CHAPTER II. INSOMNIA, OR WAKEFULNESS. Sleep, gentle sleep, Nature’s soft nurse, how have I frighted thee That thou no more wilt weigh mine eyelids down And steep my senses in forgetfulness? --KING HENRY IV, SECOND PART. We have seen that the condition of normal sleep is determined by a peculiar molecular state of the substance of the brain--a modification regularly alternating with that by means of which the condition of wakefulness is sustained. We have also seen that sleep is liable to variations in its intensity, and that its course may be partially interrupted by dreams, or even by a more or less complete resumption of the movements of locomotion, constituting the different varieties of somnambulism. Our attention must now be directed to the consideration of those greater disturbances of sleep which either serve to prevent its full development, or else to actually interrupt its course, rendering it incomplete and fragmentary, or even abolishing it altogether. But, inasmuch as the healthy brain, when associated with a healthy body, can only by an extraordinary effort of the will be kept awake beyond a certain period, and then only for a short time beyond the ordinary interval of wakefulness, it follows that the study of the usual causes of insomnia must be an investigation of morbid conditions of the bodily functions. Sleeplessness, therefore, must result, 1st, from a disturbance of the peripheral sensory organs of the nervous system; 2nd, from disordered conditions of the sensory nerves and nerve tracts; 3rd, from morbid states of the brain; 4th, from any or all of these conditions operating in association with each other. We may, therefore, consider, I, _Insomnia caused by irritation of the peripheral portions of the sensory apparatus_; and, II, _Insomnia caused by morbid states of the central nervous organs_. I. INSOMNIA CAUSED BY IRRITATION OF THE PERIPHERAL PORTIONS OF THE SENSORY APPARATUS. Irritation of the sensory apparatus may be ranked in three classes: 1. Affections of the organs of special sense. 2. Affections of the nerves of common sensation. 3. Affections of the sympathetic nerves.