It is in connection with staleness that we may consider that daily and deadly enemy of strong and vigorous living--namely, Fatigue. If fatigue were nothing more than the natural and logical outcome of sound and healthy exercise of mind and body, the question would be simple enough, and we could take it offhand as one of nature’s danger signals, enjoining rest. For it is one of the primary postulates that exertion must be succeeded by rest, and thus fatigue would be only the instinctive demand of the tired organs. But it will occur at once to most readers that the feeling of fatigue, anyhow, is in no way proportionate to the amount of exertion, bodily or mental, that has been undergone; more than this, bodily exercise, and in many cases mental exercise, instead of producing, actually seems to remove the feeling of fatigue. Clearly, then, the mere sensation of fatigue does not necessarily indicate the need of rest, or sleep, or food. A man, especially if he has dined late and heavily, may awake in the morning, even though he has had a good spell of sleep in an ill-ventilated room, feeling tired. Yet that feeling of tiredness is removed, not by further dozing, but by exercise, fresh air, or often brain work. But the contradiction is only apparent; there are, it is true, several forms of fatigue, but fatigue generally can be properly considered under one head. It will be necessary, however, to go back to a few simple physiological principles in order to make this quite clear. In several of the simplest phenomena incident to life, we are utterly ignorant of processes. We know, for instance, for certain that we breathe in air rich in oxygen, and breathe out air full of carbonic acid gas. We know, again, that the proteid which we eat and digest and assimilate in our food becomes body-cells. We know that when we use our muscles we use up these body-cells, break them down and turn them into waste products; that when we use our brains some similar exertion of nerve is implicated. How these changes take place we do not know; that they do take place is absolutely certain, and it is in connection with these changes that fatigue occurs. A deficiency in the supply of oxygen given to the lungs is parallel to a deficiency in the supply of proteid given to the digestion; in both the organs are starved. Thus, to take the instance already given, the gentleman who feels fatigued in the morning, had better go out of doors; the longer he lies in his stuffy bedroom the more tired he will become. This is _one_ of the causes of the fatigue from which he is suffering--namely, a starvation of his body in point of oxygen. He would feel fatigued in the same way, if he was starved in point of proteid. His fatigue, though he has taken no bodily or mental exercise for hours, and has been resting, is genuine fatigue consequent on an insufficient supply of oxygen. Therefore, he had better get out of doors, and take exercise so as to make up the deficit.