People fail to remember that the amount of blood in the body is a fixed quantity, and if an excess of it is called to one portion, the supply is lessened to other portions. The regular work of the body in keeping up the heart action and the circulation requires a certain amount of energy produced by a certain amount of oxidized foodstuffs. The system in normal condition, with normal breathing, readily furnishes this energy. If more than the normal amount is used in increased work, greater combustion is necessary. The extra amount of waste which has been liberated by this extra work must also be carried away. If combustion does not take place, the extra energy is not supplied, and that required for the constant bodily needs is called on. If the waste is not removed from the system and the energy not resupplied to the parts doing the extra work, the muscles, nerves, and tissues are then in the state termed “tired.” They remain so until the circulation has carried the waste to the eliminating organs and has brought more foodstuffs to the tissues, thus restoring more energy than is needed for the work constantly going on in the body. It must be remembered that for combustion oxygen is required and if undue energy is necessary deep breathing is imperative. _The relief, then, from the state of the body we call fatigue is in equalizing the circulation through exercise or rest, according to the occupation, and supplying oxygen through full breathing._ This more forceful circulation calls the blood from the unduly distended capillaries, removes the waste, and brings a new supply of energy-building foodstuffs. In mental work, the nerves and the brain call for the surplus energy, while in muscular work the tissues require it, hence undue work, either mental or physical, expresses itself in bodily fatigue, until the demand in all parts of the body is equalized. When equilibrium is restored, the body is “rested.” The relief from fatigue due to mental activity is in exercise and deep breathing. Carbon dioxid dulls the nerves of sensation and the brain action and may produce more or less stupor. It may be because the circulation in some part of the body is sluggish (most often the portal circulation through the liver), so that sufficient oxygen is not carried to that part. Relief from this “inertness” is experienced most quickly by exercise to quicken the circulation and supply the oxygen. Exercise in one’s room by the open window, or at least with the air in the room pure, is often preferable to outdoor exercise, because the body can be nude, or so loosely clothed that the oxygen may not only enter the lungs but also circulate about the skin. Fifteen minutes of brisk exercise in one’s room is better than a five-mile walk, because if the exercises are intelligently selected, every organ and tissue is used, while walking exercises only about one-fourth of the muscles.