FOOT, STRUCTURE OF THE. In describing the structure of the F., it is expedient to commence with a brief notice of the bones which occur in it. In man, these are 26 in number; and are arranged in three natural groups—viz., the tarsal bones, which are the hindermost; the metatarsal bones, which occupy the _middle portion; and the pha langes of the toes anteriorly. The tarsal bones, 7 in number, are short and thick, and form the heel and the hinder part of the instep: The uppermost is called the astrag alus, from its supposed resemblance to the dice used by the Romans. Above, it is articulated or is jointed with the two bones of the leg, the tibia and fibula, and through these bones the whole weight of the body is thrown upon the two astragal/. Behind, it is connected with and rests upon the Os calcis, or heel-bone, which is the largest bone of the foot. Immediately in front of it, and supporting it in this direction, is the scaphoid or boat-like bone. In front of the scaphoid bone are the 3 cuneiform or wedge bones; and on the outer side of the cuneiform bones, and in front of the os calcis, is the cuboid bone. The front row of tarsal bones is composed of the 3 cuneiform bones on the inner side of the F., and of the cuboid bone externally. There are 5 metatarsal bones pass ing forward, one for each toe. Each cuneiform is connected with one, and the cuboid bone with two, of these metatarsal bones. Behind, they are close together, but as they run forwards, they diverge slightly from one another, and their anterior ends rest upon the ground, and form the balls of the toes. They constitute the forepart of the instep. The remaining bones are those of the toes, and are named the phalanges, each toe three of these bones, excepting the great toe, which has only two. (A. similar law holds for the bones of the hand, each finger having three phalanges, but the thumb only two.) The instep is composed of the 7 tarsal and the 5 metatarsal bones, which are so arranged and connected as to form an arch from the extremity of the heel-bone to the balls of the toes. This is called the plantar arch, from planta, the sole of the foot. The astragalus forms the summit or keystone of this arch, and transmits the weight which it receives posteriorly to the heel, and anteriorly to the balls of the toes. The arrangement of the fibers and laminae in the interior of the bones, is such that the greater number of them, in each bone, follow the directions of the two pillars of the arch, and thus give the greatest strength to the bones in the directions in which it is most required.
The bones, where they articulate with one another, are covered with a tolerably thick layer of highly elastic cartilage, and by this means, together with the very slight movements of which each bone is capable, a degree of elasticity is given to the F., and consequently to the step, which would be altogether wanting if the plantar arch were of one single mass of bone. This elasticity is far greater in the anterior
pillar of the arch, which is composed of fiv, comparatively long bones sloping grad ually to the ground, than in the posterior pillar, which is short, narrow, and composed of a single bone, which descends almost vertically from ..,the ankle to the ground. Hence, in jumping from a height, we always endeavor to alight upon the balls of the toes, and thus break the shock which we should feel if, by accident, we descended upon the heels.
A reference to any standard work on anatomy-(see, for example, Gray's Anatomy, pp. 178-81) will show that the ligaments which unite these bones ,to one another, and by which the movements of each bone upon the others are limited, are very numerous. 'We shall merely notice two of,these ligaments, selecting those whose action is especially obvious in maintaining the shape, of the plantar arch. One, the, plantar ligament. of great strength, 'passes frOixt the under surface of the heel-bane, near its extremity, forwards to the ends of the metatarsal bones, according to Dr. Humphry (The human Foot and the Human Hanel, 1861, p. 25). Most anatomists do not trace it. (lithe so far forwards. "In other words"(we quote from Dr. Humphry's volume), "it extends between the lowest points of the two pillars of the arch, girding or holding them in their places, and preventing their being thrust asunder when pressure is made upon the key-bone, just as the ' tie-beam' of a roof resists the tendency to outward yielding of the sides when weight is laid upon the summit. The ligament, however, has an advantage which no tie-beam can ever possess, inasmuch as a quantity of muscular fibers are attached along the hinder part of its upper surface. These instantly respond to any demand that is made upon them, being thrown into contraction directly the F. touches the ground; and the force of their contraction is proportionate to the degree of pressure which is made upon the foot. In addition to its office of binding the bones in their places, the ligament serves the further purpose of protecting from pressure the tender structures—the blood-vessels, nerves, and muscles—that lie above it in the hol low of the foot. Another very strong ligament passes from the under and fore part of the heel-bone to the under parts of the scaphoid bone. It tmderlies and supports the round head of the astragalus, and has to bear a great deal of the weight which is trans mitted to that bone from the leg. It possesses a quality which the ligament just described, and most ligaments have not—viz., elasticity. This is very important, for it allows the head of the keybone to descend a little, when, pressure is made upon it, and forces it up again when the pressure is removed, and so gives very material assistance to the other provisions for preventing jars,. and for giving ease and elasticity to the step."—Hum phry, op. cit., pp. 25, 26.