rs of the second class are present in the
body.(87)

[Fig. 117]

Fig. 117--*Weight lifting levers.* Diagrams show relative distances moved
by the power and weight in levers having the weight nearer the fulcrum
than is the power. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.

[Fig. 118]

Fig. 118--*Diagram of the foot lever.* _F._ Fulcrum at ankle joint. _W._
Body weight expressed as pressure against the earth. While the muscle
power acts through the distance _ab_, the fulcrum support (body) is forced
through the distance _FE_.

*Loss of Muscular Force.*--Using a small spring balance for measuring the
power, a light stick for a lever, and a small piece of metal for a weight,
and arranging these to represent some lever of the body (as the forearm),
it is easily shown that the gain in motion causes a corresponding loss in
muscular power. (See Practical Work.) If, for example, the balance is
attached two inches from the fulcrum and the weight twelve inches, the
pull on the balance is found to be six times greater than the weight that
is being lifted. If other positions are tried, it is found that the power
exerted in each case is as many times greater than the weight as the
weight-arm is times longer than the power-arm.

Applying this principle to the levers of the body, it is seen that the
gain in motion is at the expense of muscular force, or, as we say,
_muscular force is exchanged for motion_. This exchange is greatly to the
advantage of the body; for while the ability to lift heavy weights is
important, the ability to move portions of the body rapidly and through
long distances is much more to be desired.

*Important Muscles.*--There are about five hundred separate muscles in the
body. These vary in size, shape, and plan of attachment, to suit their
special work. Some of those that are prominent enough to be felt at the
surface are as follows:

_Of the head_: The _temporal_, in the temple, and the _masseter_, in the
cheek. These muscles are attached to the lower jaw and are the chief
muscles of mastication.

_Of the neck_: The _sterno-mastoids_, which pass between the mastoid
processes, back of the ears, and the upper end of the sternum. They assist
in turning the head and may be felt at the sides of the neck (Fig. 119).

_Of the upper arm_: The _biceps_ on the front side, the _triceps_ behind,
and the