ird class_ the power is between the fulcrum and the weight.

[Fig. 115]

Fig. 115--*Classes of levers. I.* Two levers of first class showing
fulcrums in different positions. II. Lever of second class. III. Lever of
third class. _F._ Fulcrum. _P._ Power. _W._ Weight. _a._ Power-arm. _b._
Weight-arm.

*Application to the Body.*--In the body the bones serve as levers; the
turning points, or fulcrums, are found at the joints; the muscles supply
the power; and parts of the body, or things to be lifted, serve as
weights. For these levers to _increase_ the motion of the muscles, it is
necessary that the muscles be attached to the bones _near the joints_, and
that the parts to be moved be located at some distance from the joints. In
other words the (muscle) power-arm must be _shorter_ than the (body)
weight-arm.(86)

Examining Fig. 116, it is seen that the distances moved by the power and
weight vary as their respective distances from the fulcrum. That is to
say, if the weight is twice as far from the fulcrum as the power, it will
move through twice the distance, and if three times as far, through three
times the distance. Thus the muscles, by acting through short distances
(on the short arms of levers), are able to move portions of the body
(located on the long arms) through long distances. Can all three classes
of levers be used in this way in the body?

[Fig. 116]

Fig. 116--*Motion producing levers.* Diagrams show relative distances moved
by the power and weight in levers having the power nearer the fulcrum than
is the weight. _F._ Fulcrum. _P, P'._ Power. _W, W'._ Weight.

*Classes of Levers found in the Body.*--Practically all of the levers of
the body belong either to the first class or the third class. In both of
these the muscle power can be applied to the short arm of the lever,
thereby moving the body weight through a longer distance than the muscle
contracts (Fig. 116). In the levers of the second class, however, the
weight occupies this position, being situated _between_ the power and
fulcrum (Fig. 117). The weight, therefore, _cannot_ move farther than the
power in this lever. It must always move a shorter distance. While such a
lever is of great advantage in lifting heavy weights outside of the body,
it cannot be used for increasing the motion of the muscles. For this
reason no well-defined leve