“The body is therefore undergoing continued change. The hair of yesterday is not the hair of today; the muscle which extends the arm is not identically the same muscle after as before its action; old material has been removed and new has been deposited to an equal extent; and though the weight and form, the chemical constitution and histological character have been preserved, the identity has been lost.” All this is especially true of the brain. Its substance is consumed by every thought, by every action of the will, by every sound that is heard, by every object that is seen, by every substance that is touched, by every odor that is smelled, by every painful or pleasurable sensation, and so each instant of our lives witnesses the decay of some portion of its mass and the formation of new material to take its place. The necessity for sleep is due to the fact that during our waking moments the formation of the new substance does not go on as rapidly as the decay of the old. The state of comparative repose which attends upon this condition allows the balance to be restored, and hence the feeling of freshness and rejuvenation we experience after a sound and healthy sleep. The more active the mind the greater the necessity for sleep, just as with a steamer, the greater the number of revolutions its engine makes, the more imperative is the demand for fuel. The power with which this necessity can act is oftentimes very great, and not even the strongest exertion of the will is able to neutralize it. I have frequently seen soldiers sleep on horseback during night marches, and have often slept thus myself. Galen on one occasion walked over two hundred yards while in a sound sleep. He would probably have gone farther but for the fact of his striking his foot against a stone and thus awaking. The Abbé Richard states that once when coming from the country alone and on foot, sleep overtook him when he was more than half a league from town. He continued to walk, however, though soundly asleep, over an uneven and crooked road.[2]