femur, but is at such an angle that extreme extension causes a slight amount of outward motion of the leg.

6. In the ball and socket joints-the name of which implies their mechanism-the most varied movements occur. (Hip and shoulder.)

STANDING.

In order that an elongated rigid body may stand upright, it is only necessary that a line drawn vertically through its centre of gravity should pass within its basis of support, and, if the latter be sufficiently wide, the object will remain permanently in that position. The human body, in the first place, is not rigid, and in the second place the basis of support is too small to insure a satisfactory degree of steadiness. The act of standing must, therefore, be accomplished by the action of certain muscles, which are employed in preventing the different joints from bending, and in so balancing the various parts of the body as to keep the whole frame from toppling over.

In order to economize muscular energy while standing, we may lock the more important joints, and thus depend rather on the passive ligaments than upon muscular action for the rigidity of the body. With this object we are taught to place the heels together, turn out the toes, bring the legs parallel by approximating them, and, extending the knees to the utmost, to straighten and to throw back the trunk so as to render tense the anterior hip ligaments, to direct the face straight forward so as to balance the head evenly, and to let the arms fall by the sides.

In this position, as a soldier stands at attention, the knee and hip joints remain fixed, without any effort on the part of the muscles, but it is far from being the most comfortable attitude one can assume for prolonged standing, and hence the position known best by the order "stand at ease " is adopted if more complete rest is desired. In this position the weight of the body is usually allowed to rest on one leg, while the other lightly touches the ground to form a kind of stay, and relieve the muscles which surround the supporting ankle from too great an effort of balancing. At the same time the knee is extended, and the pelvis becomes somewhat oblique, so as to bring it more directly over the

head of the femur. In ordinary easy standing, the joints are not usually kept locked by the tension of the ligamentous structures, but their position is constantly being very slightly altered, so as to vary the muscles employed in preserving the balance and thus prevent fatigue.

The joints most exercised in the erect posture are the following :

1. The ankle has to support the weight of the entire body, while the joint is neither flexed nor extended to its utmost, and cannot be fixed in this position by ligamentous arrangements. The foot being placed on the ground, resting on the heel and the balls of the great and little toes, is supported in an arch-like form by strong though elastic ligaments, which allow but little motion in the numerous joints. The bones of the leg can move in the freest way, backward or forward, over the articular surface of the astragalus, which forms the roller of the hinge, lateral motion being prevented by the malleoli. The line passing through the centre of gravity of the body generally falls slightly in front of the axis of rotation of the ankle joint, so that the entire body tends to fall forward at the ankles. This tendency is checked by the powerful calf muscles, which, attached to the calcaneum by means of the strong tendo-Achillis, keep the parts in such a position that an exact balance is almost constantly kept up.

2. The knee joint, when completely extended, requires no muscular action to prevent it from bending, because the line of gravity then passes in front of the axis of rotation, and the weight of the body tends to bend the knee backward. This is impossible, on account of the strong ligaments which exert their traction behind the axis of rotation. As a rule, these ligaments are not put on the stretch in this way, but the joint is held, by muscular power, in such a position that the line of gravity passes just through, or very slightly behind, the axis of rotation of the joint, so that, if anything, there is a slight tendency for the knee to bend. This is completely checked, and the body balanced, by the powerful extensor muscles of the thigh.

3. In the hip joints, which have to support the trunk and head, the line of gravity falls just behind the line uniting the joints

when the person is perfectly erect, so that here the body has a tendency to fall backward. This is prevented by the strong iliofemoral ligament. When, however, the knee is not straightened to the full extent, so that the line of gravity passes through or a little behind the axis of rotation of that joint, then the pelvis is very slightly flexed on the femora, so that the axis of the joints lies exactly in or a little behind the line of gravity, and thus the body inclines rather to fall forward. This tendency is prevented by the powerful glutei muscles, which also enable us to regain the erect posture after bending the trunk forward.

The motions of which the pelvis and vertebral column are capable are too slight to deserve attention here. The vertebral column, wedged in as it is between the two innominate bones, may be taken, together with the pelvis, as forming a very yielding and elastic, but practically jointless pillar, the upper part of which can alone be bent to such an extent as to require mention in discussing the mechanism of station.

The individual joints between the cervical vertebræ permit but a slight amount of movement when taken separately, but by their aggregate motion they enable considerable extension and flexion of the neck to take place. These motions follow so closely, and are so inseparably associated with those of the head on the upper vertebra, that there is no need to consider them separately from the latter.

The atlanto-occipital joints admit of some little lateral movement, but that in the antero-posterior direction is much the more important, but even this would be insignificant were it not associated with the movements between the other cervical vertebræ.

The cranium has then to be balanced on the top of a flexible column, and rests immediately in a kind of socket, which can move as a double hinge around two axes at right angles one to the other. The vertical line from the centre of gravity of the cranium must vary with every forward, backward, or lateral movement of the head or neck, but in the erect posture it passes a little in front of the axis of rotation of the atlanto-occipital joint, and somewhat behind the curve of the cervical vertebræ, so that the head may be said to be poised on the apex of the verte

bral column, with some tendency to fall forward. There are no ligamentous structures which can lock the joints so as to keep the head in the erect position; therefore, without the aid of muscular force, the head will fall forward or backward, according to the position it may be in when the muscles suddenly relax, as happens in falling asleep in an upright posture.

From the foregoing facts it will be seen that there exists a kind of coördinated antagonism at work in ordinary easy standing which keeps the elastic, pliable body upright, without the rigidity adopted when standing "at attention." The muscular action is more exercised when we are not on steady ground, and varied coordination becomes necessary; for instance, when we go on board ship for the first time. Standing then takes some little time to become easy, and requires new associations of movement. The gastrocnemius and soleus relax the ankle in a degree just proportionate to the amount of flexion of the knee permitted by the quadriceps extensor cruris, while, simultaneously, the great gluteal muscle allows the body to incline forward so as to keep its centre of gravity in the proper relation to the basis of support.

WALKING AND RUNNING.

Walking is accomplished by poising the weight on one foot and then tilting the body forward with the other, which is then swung in front and placed on the ground to prevent falling. These acts are performed alternately by each leg, so that the "swinging limb" becomes the "supporting limb" of the next step. The swinging leg is described as having two phases, (1) active, while pushing off from the ground, and (2) passive, while swinging forward like a pendulum. In starting, one foot is placed behind the other, so that the line of gravity lies between the two, the hindmost limb having the ankle and knee a little bent. By suddenly straightening these joints it gives a "push off" with the toes and propels the body forward, so as to move it around the axis of motion of the fixed, or supporting ankle joint. At the end of the swing, the pendulous leg comes to the ground, and leaves the other limb in the attitude ready for the push off. Thus, on level ground walking is carried on with but small mus.

cular exercise; but in ascending an incline or going upstairs, the supporting limb has to elevate the body at each step by extending the knee and ankle joints by the thigh extensors and the calf muscles.

Running is distinguished from walking by the fact that, while in the latter both feet rest on the ground for the greater part of each pace, in the former the time that either foot rests on the ground is reduced to a minimum, and the body can never be said to be balanced on either leg, so that, in fact, there is no longer a "support limb." The legs are kept in a semiflexed position, ready for the push off or spring, which is so forcibly carried out that the body is propelled through the air without any support between each step, and has a constant tendency to fall forward. Thus, an interval of greater or less duration, according to the pace, exists during which both the feet are off the ground, because, the moment either foot comes to the ground, it at once executes a new spring without waiting for the swing of the other.