ecial 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
to exceed the limits of useful exercise. Fatigue summons us to rest long
before all the force of the motor organs has been expended, just as the
sensation of hunger warns us that we need food, long before the body has
become weak from the lack of nourishment.

We should never forget that it is highly essential to maintain an unused
reserve of power, just as a cautious merchant always keeps at the bank an
unexpended balance of money. If he overspends his money he is bankrupt,
and the person who overspends his strength is for the time physically
bankrupt. In each case the process of recovery is slow and painful.

87. Rest for the Muscles. Rest is necessary for the tissues, that
they may repair the losses sustained by work; that is, a period of rest
must alternate with a period of activity. Even the heart, beating
ceaselessly, has its periods of absolute rest to alternate with those of
work. A steam-engine is always slowly, but surely, losing its fitness for
work. At last it stops from the need of repair. Unlike the engine, the
body is constantly renewing itself and undergoing continual repair. Were
it not for this power to repair and renew its various tissues, the body
would soon be worn out.

This repair is really a renovation of the structure. Rest and work are
relative terms, directly opposed to each other. Work quickens the pulse
and the respiration, while rest slows both. During sleep the voluntary
muscles are relaxed, and those of or