certain groups of muscles. The muscles of the calf of the leg, acting on
the thigh bone, above the knee, keep the body from falling forward, while
another set in front of the thigh helps hold the leg straight. These thigh
muscles also tend to pull the trunk forward, but in turn are balanced by
the powerful muscles of the lower back, which help keep the body straight
and braced.

The head is kept balanced on the neck partly by the central position of
the joint between the atlas and axis, and partly by means of strong
muscles. Thus, the combined action of these and other muscles serves to
balance the body and keep it erect. A blow on the head, or a sudden shock
to the nervous system, causes the body to fall in a heap, because the
brain has for the time lost its power over the muscles, and they cease to
contract.

[Illustration: Fig. 36.--Diagram showing the Action of the Chief Muscles
which keep the Body Erect. (The arrows indicate the direction in which
these muscles act, the feet serving as a fixed basis.) [After Huxley.]

_Muscles which tend to keep the body from falling forward._

A, muscles of the calf;
B, of the back of the thigh;
C, of the spinal column.

_Muscles which tend to keep the body from falling backward._

D, muscles of the front of the leg;
E, of the front of the thigh;
F, of the front of the abdomen;