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 organic life work with less energy. The
pulse and the respiration are less frequent, and the temperature lower
than when awake. Hence sleep, "tired Nature's sweet restorer," may be
regarded as a complete rest.

The periods of rest should vary with the kind of exercise. Thus exercise