=A Sliding Scale.= If all this be true, real fatigue can only be the result of recent effort.  If one is still alive, the results of earlier effort must long since have disappeared.  The tissue-cells retain not the slightest trace of its effects.  Fatigue cannot possibly last, because it either kills us or cures itself.  Up to a certain point, far beyond our usual high-water mark, the more a person does the more he can do.  As Professor James has pointed out, the rate of repair increases with the rate of combustion.  Under unusual stress, the rate of the whole machine is increased:  the heart-pump speeds up, respirations deepen and quicken, the blood flows faster, the endless chain of filling and emptying buckets hurries the interchange of oxygen and carbon dioxid, until the extreme capacity is reached and the organism refuses to do more without a period of rest.

The whole arrangement illustrates the wonderful provisions of Nature.  Although each individual is continuously manufacturing enough carbonic-acid gas to kill himself in a very few minutes, he need not be alarmed for fear that he may forget to expel his own poisons.  Nobody can hold his breath for more than a few minutes.  The naughty baby sometimes tries, but when he begins to get black in the face, he takes a breath in spite of himself.  The presence of carbonic-acid gas in the circulation automatically regulates breathing, and the greater the amount of gas the deeper the breath.  The faster we burn the faster we blow.  As with breathing, so with all the rest of elimination and repair.  The body dares not get behind.

="Second Wind."= A city man frequently sets out on a mountain tramp without any muscular preparation for the trip.  He walks ten or fifteen miles when his average is not over one or two.  Sometimes after a few hours he feels himself exhausted, but a glorious view opens out before him and he goes on with new zest.  He has merely increased his rate of repair and drawn on a new stock of energy.  That night he is tired, and the next day he is likely to be stiff and sore.  There is a little fatigue left in him, but it takes only a day or two for the body to be wholly refreshed, especially if he hastens the process by another good walk.  Up to a certain point, far beyond our usual limit, the more we do, the more we can do.

One day after a long walk my little daughter said that she could go no farther and waited to be carried.  But she soon spied a dog on ahead and ran off after him with new zest.  She followed the dog back and forth, running more than a mile before she reached home, and then in the exuberance of her spirits, ran around the house three times.