This most important question, as to how a man shall know when he has sufficiently tasked his brain, demands a longer answer than I can give it here; and, unfortunately, there is no popular book since Ray's clever and useful "Mental Hygiene," and Feuchtersleben's "Dietetics of the Soul," both out of print, which deals in a readable fashion with this or kindred topics.[1] Many men are warned by some sense of want of clearness or ease in their intellectual processes. Others are checked by a feeling of surfeit or disgust, which they obey or not as they are wise or unwise. Here, for example, is in substance the evidence of a very attentive student of his own mental mechanism, whom we have to thank for many charming products of his brain. Like most scholars, he can scarcely say that he ever has a sense of "brain-tire," because cold hands and feet and a certain restlessness of the muscular system drive him to take exercise. Especially when working at night, he gets after a time a sense of disgust at the work he is doing. "But sometimes," he adds, "my brain gets going, and is to be stopped by none of the common plans of counting, repeating French verbs, or the like." A well-known poet describes to me the curious condition of excitement into which his brain is cast by the act of composing verse, and thinks that the happy accomplishment of his task is followed by a feeling of relief, which shows that there has been high tension. [Footnote 1: See, now, "Brain-Work and Overwork," by H.C. Wood, M.D.; also, "Mental Overwork and Premature Disease among Public and Professional Men," by Ch. K. Mills, M.D.; also, "Overwork and Sanitation in Public Schools, with Remarks on the Production of Nervous Disease and Insanity," by Ch. K. Mills, M.D.,--_Annals of Hygiene_, September, 1886.] One of our ablest medical scholars reports himself to me as having never been aware of any sensation in the head, by which he could tell that he had worked enough, up to a late period of his college career, when, having overtaxed his brain, he was restricted by his advisers to two or three hours of daily study. He thus learned to study hard, and ever since has been accustomed to execute all mental tasks at high pressure under intense strain and among the cares of a great practice. All his mind-work is, however, forced labor, and it always results in a distinct sense of cerebral fatigue,--a feeling of pressure, which is eased by clasping his hands over his head; and also there is desire to lie down and rest.