oxygen. When sufficient hours have been passed in sleep, slight stimuli are enough to wake him, such as a trilling noise or a light; nay, he may wake or seem to wake "of his own accord." The blood returns to the brain highly oxygenized, and the brain is alive and energetic, ready to expend in action the force it has accumulated in the period of its rest.
Now, be it observed, this force is accumulated by the brain in sleep, when the blood-supply is at its minimum and contains the least oxygen. Oxidation of brain, then, implies expenditure, not accumulation of force. Stimulation of brain increases the blood-flow and activity, ubi stimulus ibifluxus. But this activity cannot go on long, and material for new work cannot be provided, unless the blood-flow be reduced to the sleeping-point, and the oxygen in the blood cease to be consumed.
In the creation and restoration of nerve-force, food and heat are to sleep both the supplement and the complement; without all these the full energy of brain-life cannot manifest itself, except for a very limited time, and each will vary in amount according as the other two are supplied in greater or less quantities. To resist the cold of a northern climate, the Esquimaux consumes at a meal that which would feed a Hindoo for a month. If he did not, the bitter winter would bring to him, no less than to the animals hybernating around him, sleep from which he would not wake again. The intense desire for sleep felt by persons exposed to great cold is closely akin to that produced by overwhelming fatigue: the whole nerve-force is consumed in either case and cannot be replaced. In those suffering from cold, the loss may be met by warmth or by food; in those worn out by fatigue, sleep alone is the restorer. How completely the brain is upset by cold we may learn from the striking narrative of the Arctic voyager Dr. Kane,[1] who tells us, after a journey of eighty or ninety miles over the ice at a mean temperature of minus 41°.2: "We were quite delirious and had ceased to entertain a sane apprehension of the circumstances about us." "Our strength failed us, and we began to lose our self-control. . . We fell half-sleeping on the snow. I could not prevent it. Strange to say, it refreshed us. I ventured upon the experiment myself, making Riley wake me at the end of three minutes; and I felt so much benefited by it that I timed the men in the same way. They sat on the runners of the sledge, fell asleep instantly, and were forced to wakefulness when their three minutes were out."
The fact, that pleasure and pain depend on fatigue and the consumption of this nerve-force, is closely connected with two other phenomena: one, that the stimulation of any nerve-centre, if repeated, loses somewhat of its effect; the other, that the same stimulus, if prolonged or intensified, may cause every variety of feeling from
- ↑ 1 "Arctic Explorations," i, 198.
