(1) The duration of life after excision of the liver is about the same as after adrenalectomy--approximately eighteen hours. (2) The amount of glycogen in the liver was diminished in all the experiments showing brain-adrenal activity; and when the histologic changes were repaired, the normal amount of glycogen was again found. (3) In crossed circulation experiments changes were found in the liver of the animal whose brain received the stimulus. From these premises we must consider that the brain, the adrenals, and the liver are mutually dependent on one another for the conversion of latent into kinetic energy. Each is a vital organ, each equally vital. It may be said that excision of the brain may apparently cause death in less time than excision of the liver or adrenals, but this statement must be modified by our definition of death. If all the brain of an animal be removed by decapitation, its body may live on for at least eleven hours if its circulation be maintained by transfusion. An animal may live for weeks or months after excision of the cerebral hemispheres and the cerebellum, while an overtransfused animal may live many hours, days even, after the destruction of the medulla. It is possible even that the brain actually is a less vital organ than either the adrenals or the liver. In our research to discover whether any other organs should be included with the brain, the adrenals, and the liver in this mutually interdependent relation, we hit upon an experiment which throws light upon this problem. Groups of rabbits were gently kept awake for one hundred hours by relays of students, an experiment which steadily withdrew energy but caused not the slightest physical or emotional injury to any of them; no drug, toxin, or other agent was given to them; they were given sufficient food and drink. In brief, the internal and external environments of these animals were kept otherwise normal excepting for the gentle stimuli which insured continued wakefulness. This protracted insomnia gradually exhausted the animals completely, some to the point of death even. Some of the survivors were killed immediately after the expiration of one hundred hours of wakefulness, others after varying intervals. Histologic studies were made of every tissue and organ in the body. Three organs, the brain, the adrenals, and the liver, and these three only, showed histologic changes. In these three organs the histologic changes were marked, and were almost wholly repaired by one seance of sleep. In each instance these histologic changes were identical with those seen after physical exertion, emotions, toxins, etc.[*] It would appear, then, that these three organs take the stress of life-- the brain is the "battery," the adrenals the "oxydizer," and the liver the "gasoline tank." This clear-cut insomnia experiment corresponds precisely with our other brain-adrenal observations.