3. _Affections of the sympathetic nerves._--So much still remains to be learned concerning the pathological functions of the sympathetic nerves that it is impossible to assign with any great degree of precision the exact amount of interference with sleep that may depend upon disordered conditions of this portion of the nervous system. Since their principal functions consist in the regulation of the flow of blood and lymph throughout the body, and in the control of the processes of nutrition, calorification, secretion and excretion, it follows that any considerable derangement of their healthy action must be represented by a corresponding disturbance of the brain. This may reach the field of consciousness in the form of pain, and thus may become a cause of sleeplessness. In all the phenomena of inflammation sympathetic nerves play an active part. In certain portions of the body, as in the principal viscera, and in the periosteal covering of the bones, they are the interstitial nerves of the structures. When the body is in a healthy condition these nerves convey impulses of a sensory character which do not reach the cerebral organ of conscious sensation. But in certain morbid states they become inordinately sensitive, and they then serve to convey and probably also to magnify sensations to an extent that may cause exquisite pain with all its consequences. Witness the pain experienced during the various forms of colic. Rheumatic inflammations, pleurisy, pericarditis, peritonitis, cystitis, metritis, ovaritis, gastro-enteritis, and other similar diseases owe their principal suffering to the affection of the sympathetic nerves connected with the respective organs which become the seat of pain. Including with the sympathetic nerve the pneumogastric nerve, which occupies a functional position between the strictly peripheral and the ganglionic nerves, all the various forms of pain and uneasiness experienced in the region of the heart and of the lungs may be assigned to this system of nerves. Thus the various species of respiratory disturbance, such as asthma and dyspnœa from any cause, and the forms of palpitation and other cardiac disorder may become causes of wakefulness. In like manner the vague and uneasy sensations associated with certain varieties of dyspepsia are frequent sources of sleeplessness, not merely by reason of the pain which they occasion, but also because of the general disorders of nutrition with which they are associated. II. INSOMNIA CAUSED BY MORBID STATES OF THE CENTRAL NERVOUS ORGANS. So intimately connected are the spinal cord and the brain that their disorders may properly be considered together. These may be classified as: 1. Disorders of circulation and nutrition. 2. Inflammations and degenerations. 3. Neoplastic encroachments.