When a man uses his muscles, after a time comes the feeling called fatigue--a sensation always referred to the muscles, and due most probably to the deposit in the tissues of certain substances formed during motor activity. Warned by this weariness, the man takes rest--may indeed be forced to do so; but, unless I am mistaken, he who is intensely using the brain does not feel in the common use of it any sensation referable to the organ itself which warns him that he has taxed it enough. It is apt, like a well-bred creature, to get into a sort of exalted state under the stimulus of need, so that its owner feels amazed at the ease of its processes and at the sense of _wide-awakefulness_ and power that accompanies them. It is only after very long misuse that the brain begins to have means of saying, "I have done enough;" and at this stage the warning comes too often in the shape of some one of the many symptoms which indicate that the organ is already talking with the tongue of disease. I do not know how these views will be generally received, but I am sure that the personal experience of many scholars will decide them to be correct; and they serve to make clear why it is that men may not know they are abusing the organ of thought until it is already suffering deeply, and also wherefore the mind may not be as ruthlessly overworked as the legs or arms. Whenever I have closely questioned patients or men of studious habits as to this matter, I have found that most of them, when in health, recognized no such thing as fatigue in mental action, or else I learned that what they took for this was merely that physical sense of being tired, which arises from prolonged writing or constrained positions. The more, I fancy, any healthy student reflects on this matter the more clearly will he recognize this fact, that very often when his brain is at its clearest, he pauses only because his back is weary, his eyes aching, or his fingers tired.