The second point, then, that one wants to make about fatigue is, that it is the commonest cause of infectious disease. Pasteur's great discovery, which set modern medicine upon the right bases, sometimes gets twisted out of perspective. Sometimes we fail to realize that the seed may fall upon stony ground. The seed, of course, is bacteria, and its discovery was Pasteur's immense service to humanity. But Pasteur was so busy that he did not emphasize the truth that a seed can fall upon good ground or upon bad ground. When bacteria fall upon bad ground, that is, upon healthy tissue, they do not grow, they do not spring up and multiply. Tired tissues, as has been abundantly proved by animal experimentation, are prone to infection. They are good soil for the growth of bacteria. It is true generally; it is true locally. A part that has been injured, for instance, a part that has been bruised without any break in the skin, without the entrance of any infection from the outside, is damaged by something that hurts its resisting power as fatigue does. Such a part will often become inflamed, will often become subject to the action of bacteria which must have been in the body already, but which had been kept on the frontier by our powers of resistance.
Our "powers of resistance," then, which we cannot more definitely name, which we do not as yet know to be identified with leucocytes or with anything else, can get tired. When they get tired we "catch" a cold or a diarrhea, or a hundred things which seem to have nothing to do with fatigue, but have nevertheless.
Accumulated fatigue or physical debt. If you go up a long flight of steps at a moderate rate, you can get to the top without being tired; if you go up at a rapid rate, as most of us do, you are tired at the top. Physically you put out the same amount of energy, I suppose. I do not see that there can be any considerable difference in the energy consumed by the performance of that act whether we do it slowly or quickly. The difference is that in the first case we rest between each two steps as we rest between each two days at night. When our activities are so balanced as not to run in debt, we rest between each two steps. You and I can walk at our individual peculiar gait on the level for a long time without any accumulation of fatigue, often with refreshment. But push us and we are soon exhausted. Suppose that our normal walking rate is three and a half miles an hour; push us to four, and it may not be a quarter of a mile before we are done up, because we have not been able to avoid accumulated fatigue by resting between each two steps. It has been said that in rowing the crew that wins is the crew that rests between each two strokes. The person who does not get tired is the person who rests between each two days. He does not accumulate fatigue. It is the accumulation that finally breaks you, makes you bankrupt. It is the little unnoticed bit added day by day, week by week, month by month, that makes the break.
Fatigue we should think of as running in debt. One of the figures of speech that has served me best in teaching patients how to live is that figure of income and outgo. I have often said to people, "Physically you are spending more than you earn, not to-day merely, but right along. You must earn more than you spend. You must get a plus balance in the bank. Then you can run along with fatigue or illness."
That figure of speech helps us also to express another fact about fatigue, which is important to recognize in ourselves and in our patients, because otherwise we get thrown off the track: delayed fatigue. The first day that your income begins to be less than your expenditures, nothing necessarily happens. The bank does not proclaim that there is no deposit there. It is some days later, usually, that you begin to reap your troubles. It is the same in physical fatigue. Patients say to us, "I slept ten hours last night. I spent a virtuous Sunday. Why should I be tired to-day?" We should answer, "Because of something you did last Tuesday or thereabouts." We all are familiar with this in relation to sleep. It is not the day after a bad night, but several days later that its effects depress us.
Delayed fatigue, then, is an important thing to notice in ourselves and to bring home to the people that we are trying to help. I suppose one could say that a great part of our business in social work is to call people's attention to things; if they have recognized them before, they will perhaps get a lesson out of what we say. Such matters are referred fatigue, delayed fatigue, accumulated fatigue,—familiar enough, only the person does not act on them because he does not notice them.
The fatigue-rest rhythm, the alternation of fatigue and rest, I have already phrased by the metaphor of earning and spending. You can phrase it also by a metaphor very close to the physical facts as we know them, the metaphor of building up and tearing down. During the daytime, from the point of view of physiology and the workings of the body, we burn up tissue. In us oxidation processes are going on which are really burning, as really as if we saw the flame. Tissue is being destroyed, broken down, going off in the form of heat, energy, and life. That is good in case it is followed, as it should be, by a period of rest in which we build up. Presumably, if we could see with adequate powers of the microscope or powers of observation of some sort, what goes on during rest, we should see a perfect fever of rebuilding all that we have torn down during the day. People often say, "Shall I take exercise?" Yes, but remember that half of the process of taking exercise is getting rested afterwards. It will do you good provided you rest after it, provided what has been torn down in exercise is replaced by sufficient tissue or fresh power in rest.
The English studies of fatigue to which I have referred have been of great importance because, so far as I know, they are the first attempt we have had in the way of testing when men or women in industry are too tired and how much too tired they are. I do not suppose that any employer of labor would want for his own profit or for more than a short time to overwork people in this sense, if he had the facts called to his attention. If he realized what he was doing, he would not want to break down his working force any more than he would to spoil his machinery. But some employers are careful of their steel machinery and careless of their human machinery. They will continue to be so, I fear, until we know more about fatigue.
It is one of the most difficult things to measure that I know. Take it in your own case: what tires you one day does not tire you another day. The individuality of it, the disturbing factors when we try to measure it, are perfectly extraordinary. Such a disturbing factor in our calculations is "second wind"—mental or physical. A number of men marching along will grow less tired as time goes on by the acquisition of what we call "second wind." We do not know what it is. We have tried to connect it with the condition of the heart, to say that the heart finally gets to deal with the volume of blood that is running through it so that there is no overplus of blood stored in any one chamber at any moment. But we do not really know anything about that. We do not know what second wind is; but it is important to know that it exists.
Moreover, as Professor William James pointed out in that essay called "The Energies of Men," there are "mental second winds." Just when a man is worn out he often finds new strength. He often cannot get his best strength until he pushes himself even to despair. In the spiritual experiences of the world's saints and heroes we find that it was just when it seemed as if they were about to go under that this second wind, or third wind, for it sometimes comes again and again, this mitigation of fatigue without rest, comes to them. This is a most disturbing fact. If we were like a pitcher which is emptied out and filled up, we should know all about fatigue very soon. We are like a pitcher to a certain extent, but the similarity is disturbed by such factors as second wind, and disturbed, moreover, by mental and emotional intruders like music. A military band coming upon a body of marching men will give them strength when they had no strength. That is not a sentimental but a practical fact which army men have to take advantage of. Then the fact that many people can rest by change of work without stopping, is also disconcerting. We say to a person, "You have been working hard all day; you must stop, lie down, go to bed." That person disobeys, keeps going on something different, is altogether fresh next morning, and we have to confess that we were wrong.