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 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 which produces breathlessness requires frequent but short rests. The trained runner, finding his respiration embarrassed, stops a moment to regain his breath. Exercises of endurance cause fatigue less quickly than those of speed, but require longer rest. Thus a man not used to long distances may walk a number of hours without stopping, but while fatigue is slow to result, it is also slow to disappear. Hence a lengthy period of rest is necessary before he is able to renew his journey. 88. Amount of Physical Exercise Required. The amount of physical exercise that can be safely performed by each person, is a most important and practical question. No rule can be laid down, for what one person bears well, may prove very injurious to another. To a certain extent, each must be guided by his own judgment. If, after taking exercise, we feel fatigued and irritable, are subject to headache and sleeplessness, or find it difficult to apply the mind to its work, it is plain that we have been taxing our strength unduly, and the warnings should be heeded.