Again, I have proceeded, in discussing the subject, on the supposition that the nervous system is a unit; that, though the functions of the several parts are of diverse character, such as motion, sensation, and thought, yet the same laws of healthy activity pertain to all portions alike; and that we could safely reason, in reference to the effects of unfavorable influences, from the observed and known to the less known, from the simple to the more complex processes of nerve-function; that what is known to be injurious to the one must be so to the other. I have endeavored to show that as too great or too little activity of the various portions of the nervous system result in irregular activity or in failure of activity, so, also, too much stimulation to the brain, as well as too little exercise of function, both result in failure in some degree; that through these two channels, and also from the effects of poisons acting on the brain, comes the largest danger to its integrity of activity. While some of the causes of insanity, however, are of such a character as has been pointed out, and consequently preventable, yet it will readily be perceived how difficult it will be to educate society so that it may be avoided. The conditions of its existence pertain in many cases to all classes of society, and ramify in the customs and habits alike of the rich and the poor. In many other forms of disease there exists some degree of unity in etiology, and we are able to discover their immediate hygienic conditions with considerable certainty, and these conditions can in many cases be avoided without much inconvenience; but those of insanity are so multifarious, they are so interwoven with the very texture of our modern civilization, that any warning which we can give, any words of help, or of caution even, all are only too likely to fall on ears which are dull of hearing. The ruling tendencies in our modes of living and of conducting the great business enterprises of life, some of which are inherited, and others learned in the years of early life, lie directly athwart its path. Again, in many other forms of disease, we approach toward their nature and causes by examination of the secretions and excretions of the body; we use our chemical tests; we percuss and auscultate; we reason from the pathological conditions existing after death, to those which must have existed prior to death; but in cases of insanity, these modes of procedure have so far availed us very little. While we find slight degrees of difference in the secretions of the insane at times, yet these changes do not appear to be pathognomonic. They may be found to exist equally with the sane and insane, and therefore avail little or not at all in determining any change which has taken place in the brain. Nor can we determine the nature of those vibratory movements which are supposed to take place during the processes of reasoning or the experiences of sensation.