Insomnia.--The brains of rabbits which had been kept awake for one hundred hours showed precisely the same changes as those shown in physical fatigue, strychnin poisoning, and exhaustion from emotional stimulation. Eight hours of continuous sleep restored all the cells except those that had been completely exhausted. This will explain the permanent ill effect of long-continued insomnia; that is, long-continued insomnia permanently destroys a part of the brain-cells just as do too great physical exertion, certain drugs, emotional strain, exophthalmic goiter, and hemorrhage. We found, however, that if, instead of natural sleep, the rabbits were placed for the same number of hours under nitrous oxid anesthesia, not only did the brain-cells recover from the physical deterioration, but that 90 per cent. of them became hyperchromatic. This gives us a possible clue to the actual chemical effect of sleep. For since nitrous oxid owes its anesthetic effect to its influence upon oxidation, we may infer that sleep also retards the oxidation of the cell contents. If this be true, then it is probable that inhalation anesthetics exert their peculiar influence upon that portion of the brain through which sleep itself is produced. If nitrous oxid anesthesia and sleep are chemically identical, then we have a further clue to one of the primary mechanisms of life itself; and as a practical corollary one might be able to produce artificial sleep which would closely resemble normal sleep, but which would have this advantage, that by using an anesthetic which interferes with oxidation the brain-cells might be reconstructed after physical fatigue, after emotional strain, or after the depression of disease. In the case of the rabbit in which nitrous oxid was substituted for sleep, the appearance of the brain-cells resembled that in but one other group experimentally examined--the brain-cells of hibernating woodchucks.