in kicking.

HOW WE STAND ERECT.--The joints play so easily, and the center of gravity
in the body is so far above the foot, that the skeleton can not of itself
hold our bodies upright. Thus it requires the action of many muscles to
maintain this position. The head so rests upon the spine as to tend to
fall in front, but the muscles of the neck steady it in its place.
[Footnote: In animals the jaws are so heavy, and the place where the head
and spine join is so far back, that there can be no balance as there is in
man. There are therefore large muscles in their necks. We readily find
that we have none if we get on "all fours" and try to hold up the head. On
the other hand, gorillas and apes can not stand erect like man, for the
reason that their head, trunk, legs, etc., are not balanced by muscles, so
as to be in line with one another.] The hips incline forward, but are held
erect by the strong muscles of the back. The trunk is nicely balanced on
the head of the thigh bones. The great muscles of the thigh acting over
the kneepan tend to bend the body forward, but the muscles of the calf
neutralize this action. The ankle, the knee, and the hip lie in nearly the
same line, and thus the weight of the body rests directly on the keystone
of the arch of the foot. So perfectly do these muscles act that we never
think of them until science calls our attention to the subject, and yet to
acquire the necessary skill to use them in our infancy needed patient
lessons, much time, and many hard knocks.

FIG. 21.

[Illustration: _Action of the Muscles which keep the body erect._]

HOW WE WALK.--Walking is as complex an act as standing. It is really a
perilous performance, which has become safe only because of constant