In order that the foot may support the weight of the body in the erect posture in the most economical and efficient manner, it is constructed of a series of arches formed by the tarsal and metatarsal bones, and strengthened by the ligaments, tendons and fascia of the foot.

The main arch is the anteroposterior arch, which may be regarded as divisible into two parts- a medial and a lateral.

The medial arch (fig. 577) is made up of the calcaneum, the talus, the navicular, the three cuneiform,. and the first, second and third metatarsal bones. Its summit is at the superior articular surface of the talus, and its two extremities or piers, on which it rests in standing, are the tubercles on the plantar surface of the calcaneum posteriorly, and the heads of the first, second and third metatarsal bones anteriorly. The chief characteristic of this arch is its resilience, due to its height and to the number of joints between its component parts.

Figure 577
Left foot bones medial view - Figure 577
Its weakest part, i.e. the part most liable to yield from over-pressure, is the joint between the talus and the navicular bone; this is braced by the plantar calcaneonavicular ligament,, which is elastic and is thus able to restore the arch quickly when the disturbing force is removed. The arch is not dependent for its maintenance on, this ligament alone, for it is supported inferiorly by the tendons of the tibialis posterior, the flexor digitorum longus and the flexor hallucis longus. It is further supported by the plantar aponeurosis, by the small muscles in the sole of the foot, by the tendon of the tibialis anterior, and by the plantar ligaments of all the joints involved.

Figure 578
Left foot bones lateral view - Figure 578
The lateral arch (fig. 578) is composed of the calcaneum, the cuboid, and the fourth and fifth metatarsal bones. Its summit is at the talocalcaneal articulation, and its chief joint is the calcaneocuboid, which allows only a limited movement. The most marked features of this arch are its solidity and its slight elevation; two strong ligaments (the long and short plantar), the tendon of the peroneus longus and the short muscles of the little toe, preserve its integrity.

In addition to the longitudinal arches the foot presents a series of transverse arches. At the posterior part of the metatarsus and the anterior part of the tarsus the arches are complete, but in the middle of the tarsus they present more the characters of half-domes the concavities of which are directed downwards and medially, so that when the medial borders of the feet are placed in apposition a complete tarsal dome is formed. The transverse arches are strengthened by the interosseous and the plantar ligaments, by the short muscles of the first: and fifth toes (especially the transverse head of the adductor hallucis), and by the peroneus longus, the tendon of which stretches between the piers of the arches.

It should be observed that in a normal foot the arches become slightly flattened when the erect posture is assumed, and are restored when the weight of the body is taken off the feet. This resilience accounts for the suppleness of the normal foot and enhances the value of the arches by rendering possible such rapid and sudden movements as running and jumping.

Applied Anatomy.-Pes planus or flat-foot is a very common deformity due to the disappearance of the arches of the foot, from subsidence dependent on stretching of the plantar ligaments. In its early stages, before the deformity is marked, it is accompanied by considerable pain from pressure on the plantar nerves, but when the condition is fully formed and the head of the talus has descended completely, the condition is painless but irremediable.

The stretching of the ligaments in this condition is most often due to muscular weakness in young adults who may have to stand for long periods daily. The deep muscles, especially the posterior tibial group, by their tonicity are the most potent in maintaining the longitudinal arch, and when overtired they relax and throw the strain on the plantar ligaments, which yield secondarily. All three posterior tibial muscles are effective in thus supporting the longitudinal arch, but pea-hags the most important one is the flexor hallucis longus, which normally stretches like a bow-string across the whole anteroposterior length of the arch.

The transverse arch subsequently disappears, this having been largely held-up by the peroneus longus. In the treatment of this static form of flat-foot, muscular rest is essential in the early stages, as once the ligamentous tissues are stretched they are unable to return to their original condition.