outstretched horizontally even for a few moments. A single muscle, the
deltoid, in this case does most of the work. Even in a vigorous man, this
muscle can act no longer than four to six minutes before the arm drops
helpless. We may prolong the period by a strong effort of the will, but a
time soon comes when by no possible effort are we able to hold out the
arm. The muscle is said to be fatigued. It has by no means lost its
contractile power, for if we apply a strong electric stimulus to it, the
fatigue seems to disappear. Thus we see the functional power of a muscle
has a definite limit, and in fatigue that limit is reached.

[Illustration: Fig. 42.--A Well-Equipped Gymnasium. (From a photograph.)]

The strength of the muscle, its physical condition, the work it has done,
and the mental condition of the individual, all modify the state of
fatigue. In those difficult acts which involve a special effort of the
will, the matter of nerve exhaustion is largely concerned. Thus, the
incessant movements in St. Vitus' dance result in comparatively little
fatigue, because there is no association of the brain with the muscular
action. If a strong man should attempt to perform voluntarily the same
movements, he would soon have to rest. None of the movements which are
performed independently of the will, as the heart-beats and breathing
movements, ever involve the sensation of fatigue. As a result of fatigue
the normal irritability of muscular tissue becomes weakened, and its force
of contraction is lessened. There is, also, often noticed in fatigue a
peculiar tremor of the muscles, rendering their movements uncertain. The
stiffness of the muscles which comes on during severe exercise, or the day
after, are familiar results of fatigue.

This sense of fatigue should put us on guard against danger. It is a kind
of regulator which serves in the ordinary actions of life to warn us not