Various theories have at one time or another been suggested to account for sleep. Some are both bewildering and absurd. There was a time when it was seriously urged that sleep has in the thyroid gland its special organ, but when someone in the interest of the theory excised the thyroid gland, only to increase in certain instances the tendency to go to sleep and stay asleep, the theory was at once abandoned even by its staunchest advocates.

Finding that sleep usually follows fatigue, and that fatigue is a chemical phenomenon, the so-called chemical theory was next set up, and Sommer was quite sure that sleep comes as a consequence of the exhaustion of the reserve of oxygen in the tissues and the blood, and its replacement by carbonic acid during sleep. But here, too, experimentation has been both inadequate and inconclusive.

The vaso-motor theory, as modified by Howell, that sleep is due to the anÆmia of the cortical layer of the brain, which invariably takes place when the blood pressure in the arteries at the base of the brain falls, has had a larger and a longer following. But convincing proof is yet to be secured, and Dr. Percy G. Stiles of the Bellevue Hospital ends his discussion of the subject with a guarded inference that there may be truth in both the theories, and that eclecticism is in consequence the wisest policy for the histologist.1