible to walk or to stand. Its name refers to the Greek hero who was vulnerable only on his heel. Both flexors and extensors must be intact even for standing at rest, for a joint can be held firm only by the intertension of both sets of muscles.
The build of the third shin muscle, the soleus, together with that of the Achilles' tendon, determines the shape of the foot: the shorter the heel bone the shorter and thicker the shin musculature. Negroes, who have very long heel bones, have shin muscles extending long and thin almost to the heel.
The mobility of the foot is governed by the activity of the upper and lower ankle joint. The upper connects the ankle bone and tibia (the fibula, like the ulna, is not attached to the joint); the lower connects the ankle and heel bones. It is arranged so that it is bent inwards however far the toes point outwards: the typical position of dancers and jumpers. The toes, again like the fingers, are moved by long tendons which can be seen plainly when the right muscles are tensed. As in the hand, these tendons are held in their curving path by cross bands and projections of bone.
It is in walking that the continual and versatile co-operation of all parts of the body in the act of balancing becomes most manifest, since the foot has a relatively small bearing surface. In walking and in keeping one's balance, not only do the muscles of legs and trunk play their part, but the swinging of the arms also helps towards an easy gait. Thus, the swing of the arms is always opposed to tlie leg movements.
If one side of the body is carrying a heavy weight, the arm on the other side instinctively lifts away from the body, but without discontinuing the swinging movement. Similar actions can be observed in athletics - javelin throwing, for example, or the discus, or shot putting - where an explosively sudden weight shift has to be achieved.
Movement also shows up a person's general physical condition. Strenuous exertion of almost all muscles (even facial muscles are distorted in conditions of stress) presumes normal tone. One who is in poor condition betrays this by a dragging gait. And one's general condition depends to some extent on one's state of mind: you can see people walking along in bright and cheerful mood, self-assured and full of confidence, or sadly, with a depressed or care-worn slouch. If the foot is injured, even if only slightly, the gait becomes a painful limp; chronically unsound feet will permanently destroy the beauty of an upright gait, the most "human" of all man's movements.