Observation of nervous patients shows us that their small stock of available nerve-force may be attracted to one part to the detriment of another. For this reason nervous disease is specially difficult to understand. One patient told me he could get on fairly well if he used his body and not his brain, or if he used his brain and not his body; but if he tried to use both in the same day, he became ill. Of course, he did in reality use both at the same time; what he meant was that, if he exerted a fair amount of activity in the brain, he had only strength to exert a very small amount of activity in the rest of the body, and _vice versâ_. He was right in saying that his available store of energy was so small that he could not expend even a moderate amount of it in both physical and mental labour during the same day. Others have told me that if they devoted themselves for a few days to book-work and abstained from bodily labour, their studies became easy and were performed without fatigue; but that if they began to take exercise, they were compelled by exhaustion to abandon the book-work. Moreover, the physical exertion caused great fatigue for a day or two, until they grew accustomed to the altered mode of expending energy; and when it had become easy to them, the same weariness was experienced for a short time on their return to the book-work, even though the bodily exercise was then totally discontinued. By no effort of will could the two modes of activity be carried on together, unless only a very small amount of power was expended in each, the rest of the day being given up to repose. Total collapse followed the attempt, for the necessary nerve-force was wanting.