We only slowly learn to stand in the year or two after birth, and though we finally come to do it without conscious attention, standing always requires the co-operation of many muscles, guided and controlled by the nervous system. The influence of the latter is shown by the fall which follows a severe blow on the head, which has fractured no bone and injured no muscle; " the concussion of the brain" stuns the man, and until it has passed off he cannot stand.

Why is the power-arm in the body usually short? What kind of pulley is used in the body? Is any mechanical advantage gained from it? What is it used for? Give an example. Is standing a simple process?

When we stand erect, with the arms close by the sides and the feet together, the centre of gravity of the whole adult body lies at the articulation between the sacrum and the last lumbar vertebra, and a perpendicular drawn from it will reach the ground between the feet. In any position in which this perpendicular falls within the space bounded by a line drawn close around both feet, we can stand. When the feet are together the area enclosed by this line is small, and a slight sway of the trunk would throw a perpendicular dropped from the centre of gravity of the body outside it; the more one foot is in front of the other the greater the sway back or forward which will be compatible with safety, and the greater the lateral distance between the feet the greater the lateral sway which is possible without falling. Consequently, when a man wants to stand very firmly he advances one foot obliquely, so as to increase his base of support both from before back, and from side to side.

In consequence of the flexibility of its joints a dead body cannot be balanced on its feet as a statue can. When we stand, the ankle, knee, and hip-joints, if not braced by the muscles, would give way, and the head also fall forward on the chest. But (Fig. 39) muscles, 1, in front of the ankle-joint, and others, I, behind it, both contracting at the same time, keep the joint from yielding; similarly muscles (2) in front of the knee and hip-joints are opposed by others (II) behind them, and when we stand both contract to a certain extent and keep those joints rigid; and the muscles (III), which run from the pelvis to the back of the head similarly pull against others, 3 and 4, which run from the pelvis to the lower end of the breastbone, and from the upper end of the breastbone to the anterior part of the skull, and their balanced contraction keeps the head erect. Since the degree to which each muscle concerned contracts when we stand must be accurately adjusted to the contraction of its antagonist on the opposite side of the joint, we may easily comprehend why it takes us some time to learn to stand, and why a stunned man, whose muscles have lost guidance from the nervous system, falls.

Illustrate the influence of the nervous system in connection with standing.

Where is the centre of gravity of the body when we stand erect? Where does a perpendicular from it reach the ground? Why do we separate the feet when we want to stand firmly?

Why cannot a dead body be balanced on its feet? What prevents our knee, and hip-joints from bending when we stand?

Fig. 39. Diagram illustrating the muscles (drawn in thick black lines) which pass before and behind the joints, and by their balanced activity keep the joints rigid and the body erect.

Locomotion includes all movements of the body in space, dependent on its own unaided muscular efforts, such as walking, running, leaping, and swimming.

Explain how the different joints concerned are " braced " in standing. Why does it take a child some time to learn to stand? What is meant in physiology by locomotion?