. This class of levers
is not common in the body. Standing on tiptoe is, however, an example.
Here the toes in contact with the ground are the fulcrum, the power is the
action of the muscles of the calf, and between these is the weight of the
body transmitted down the bones of the leg to the foot.

Levers of the third class are those in which the power is applied at
a point between the fulcrum and weight. A familiar example is where a
workman raises a ladder against a wall. This class of levers is common in
the body. In bending the forearm on the arm, familiarly known as "trying
your muscle," the power is supplied by the biceps muscle attached to the
radius, the fulcrum is the elbow joint at one end of the lever, and the
resistance is the weight of the forearm at the other end.

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 f