_Exhaustion of the nervous System._--Sleep, then, appears to be primarily an affection of the nervous system, the result of its exhaustion. By the law of nature, it cannot continue always active; repose must succeed to effort. Hence, the more rapid the exhaustion of the nervous system, from any cause, the more sleep is demanded. This we know to be the fact. The more sensitive the system, as in childhood, or with the gentler sex, as in men of great sensibility also, poets, artists, and others, the more sleep. On the other hand, those sluggish natures which allow nothing to excite or call into action the nervous system, sleep from precisely the opposite cause; not the exhaustion of nervous activity, but its absolute non-existence. If both our systems, the animal and the vegetative or nutritive, should sleep at once, says Rauch, there would be nothing to awaken us. That would be death. "In sleep, every man has a world of his own," says Heraclitus; "when awake, all men have one in common." Sleeping and waking, it has been beautifully said by another, are the ebb and flood of mind and matter on the ocean of our life. § II.--DREAMS. _Resumè of previous Investigation._--It has been shown in the preceding section, that sleep is primarily and chiefly an affection of the nervous system, in which, through exhaustion, the senses become inactive, and, as it were, dead, while, at the same, the nutritive system and the functions essential to life go on; that in consequence of this inactivity of the sensorium, there results, 1. Loss of consciousness, so far, at least, as regards all connection with, and relation to, external things; 2. Loss of voluntary power over the physical and muscular frame; 3. Loss of voluntary control over the operations of the mind; the mind still remaining active, however, and its operations going on, uncontrolled by the will. We are now prepared to take up, more particularly, that specific form of mental activity in sleep, called dreaming; a state which admits of easy explanation on principles already laid down.