As nations advance in civilization and refinement, affections of the nervous system become more frequent, because progress in these directions is necessarily accompanied by an increase in the wear and tear of those organs through which perceptions are received and emotions excited; and, in addition, the mode of life, as regards food, clothing, occupation, and habits, is being constantly removed farther from that standard which a regard for hygienic considerations would establish as most advantageous. If, as we have every reason to believe, each thought involves the destruction of a certain amount of nervous tissue, we can very well understand why, as we go forward in enlightenment and in all the elements of material and intellectual progress, we are at the same time, unless we also advance in the knowledge of the laws of our being, hurrying ourselves with rapid strides to a state of existence in which there is neither waste nor repair. I am far, however, from desiring to be understood as intimating that a high state of civilization is antagonistic to long life or health. What is lost in these directions as regards the nervous system is more than made up by the increased provision afforded for comfort in other ways. But while we have improved the hygienic condition of our cities and dwellings; while we as a rule clothe our bodies according to the principles of sanitary science and common sense; and while cleanliness of person has become the rule, and filthiness the exception, we have made little or no progress in the hygienic management of those organs which place us in relation with the world, and a healthy condition of which is so essential to our happiness. Among the many derangements in the normal operation of the nervous system, induced by irregular or excessive cerebral action, those which relate to the function of sleep are certainly not the least in importance, whether regard be had to the actual comfort of the individual or to the serious consequences to which they may give rise. To the consideration of some of these morbid conditions I propose to devote the remainder of the present volume, and first to inquire into the most important of them, wakefulness or insomnia.