from the shoulder by a short neck, as in the thigh-bone (femur). Like the thigh-bone it is twisted upon itself and forms a screw. The inferior extremity of the arm bone is furnished with spiral articular surfaces resembling those found at the knee. The spiral articular surfaces of the arm bone are adapted to similar surfaces existing on the superior extremities of the bones of the forearm, to wit, the radius and ulna. These bones, like the bones of the leg, are spirally disposed with reference to each other, and form a screw consisting of two parts. The bones of the forearm are united to those of the wrist (carpal) and hand (metacarpal and phalangeal) by articular surfaces displaying a greater or less degree of spirality. From this it follows that the superior extremities of man greatly resemble his inferior ones; a fact of considerable importance, as it accounts for the part taken by the superior extremities in locomotion. In man the arms do not touch the ground as in the brutes, but they do not on this account cease to be useful as instruments of progression. If a man walks with a stick in each hand the movements of his extremities exactly resemble those of a quadruped.

The bones of the human extremities (superior and inferior) are seen to advantage in fig. 26; and I particularly direct the attention of the reader to the ball-and-socket or universal joints by which the arms are articulated to the shoulders (x, x), and the legs to the pelvis (a, a'), as a knowledge of these is necessary to a comprehension of the oscillating or pendulum movements of the limbs now to be described. The screw configuration of the limbs is well depicted in the left arm (x) of the present figure. Compare with the wing of the bird, fig. 6, and with the anterior extremity of the elephant, fig. 7, p. 28. But for the ball-and-socket joints, and the spiral nature of the bones and articular surfaces of the extremities, the undulating, sinuous, and more or less continuous movements observable in walking and running, and the twisting, lashing, flail-like movements necessary to swimming and flying, would be impossible.

The leg in the human subject moves by three joints, viz., the hip, knee, and ankle joints. When standing in the erect position, the hip-joint only permits the limb to move forwards,

the knee-joint backwards, and the ankle-joint neither backwards nor forwards. When the body or limbs are inclined

FIG. 26. Skeleton of Man. Compare with fig. 4, p. 21, and fig. 24, p. 47.-Original.

obliquely, or slightly flexed, the range of motion is increased.

The greatest angle made at the knee-joint is equal to the sums of the angles made by the hip and ankle joints when these joints are simultaneously flexed, and when the angle of inclination made by the foot with the ground equals 30°.

From this it follows that the trunk maintains its erect position during the extension and flexion of the limbs. The step in walking was divided by Borelli into two periods, the one corresponding to the time when both limbs are on the ground; the other when only one limb is on the ground. In running, there is a brief period when both limbs are off the ground. In walking, the body is alternately supported by the right and left legs, and advanced by a sinuous movement. Its forward motion is quickened when one leg is on the ground, and slowed when both are on the ground. When the limb (say the right leg) is flexed, elevated, and thrown forward, it returns if left to itself (i.e. if its movements are not interfered with by the voluntary muscles) to the position from which it was moved, viz. the vertical, unless the trunk bearing the limb is inclined in a forward direction at the same time. The limb returns to the vertical position, or position of rest, in virtue of the power exercised by gravity, and from its being hinged at the hip by a ball-and-socket joint, as explained. In this respect the human limb when allowed to oscillate exactly resembles a pendulum,—a fact first ascertained by the brothers Weber. The advantage accruing from this arrangement, as far as muscular energy is concerned, is very great, the muscles doing comparatively little work.1 In beginning to walk, the body and limb which is to take the first step are advanced together. When, however, the body is inclined forwards, a large proportion of the step is performed mechanically by the tendency which the pendulum formed by the leg has to swing forward and regain a vertical position, an effect produced by the operation of gravity alone. The leg which is advanced swings further forward than is required for the step, and requires to swing back a little before it can be deposited on the ground. The pendulum

1 The brothers Weber found that so long as the muscles exert the general force necessary to execute locomotion, the velocity depends on the size of the legs and on external forces, but not on the strength of the muscles.

movement effects all this mechanically. When the limb has swung forward as far as the inclination of the body at the time will permit, it reverses pendulum fashion; the back stroke of the pendulum actually placing the foot upon the ground by a retrograde, descending movement. When the right leg with which we commenced is extended and firmly placed upon the ground, and the trunk has assumed a nearly vertical position, the left leg is flexed, elevated, and the trunk once more bent forward. The forward inclination of the trunk necessitates the swinging forward of the left leg, which, when it has reached the point permitted by the pendulum movement, swings back again to the extent necessary to place it securely upon the ground. These movements are repeated at stated and regular intervals. The retrograde movement of the limb is best seen in slow walking. In fast walking the pendulum movement is somewhat interrupted from the limb being made to touch the ground when it attains a vertical position, and therefore before it has completed its oscillation.1 The swinging forward of the body may be said to inaugurate the movement of walking. The body is slightly bent and inclined forwards at the beginning of each step. It is straightened and raised towards the termination of that act. The movements of the body begin and terminate the steps, and in this manner regulate them. The trunk rises vertically at each step, the head describing a slight curve well seen in the walking of birds. The foot on the ground (say the right foot) elevates the trunk, particularly its right side, and the weight of the trunk, particularly its left side, depresses the left or swinging foot, and assists in placing it on the ground. The trunk and limbs are active and passive by turns. walking, a spiral wave of motion, most marked in an anteroposterior direction (although also appearing laterally), runs through the spine. This spiral spinal movement is observable in the locomotion of all vertebrates. It is favoured in man by the antero-posterior curves (cervical, dorsal, and lumbar) existing in the human vertebral column. In the effort of walking the trunk and limbs oscillate on the ilio-femoral 1 "In quick walking and running the swinging leg never passes beyond the vertical which cuts the head of the femur."

In

articulations (hip-joints). The trunk also rotates in a forward direction on the foot which is placed upon the ground for the time being. The rotation begins at the heel and terminates at the toes. So long as the rotation continues, the body rises. When the rotation ceases and one foot is placed flat upon the ground, the body falls. The elevation and rotation of the body in a forward direction enables the foot which is off the ground for the time being to swing forward pendulum fashion; the swinging foot, when it can oscillate no further in a forward direction, reversing its course and retrograding to a slight extent, at which juncture it is deposited on the ground, as explained. The retrogression of the swinging foot is accompanied by a slight retrogression on the part of the body, which tends at this particular instant to regain a vertical position. From this it follows that in slow walking the trunk and the swinging foot advance together through a considerable space, and retire through a smaller space; that when the body is swinging it rotates upon the ilio-femoral articulations (hip-joints) as an axis; and that when the leg is not swinging, but fixed by its foot upon the ground, the trunk rotates upon the foot as an axis. These movements are correlated and complementary in their nature, and are calculated to relieve the muscles of the legs and trunk engaged in locomotion from excessive wear and tear.

Similar movements occur in the arms, which, as has been explained, are articulated to the shoulders by ball-and-socket joints (fig. 26, x x', p. 55). The right leg and left arm advance together to make one step, and so of the left leg and right arm. When the right leg advances the right arm retires, and vice versa. When the left leg advances the left arm retires, and the converse. There is therefore a complementary swinging of the limbs on each side of the body, the leg swinging always in an opposite direction to the arm on the same side. There is, moreover, a diagonal set of movements, also complementary in character: the right leg and left arm advancing together to form one step; the left leg and right arm advancing together to form the next. The diagonal movements beget a lateral twisting of the trunk and limbs; the oscillation of the trunk upon the limbs or feet, and the oscillation