Experiment 22. To illustrate how the muscles use the bones as levers. First, practice with a ruler, blackboard pointer, or any other convenient object, illustrating the different kinds of levers until the principles are familiar. Next, illustrate these principles on the person, by making use of convenient muscles. Thus, lift a book on the toes, by the fingers, on the back of the hand, by the mouth, and in other ways.
These experiments, showing how the bones serve as levers, may be multiplied and varied as circumstances may require.

75. The Erect Position. The erect position is peculiar to man. No other animal naturally assumes it or is able to keep it long. It is the result of a somewhat complex arrangement of muscles which balance each other, some pulling backwards and some forwards. Although the whole skeleton is formed with reference to the erect position, yet this attitude is slowly learned in infancy.

In the erect position the center of gravity lies in the joint between the sacrum and the last lumbar vertebra. A line dropped from this point would fall between the feet, just in front of the ankle joints. We rarely stand with the feet close together, because that basis of support is too small for a firm position. Hence, in all efforts requiring vigorous muscular movements the feet are kept more or less apart to enlarge the basis of support.

Now, on account of the large number and flexibility of the joints, the body could not be kept in an upright position without the cooperation of certain groups of muscles. The muscles of the calf of the leg, acting on the thigh bone, above the knee, keep the body from falling forward, while another set in front of the thigh helps hold the leg straight. These thigh muscles also tend to pull the trunk forward, but in turn are balanced by the powerful muscles of the lower back, which help keep the body straight and braced.

The head is kept balanced on the neck partly by the central position of the joint between the atlas and axis, and partly by means of strong muscles. Thus, the combined action of these and other muscles serves to balance the body and keep it erect. A blow on the head, or a sudden shock to the nervous system, causes the body to fall in a heap, because the brain has for the time lost its power over the muscles, and they cease to contract.

Fig. 36.—Diagram showing the Action of the Chief Muscles which keep the Body Erect. (The arrows indicate the direction in which these muscles act, the feet serving as a fixed basis.) [After Huxley.]