A psychological analysis of the subjective side of fatigue will make its true nature more apparent. Probably the first thing we find in the mind when fatigued is a large mass of sensations. They are referred to various parts of the body, mostly the part where muscular activity has been most violent and prolonged. Not all of the sensations, however, are intense enough to be localizable, some being so vague that we merely say we are "tired all over." These vague sensations are often overlooked; nevertheless, as will be shown later, they may be exceedingly important. But sensations are not the only contents of the mind at time of fatigue. Feelings are present also, usually of a very unpleasant kind. They are related partly to the sensations mentioned above, which are essentially painful, and they are feelings of boredom and ennui. We have yet to examine the ideas in mind and their behavior at time of fatigue. They come sluggishly, associations being made slowly and inaccurately, and we make many mistakes. But constriction of ideas is not the sole effect of fatigue. At such a time there are usually other ideas in the mind not relevant to the fatiguing task of the moment, and exceedingly distracting. Often they are so insistent in forcing themselves upon our attention that we throw up the work without further effort. It is practically certain that much of our fatigue is due, not to real weariness and inability to work, but to the presence of ideas that appear so attractive in contrast with the work in hand that we say we are tired of the latter. What we really mean is that we would rather do something else. These obtruding ideas are often introduced into our minds by other people who tell us that we have worked long enough and ought to come and play, and though we may not have felt tired up to this point, still the suggestion is so strong that we immediately begin to feel tired. Various social situations can arouse the same suggestion. For example, as the clock nears quitting time, we feel that we ought to be tired, so we allow ourselves to think we are. Let us now examine the bodily conditions to see what fatigue is objectively. "Physiologically it has been demonstrated that fatigue is accompanied by three sorts of changes. First, poisons accumulate in the blood and affect the action of the nervous system, as has been shown by direct analysis. Mosso ... selected two dogs as nearly alike as possible. One he kept tied all day; the other, he exercised until by night it was thoroughly tired. Then he transfused the blood of the tired animal into the veins of the rested one and produced in him all the signs of fatigue that were shown by the other. There can be no doubt that the waste products of the body accumulate in the blood and interfere with the action of the nerve cells and muscles. It is probable that these accumulations come as a result of mental as well as of physical work.