The Arch of the Foot
The seven tarsal and the five metatarsal bones—that is, the twelve bones of the instep—are arranged and jointed together so as to form an arch from the point of the heel to the balls of the toes. This is called the “plantar arch,” from the Latin word planta, the sole of the foot. The astragalus forms the summit, or key-bone, of the arch. It receives the weight from the leg, and transmits it, through the hinder pillar of the arch, to the heel, and, through the front pillar of the arch, to the balls of the toes.
The drawing represents a section, from behind forwards, of the lower end of the leg-bone, and of the bones lying along the inner side of the plantar arch. Behind it extends through the heel-bone, and in front through the great toe. It exhibits the arrangement of the fibres and plates in the interior of the bones, and shows that the greater number of them, in each bone, follow the direction of the two pillars of the arch; that is to say, they descend from the summit of the arch where it supports the leg-bone, backwards and downwards, to the heel, and, forwards and downwards, to the balls of the toes. Their arrangement is, therefore, such as to give resisting strength to the bones in the directions in which it is most required.
You may think that the arch of the foot would have been a much simpler structure, as well as stronger, if it had been composed of one bone instead of several. But it must be remembered that it would, then, have been liable to be cracked and broken by the sudden and violent manner in which, during running and jumping, the weight of the body is thrown upon it. Moreover, the several bones, where they touch one another, are covered with a tolerably thick layer of highly elastic gristle or cartilage (represented by the clear line left in the drawing along the contiguous edges of the bones); and this provision, together with the slight movements which take place between these bones, gives an elasticity to the foot and to the step, and serves to break the jars and shocks which are caused by the sudden contact of the foot with the ground.
This last is a very important point; and we find numerous contrivances in different parts of the body to protect the brain and other delicate organs from jars. So efficient are these contrivances, and so exact is the adaptation of the mechanism of the limbs and the trunk to the texture of the internal organs, that, while these are in a healthy state, we are able to run, to jump, and to leap from a considerable height, without inconvenience. But, if the organs be inflamed, or if the nervous system be over sensitive, as in common headache, the provisions, which are calculated for the normal state, are insufficient; ordinary movements are then painful, and to jump is intolerable.
The muscles play a very essential part in this work. First, they place the limbs in the most favourable position. Thus, when we alight upon the ground, from a height, we always contrive to do so with the knees and hips a little bent, so that the limbs readily yield at the joints, and act as springs to break the jar. Elderly persons commonly keep the limbs bent, even when walking quietly along. They do this because they need all the benefit which position will afford to make amends for the loss of elasticity consequent on the thinning and drying of the cartilages, and other changes that take place in the body with advancing years. Secondly, the muscles brace the limbs and joints in the position in which they have placed them. We experience the effect of the want of this salutary influence when we kick against an unseen object, or fall suddenly, or receive any blow or shock for which we are unprepared. How disagreeable, to say the least, it is to make the step for an additional stair when we have arrived at the top of a staircase, or, still worse, to meet with an unseen stair when we think that we have got to the bottom.
You perceive from the drawing (fig. 6) that there is a great difference between the two pillars of the plantar arch. The hinder pillar is comparatively short, and narrow, and descends suddenly, almost in a vertical direction, from the ankle, to the ground; and it is composed of only one bone—the heel-bone—which is jointed directly with the astragalus: whereas the fore pillar is longer and broader, is composed of several bones jointed together, and slopes much more gradually to the ground. There is, therefore, far less elasticity in the hinder part of the foot than in the fore part. Hence, when we descend from a height upon the ground, we always alight upon the balls of the toes, and thus gain the advantage which the several bones and joints afford in breaking the shock. If, after going up stairs this evening, you take the trouble to come down again, you will find that you alight upon each stair on the balls of the toes and experience no inconvenience, however quickly the descent is made. But, if you change the mode of proceeding, and descend upon the heels, the feeling will be by no means agreeable; and the various organs of the body, being disturbed from their accustomed repose, will raise such remonstrances against your infringement upon nature’s ways, that you will scarcely be able to continue the experiment. Proportionately more distressing is the sensation caused by jumping from a chair upon the heels. Indeed, this is not done altogether without risk; and the trial of it is scarcely to be recommended to persons who have attained to that sober period of life at which we are willing to concede that, in some things, nature is wiser than ourselves. Only a short time since I saw a gentleman, who, in jumping down some steps into a back yard, accidentally came upon his heels, and jarred one hip so severely that he was confined to his sofa for several days in consequence.
But, you may say, “in walking we do place the heel upon the ground first and experience no inconvenience.” True, because the force with which the foot descends in walking is very slight; and the weight is directed upon the heel, obliquely, in such a manner as to bring the toes very quickly to the ground, and really to throw nearly the whole force in that direction. Moreover, you may observe that when we walk, the weight of the body is partly sustained by the fore part of the one foot till the whole of the other foot is on the ground. I will, however, revert to the disposition of the feet in walking and running presently.
The arch of the foot has to bear great weight and at great disadvantage; and there is very little in the shape of the bones to maintain its integrity. Indeed, they all fall asunder when the other structures are removed, the key-bone dropping through of its own weight. And the same thing may be remarked throughout the skeleton. Wherever two or more bones move upon one another, their surfaces are so constructed that they do not hold together without some assistance from the soft parts. There are joints in the body which we call “hinge-joints,” and others which we call “ball-and-socket joints;” but in none of them is there such a holding and locking of one part in the other as you have in the hinge and the ball-and-socket of the mechanic. In every case the bones are held together, not by their own shape, but by ligaments and muscles. Consequently, any one of the bones may be dislocated from those next it without breakage; and when the muscles and ligaments are cut through, or have been destroyed by maceration, all the bones, between which any movement was possible during life, separate from one another.
Not only is this so, but in no instance are the movements of joints limited simply by the shape of the bones—that is to say, they are never brought to a stop by a part of one bone coming into contact with the edge of another. Such a contact would have caused a sudden check; and this would have been attended with more or less jar and with some danger of chipping and breaking the articular edges. The range of movement of a joint is always regulated by the ligaments or the muscles, not, directly, by the bones; and the restraint thus imposed upon the movements is brought to bear, not suddenly, but gradually; somewhat like the effect of the “break” upon a railway-train; while the cartilages between the bones may be compared with the “buffers” between the carriages.
It is chiefly by means of strong Ligaments, or sinewy bands, passing from bone to bone, that the shape of the plantar arch is maintained and the movements of the bones upon one another are regulated and limited. These ligaments are numerous; but I will mention only two.
One, the Plantar Ligament (A, fig. 7), of great strength, passes from the under surface of the heel-bone, near its extremity, forwards, to the ends of the metatarsal bones; in other words, 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 (D); 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 fibres 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 foot touches the ground; and the force of their contraction is proportionate to the degree of pressure which is made upon the foot. Thus they add a living, self-acting, self-regulating power to the passive resistance of the ligament. 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 hollow of the foot, under the shelter of the plantar arch.
Another very strong ligament (B in the wood-cut) passes from the under and fore part of the heel-bone (F) to the under part of the scaphoid bone (E). It underlies and supports the round head of the astragalus, and has to bear a great deal of the weight which is transmitted to that bone from the leg. It does not derive the same assistance from a close connexion with muscular fibres as the ligament just described; but it possesses a quality, which that and most other ligaments do not have, viz. elasticity. This is very important, for it allows the head of the key-bone (D) 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.
A glance at the drawing will show you that here is a weak point in the foot. The head of the key-bone receives great weight from the leg, but is comparatively unsupported; and there is a considerable strain upon this part when the heel is being raised in walking. Moreover, a good deal of movement takes place between the key-bone (D) and the scaphoid bone (E), more than between any other two bones of the instep; and freedom in the range of movement is generally attended with some sacrifice of strength. The strong elastic ligament comes in therefore with peculiar advantage at this point; and it is underlaid, and additional support is afforded exactly when it is most required, by the tendon (b in fig. 12) of a strong muscle, the especial office of which is to assist in raising the heel and bending the instep, and which runs, from the back of the leg, behind the inner ankle, to the scaphoid bone.