The existence or the prevalence of insanity, however, does not depend on any such conditions as relate to zymotic diseases, at least in the vast majority of cases. Our study of its causes, therefore, has been in other directions; and if our views of the influences which lead to degeneration of nerve tissue are correct, these are even more easily appreciable than are the causes of some other forms of disease, and consequently may be avoided. This must come largely, primarily, from the education of home and school life, and from the regulation of daily conduct in its relation to the brain; and, as the nervous system presides over and controls the body and its several members in the discharge of their functions, an understanding of its physiological action, at least in some degree, is of great importance to everybody. While we cannot do much to lessen the amount of brain-work or check the ambitions of adult life, as they have become so intensified by modern modes of living and the requirements of business, yet we may hope to do something by the judicious training and education of the young. That this may be effectually done, we must study the action of the brain and nervous system when under the influence of different external conditions and agencies. We have been accustomed to draw up long lists of experiences and occurrences in the lives of those who have become insane, as _causes_, such as _shock_, _grief_, _loss of friends_, _fever_, etc., etc. but it should be borne in mind that the vast majority of persons pass through these or similar experiences, and yet do not become insane. It becomes necessary, therefore, to go back of these experiences or antecedents, and inquire as to the causation of that peculiar condition of the nervous system which renders it susceptible of the effects of such secondary causes. This I have endeavored to do, more especially in the chapter on the Insane Diathesis. I have sought particularly to draw attention to the delicacy and great susceptibility of the brain while in childhood and youth to external influences and impressions, and to show that if much in the way of stimulation of any kind is added to its daily experiences, the effect upon its future development and character may prove to be most unfavorable. We have seen how at this early period of life it is moulded and changed in no small degree by its experiences, and that if these are of a disturbing character from any cause whatever, there can but result such an influence on the brain-cells as will be incorporated into their growth, and manifest itself in after-life in uncertainty and liability to irregularity of brain-action.