Having thus illustrated the first order of levers at the head, the third order at the elbow, we can see an excellent example of the second order at the ankle joint. In this case the foot is the lever, and we shall consider the mechanism of the foot and leg in walking. A step forwards has been taken by the left and the heel of the right foot is being raised to propel the body forwards. The pad of the foot and balls of the toes represent the fulcrum of the right foot; the power is applied to the rising heel by the muscles of the calf of the leg. The weight of the body falls on the foot at the ankle joint between the fulcrum at the toes and the power at the heel, but much nearer the power than the fulcrum. The nearer the weight is to the power, or putting the matter more briefly—the shorter the heel, the smaller is the purchase and the greater the muscle power required. Hence the races with short feet, high insteps and steeply set heels, have large calves. The foot, however, is a peculiar kind of lever; it is arched with the weight falling on the keystone of the arch. If the foot is long and slender, and especially if the weight falls near the centre of the arch, the more likely is the arch to break down.
It is sometimes said that the collapse of the arch of the foot is due to a giving way of the ligaments which bind the bones of the arch together. Those who hold this view of " flatfoot " overlook a very important function of muscles and the true nature of ligaments. Our bones are really not bound and kept together at joints by passive fibrous ligaments. There is only one joint in the body where this is the case, and that is at the knee joint. As we take a step forward we bend the knee, the bones being then held in apposition by the active muscles which end round the knee; then as we bring the weight of the body upon the outstretched leg the knee is straightened out, the ligaments becoming tight and pressing the bones together, thus securing the joint as it supports the weight of the body. At the very end of the movement the lower end of the thigh-bone undergoes a peculiar rotation inward and backward which locks the joint and secures it more firmly. At all other joints muscles keep the bones in apposition except when the movements become forced or extreme; it is only in these extreme movements that the ligaments take a part in limiting the action and securing the joint. This is also the case with the ligaments of the arch of the foot. When we stand, the arch is maintained by the steady and continuous action of muscles. Some of these pass along the sole of the foot from heel to toes, and act like the tense string of a bow, but the chief supporting muscles are situated in the leg, and sustain the arch of the foot by means of long tendons. Every one knows how tiresome a thing it is to stand still; it fatigues infinitely more than walking, because in standing these muscles of the sole are continuously in action, whereas in walking they have alternate phases of action and rest.
Few people realize how complex is the act of standing and the great number of joints and muscles which are involved. Not a muscle of the foot is asleep then, all are in action binding the various parts of the foot together to form a solid supporting base for the upright body. A dead man, however stark death may have rendered the trunk, cannot be maintained upright without support. Even a statue needs to have its feet firmly bolted to the pedestal. In the living standing man all the muscles of the leg are in quivering action, balancing and stiffening the legs on the feet. All the time messages are passing from the muscles to the centres in the spinal cord, from which other messages are being issued to regulate and co-ordinate the muscles in their action. The knees may be locked by the mechanism just mentioned when we stand at " attention," but most of us prefer to stand with the knees slightly bent, with the knee-caps loose, and the muscles behind the knees in action, keeping the joint stiff. Every schoolboy knows how the standing posture may be upset by catching the kneecap muscles off their guard by a blow delivered behind the joint. At the hip joint there is also a mechanism which saves muscular effort. In the front of the joint there is an extremely-strong ligament (ilio-femoral); when we stand at attention we over-extend the body at the hip-joint, and this strong ligament in front of the joint becomes tense, supports the body, and thus saves the muscles. The spinal column is not stable by itself. It rests on the sacrum of the pelvis, the pelvic basin in turn resting on the thighs at the hip-joints. The vertebrae of the backbone—five in the loins, twelve in the dorsal region, seven in the neck, making twenty-four in all—are balanced one upon the other and maintained erect by the exceeding complex musculature of the back. The twelve pairs of ribs, besides serving the purposes of respiration, also act as levers for the backbone. The great sheets of muscles in the wall of the abdomen and thorax, which come into action and support the viscera when we stand up, at the same time act on the ribs and through them balance the spine and trunk. Then the head is balanced on the spine by the muscles of the neck ; every one notices how the balance of the head is lost as the seated sleeper nods. The shoulders, too, have to be kept up by the muscles ; the great muscular sheet (trapezius) which descends from the head and neck to the prominence of each shoulder, is in action every minute we are in the upright posture. In time an aching feeling of fatigue may settle on the shoulder when the contracting power of the muscles is exhausted. Thus standing is an exceedingly complex act, which involves the majority of the muscles of the body. All the muscles involved are regulated and co-ordinated unconsciously by a silently working but elaborate reflex nervous system.
One of the most marvellous adaptations of our bodies is the manner in which bones are built to meet the strain and stress to which the skeleton is exposed during life. The material of the bones is so arranged that the greatest strength is obtained at a minimum expenditure of building material. The skeleton is not a dead inert framework in the living body; although it is chiefly composed of calcium salts yet we have good reason to suppose that every particle of a bone is alive. When from illness or idleness the bones are not exposed to strain or stress, they atrophy; their tissue is partly removed and the living cells or osteoblasts which permeate every part of a bone are lessened in their building activity ; absorption of old bone takes place at a greater rate than deposition of new bone. In men who are living active lives and taking vigorous exercise and have not passed the prime of life the opposite is true; deposition or growth exceeds absorption in rate. The cells of the bone are sensitive to the forces which are brought to bear on them. This is p very well seen when the bones of the arch of the foot are laid open by a vertical section which passes from heel to toes. In the heel bone which forms the posterior pillar of the supporting arch, the building cells or osteoblasts have laid the bone down in long needles running in the direction of the transmitted weight—from the base of the heel upwards and forwards to the ankle joint. These supporting needles are bound together by fine transverse plates, the whole system of the heel bone being enclosed within a thin shell. In the front pillar of the arch of the foot, the needles of bone are arranged so that their direction is from the bones of the toes backwards and upwards to the ankle. The systems of the front and back pillars of the foot meet and cross in the keystone of the arch—the astragalus or ankle bone.