_Disorders of circulation and nutrition._--Hyperæmia of the brain is a frequent cause of wakefulness. This may be maintained by an unconscious effort of the organ of the will under the influence of any great and unusual excitement of the mind. So soon as the mental excitement is allayed, the excessive afflux of blood subsides, and the brain becomes fitted for sleep. But, if excitement be too far prolonged, the nutrition of the nervous centres suffers, and the regulative apparatus of the cerebral circulation becomes exhausted, so that the brain cannot rest, because its inhibitory centres have lost their power of control over the lower ganglia of the organ. The cerebro-spinal centres are then in a condition analogous to that of a locomotive engine on which the engineer can no longer regulate the production and distribution of steam. Such a condition is usually the result of numerous antecedent causes. Long hours of work, scanty or improper food, insufficient sleep, bad hygienic surroundings and habits, with indulgence in the use of narcotics and stimulants, are among the most common causes of the disorder. Active hyperæmia of the nervous centers has been above mentioned as the consequence of cerebral function under unfavorable conditions. But, as the disorder persists, its type undergoes a change. We still speak of the disorder as functional in its character, but it continually tends to become organic. No visible alterations, perhaps, can be detected, but, evidently, there are radical changes in the substance of the nervous tissue. Nutrition suffers throughout the body to a degree that attracts attention. The blood diminishes in quantity and quality, till the patient becomes notably anæmic. In this condition the brain is inordinately excitable. It is incapable of sustained activity, and the patient may even be oppressed by an inclination to constant drowsiness; yet he will be unable to sleep soundly, and his sleep will be continually agitated by dreams. This state is one of the constant accompaniments of slow starvation. The molecular structure of the nervous organs seems to be so slightly constructed, under such circumstances, that its equilibrium is disturbed by the most trifling incidents. It may be likened to a lofty wall of bricks laid up without mortar--“if a fox go up,” the entire structure may be thrown down with a tremendous noise. Thus the anæmic and irritable brain will react excessively under the slightest impression; consciousness is invaded by perceptions which would never arise under normal conditions of the nervous tissue; and the mind is continually aroused. This form of wakefulness is very frequent among women who have become anæmic, and among patients who are slowly convalescing from exhausting diseases.