A knowledge of the bony structure of the foot is the key which unlocks its pathology. The bones of the foot are numerous, so as to give it mobility and to lessen shocks. If the bones become ankylosed the footing becomes insecure, balancing is difficult, the gait is altered, and great care is necessary in locomotion to avoid straining and injury.

The foot is triangular in shape, being broad across the toes and narrow at the heel. Its bones compose the tarsus, metatarsus, and phalanges. Of these the first two are essential, but the third is less so. Phalanges are more or less for prehensile uses, and as man, as we see him, encases his foot in shoes he makes but little use of the toes, hence they are the least important part of the foot. They are used somewhat in walking, and to a greater degree in balancing, climbing, running, etc. They add to the efficiency of the foot, but their loss does not impair it to a great extent. Intricate and delicate movements may be interfered with, but the more deliberate firmer movements, as in walking, may remain almost normal. The big toe has only two phalanges and this increases its strength at the cost of mobility. The remaining portion of the foot is composed of the metatarsus and tarsus - five bones of the former and seven of the latter.

The foot bones are divided longitudinally into two sets, an in-, ternal and external. The main weight of the body is transmitted through the internal set, which is in relation with the tibia. It consists of the astragalus {talus), scaphoid (navicular), the three cuneiform, and the inner three metatarsal bones with their corresponding phalanges (Fig. 578). The external set is in relation with the fibula, and is composed of the os calcis, cuboid, and outer two metatarsals with their corresponding phalanges.

As has been pointed out by Quenu and Kuss (Revue de Chirurgie, Jan., 1909, p. 1), while the main function of the internal portion of the foot is support, that of the external portion is balance. They suggest that from a functional standpoint the foot may be divided into an internal portion composed of all the tarsal bones and the first metatarsal bone with its phalanges, and an external portion consisting of the outer four metatarsal bones and phalanges. They show that in dislocation of the metatarsus the line of division passes between the first and second metatarsal bones; the first metatarsal is usually displaced inwardly, while the second, third, fourth, and fifth metatarsal bones are practically always displaced outwardly, there being a considerable separation between the metatarsal bone of the big toe and the second metatarsal bone adjacent.

Fig. 578.   Anterior view of the bones of the foot showing their division into internal and external sets.

Fig. 578. - Anterior view of the bones of the foot showing their division into internal and external sets.

When there is congenital absence of the tibia the foot bones related to it are also lacking, and when the fibula is lacking there are no bones of the external set. In man both sets contribute to support, but the tibial or inner set is the more important, the fibular or outer set being in a condition of regression.

As has been stated above, the foot is dome-shaped, being arched anteroposteriorly and laterally. The anteroposterior arching has been divided into an inner and outer arch. The inner arch is composed of the os calcis, astragalus, scaphoid, three cuneiform and inner three metatarsal bones. The highest point of this arch is the midtarsal joint between the astragalus and scaphoid (Fig. 579). The outer arch is composed of the os calcis, cuboid, and outer two metatarsal bones. It is much lower than the inner arch. The highest point is between the cuboid and os calcis, and when weight is borne on the foot this outer arch becomes obliterated and comes in contact with the ground.

The lateral arch has its outer end supported by the outer edge of the foot, which through the medium of the soft parts is in contact with the ground. Its inner end is supported by the inner edge of the foot which is some distance above the ground. Thus it is seen that the weight of the body is transmitted from the body of the astragalus in three directions, viz., backward to the tuberosities of the os calcis, forward to the heads of the metatarsal bones, and laterally toward the base of the fifth metatarsal bone. The posterior pillar of the anteroposterior arch is short, thick, and composed of only two bones, the astragalus and os calcis. It is stiff and strong, but having only two parts is comparatively immovable. The anterior pillar of the arch is longer and has more bones and, while it is not so strong against static pressure as the posterior pillar, is, on account of its elasticity and mobility, far more effective against dynamic (active) pressure. Thus it is that when a person jumps from a height and alights on the sole of the foot the astragalus or os calcis of the posterior pillar is fractured while the bones of the anterior pillar escape. The internal part of the foot is more liable to give way than the external part because the external part is practically in contact with the ground while the internal part has as its support ligaments and muscles, and when these latter give way it is the inner side of the foot which sinks. This is still more favored by the position of the tuberosities of the os calcis with reference to the ankle-joint; they are not directly beneath it, but somewhat to its outer side.

FlG. 579.   The inner arch of the foot.

FlG. 579. - The inner arch of the foot.

Fig. 580.   The outer arch of the foot

Fig. 580. - The outer arch of the foot.

Fig. 581.   Lateral or transverse arch of foot.

Fig. 581. - Lateral or transverse arch of foot.