Various conjectures have been advanced as to the precise physiological change that attends the condition of sleep. Some have located the source of sleep in the heart and others in the head. It was formerly a favourite theory that the action of the heart slackened and then the blood, flowing slowly through the brain, caused a kind of congestion there. This was, in fact, to look upon sleep as a species of coma that produced unconsciousness by pressure upon the fibres of the brain. The later and better opinion is, that sleep is produced by the reverse of this process; that it is not a state of congestion but of collapse; that the blood flows _from_ the part of the brain that sleeps, which is thus left in a state of depletion, with a consequent collapse of the brain fibres. Observation of the actual brain of a man who had been trepanned and over a part of whose brain a movable silver plate was placed entirely confirmed this conjecture. In sleep, the convolutions of his brain were depressed; when awake, they resumed their normal form; when his mind was exerted, they swelled visibly. Any reader who has been suddenly wakened may recal a sensation as of swelling of the brain by the blood rushing into it. This sensation was probably the result of the rapid erection of the flaccid brain fibres. Other facts strongly support this theory. When the action of the heart is stimulated by any excitement, mental or bodily, sleep will not come. So long as the brain is busy we court sleep in vain. To induce sleep we apply remedies that tend to draw the blood from the brain to the extremities. A full meal engenders sleep; but not, as formerly supposed, by congesting the brain, but by attracting the blood to the stomach and so depleting the brain. Rapid motion in a cold wind causes drowsiness when warmth is restored. Why? The blood is borne swiftly back to the surface of the body and quits the brain. Many other instances will readily occur to the reader. Note in another the process of “falling sleep.” The eyes move more and more slowly, the eyelids descend, the head nods and droops, the limbs relax, the book falls from the hand. Usually, before positive sleep occurs, involuntary endeavours at resistance are made. The eyes open with a stare. Consciousness is regained with an effort and a start. The thread of waking thought is resumed. But it is for a moment only. Again the head nods, the eyes blink and close, the limbs relax. He is _asleep_.