It is impossible in the small limits of a section to enter more fully into the physiology of this fascinating phenomenon, and we must refer the reader who wishes to know more about it to Dr. Alexander Haig’s work on the subject,[21] and merely note that in this way exhaustion and so fatigue are brought about both by a want of food--temporary starvation--and also by an excess of food which causes excess of waste products. In the same way, too, though food may be needed, yet to bolt a heavy meal will not meet the case satisfactorily, since much of the nourishing value of the foods will be lost, as they will not be given to the stomach in a form in which it is possible for it to assimilate them, and also the digestive and excretive organs will be severely taxed, and fatigue in them will be produced. As we have said before, fatigue, even when produced healthily, so to speak--namely by the exercise of muscles in the open air--may be quite disproportionate to the work accomplished. This question is dealt with under the chapter on exercise, and it will be sufficient here merely to mention that the wrong use of muscles (_e.g._, slow heavy movements for the wrists as in many dumb-bell exercises), or the use of the wrong muscles, as, for instance, excessive employment of the muscles of the arm when what is needed is the use of the large body muscles, are common causes of unnecessary fatigue in games and athletics. There are, furthermore, many mental causes of general bodily fatigue, for in the intimate interweaving of mind and body, as we have seen, the one suffers with the other. Thus worry of the mind, nervousness, depression, make one feel physically tired, not because the muscles have been used, but because the nervous energy whereby they work has been exhausted by mental trouble. Here, if a man can bring himself, by an effort of will, to take an interest in some bodily or mental pursuit that will draw his mind off himself and his worry, he will quite certainly be the better for it, for the nerves that have been wearing and exhausting themselves over the trouble will have rest. And rest, as we have said before, is to nerves what food is to the stomach and oxygen to the lungs.