This section is from the book "Applied Anatomy: The Construction Of The Human Body", by Gwilym G. Davis. Also available from Amazon: Applied anatomy: The construction of the human body.
The foot is intended for support and locomotion. The locomotion takes place in the upright position and, in moving, the weight is shifted from one foot to the other. Hence we see that if the foot is to fulfil its function of support it must have strength, because on it rests the weight of almost the whole of the body. If a person is at rest in a standing position the foot is subject to a continuous static pressure which, if any part of the foot is abnormal, whether from congenital or acquired qualities, will eventually result in distortion and impairment of function. If a person is moving about, the foot is subjected to a pressure which is dynamic (movable) in character, and is much greater in amount than is the static pressure of the body at rest.
The movements of the foot in locomotion are not always slow, sometimes they are exceedingly rapid. A person treads on an uneven or unstable surface and the foot must adapt itself instantly or injuries will result; failure to do so results in sprains, fractures, and luxations. In running rapidly the changes in position of the component parts of the foot are instantaneous, otherwise rapid running is impossible. In jumping especially the dynamic pressure plus the inertia causes an enormous strain on the foot. The mobility demanded of the foot is not so great, however, as that of the hand because the movements are neither so intricate nor so numerous. A consideration of these facts enables one to understand: first, the method of construction of the foot; second, its injuries, diseases and deformities; third, the means necessary to employ in preventing and curing them and in obviating to as great extent as possible their consequences.
The foot is constructed with a view of possessing strength and mobility. Strength is obtained by the bones being short and solid, well compacted together in the form of a double arch, joined by strong ligaments, and supported by powerful muscles. The double arch forms the hollow of the sole of the foot. As pointed out by Ellis ("The Human Foot" ) when the two feet are placed together there is formed a " dome-shaped space " arching anteroposteriorly from the internal tuberosity of the calcaneum to the head of the first metatarsal bone, and laterally from the inner to the outer edge (Fig. 577). Mobility is obtained by the bones and joints being numerous and the muscles highly specialized.
Disease weakens the foot - sometimes, as in adolescents, the foot is weakened without any apparent disease. In other cases the bones and ligaments become affected, as in rickets, rheumatism, gout, and tuberculous disease. In still others the muscles become affected, either contracted, as in spastic diseases, or relaxed, as in infantile paralysis. When the bones and ligaments are involved they fail to bear the body weight, the arch is crushed and flat-foot and eversion results. Hence valgus is almost always a disease of weakness. If muscles become affected by spasm or paralysis all kinds of deformities are produced. There are many muscles controlling the foot and frequently only one or a few are paralyzed; this leaves the balancing muscles unopposed and they drag the part toward the healthy side. Anything that disturbs the equilibrium or balance of the various muscles results in distortions and deformities. Injuries impair the efficacy of the mechanism of the foot. A crush of the head of the first metacarpal bone destroys the anterior support of the arch and the resultant weakness is marked.
Traumatism produces flat-foot, also sprains, which, while not so deforming, are often disabling. Fractures and luxations occur and may impair the foot permanently.
Finally, many children have congenitally deformed feet which require treatment before they can fulfil their functions.
Fig. 577. - Section of toot, showing the longitudinal arch.
The foot is exceptionally-accessible both for diagnosis and treatment. The bones and joints are accessible often to both sight and touch, and one should know where to look and feel for them. Exploratory operations in this portion of the body are out of place. An accurate knowledge of the structures of the foot is absolutely essential to intelligent treatment. The deformities are dependent on muscular action, and one should know the position of the tendons and the influence of the muscles. In amputating, a knowledge of the joints is essential. The problems presented are largely of a mechanical nature, to be solved by a thorough knowledge of the structures and the application of mechanical principles to living tissues.