1st. _By long-continued or excessive intellectual action, or any powerful emotion of the mind._--Every organ of the body, the condition of which admits of being ascertained by ocular examination, invariably contains more blood in its tissues when in a state of activity than when its functions are temporarily suspended. We are hence, _a priori_, justified in assuming that the law is equally applicable to the brain, but we are not forced to rely entirely upon reasoning from analogy. It has been shown already that during sleep the circulation of blood within the cranium is at its minimum, both as regards quantity and rapidity, and that as soon as the individual awakes there is an immediate afflux of this fluid to the cerebral tissues. All of us are familiar with the facts that, during severe mental labor, or while under the influence of some exciting emotion, the vessels of the head and neck become distended, the head feels full, the face is flushed, and the perspiration of the parts in question is increased in quantity. Within certain limits the more blood there is in the brain the more actively its functions are performed, and so well known is this fact that some persons, who require to exercise the several faculties of the mind to an extreme degree, make use of stimulating ingesta for the purpose of accomplishing the object in view. A moderate degree of cerebral activity is undoubtedly beneficial. Exercise strengthens the mind and improves its faculties, if it is succeeded by a proper period of repose, during which the vessels are emptied to some extent of their contents, and are thus enabled to recover their tone. If, however, the brain is often kept for long periods on the stretch, during which the vessels are filled to repletion, they cannot contract even when the degree of cerebral activity is diminished. Wakefulness results as a necessary consequence, and every day renders the condition of the individual worse, because time also brings the force of habit into operation.