fulcrum is the hinge joint at the back, and the muscles (temporal,
_d_, and the mas'-se'ter, _e_, Fig. 14) on each side are the
power. [Footnote: We may feel the contraction of the masseter by placing
our hand on the face when we work the jaw, while the temporal can be
readily detected by putting the fingers on the temple while we are
chewing. The tendon of the muscle (digastric)--one of those which open the
jaw--passes through a pulley (_c_, Fig. 14) somewhat like the one in
the eye.] They act much closer to the resistance than those in the hand,
since here we desire force, and there, speed.

FIG. 20.

[Illustration: _The Kneejoint;_ k, _the patella;_ f, _the
tendon._]

THE ENLARGEMENT OF THE BONES AT THE JOINTS not only affords greater
surface for the attachment of the muscles, as we have seen, but also
enables them to work to better advantage. Thus, in Fig. 20 it is evident
that a muscle acting in the line _f b_ would not bend the lower limb
so easily as if it were acting in the line _f k_, since in the former
case its force would be about all spent in drawing the bones more closely
together, while in the latter it would pull more nearly at a right angle.
Thus the tendon _f_, by passing over the patella, which is itself
pushed out by the protuberance _b_ of the thigh bone, pulls at a
larger angle, [Footnote: The chief use of the processes of the spine (Fig.
6) and other bones is, in the same way, to throw out the point on which
the power acts as far from the fulcrum as possible. The projections of the
ulna ("funny bone") behind the elbow, and that of the heel bone to which
the Tendon of Achilles is attached, are excellent illustrations (Fig. 1).]
and so the leg is thrown forward with ease in walking and with great force