FIG. 19.

[Illustration: _The hand as a Lever of the third class._]

When we raise the body on tiptoe, we have an instance of the _second
class_. Here, our toes resting on the ground form the fulcrum the
muscles of the calf (gas-troc-ne'-mi-us, _j_ and so-le'-us, Fig. 14),
acting through the tendon of the heel, [Footnote: This is called the
Tendon of Achilles (_k_, Fig. 14) and is so named because, as the
fable runs, when Achilles was an infant his mother held him by the heel
while she dipped him in the River Styx, whose water had the power of
rendering one invulnerable to any weapon. His heel, not being wet, was his
weak point, to which Paris directed the fatal arrow--"This tendon," says
Mapother, "will bear one thousand pounds weight before it will break." The
horse is said to be "hamstrung," and is rendered useless, when the Tendon
of Achilles is cut. (see p. 284.)] are the power and the weight is borne
by the ankle joint.

An illustration of the _third class_ is found in lifting the hand
from the elbow. The hand is the weight, the elbow the fulcrum, and the
power is applied by the biceps muscle at its attachment to the radius (A,
Fig. 19.) In this form of the lever there is great loss of force, because
it is applied at such a distance from the weight, but there is a gain of
velocity, since the hand moves so far by such a slight contraction of the
muscle. The hand is required to perform quick motions, and therefore this
mode of attachment is desirable.

The nearer the power is applied to the resistance, the more easily the
work is done. In the lower jaw, for example, the jaw is the weight, the