The regular work of the body in keeping up the heart action and the circulation and in renewing and relieving waste, requires a certain quantity of oxygen to liberate energy. This energy the system, in normal condition, with normal breathing, readily furnishes, but when that used in undue muscular work is more than that being liberated at the time, through combustion, the energy required for the constant bodily needs is called upon, and the muscles, nerves and tissues are then in the state termed “tired.” They remain so until sufficient oxygen has liberated more potential energy than is needed for the work constantly going on in the body. When a sufficient supply of oxygen has been consumed to equal the demand, the body is in a state of rest. 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 oxygen equals the demand in all parts of the body. A torpid condition of body, producing inertness, means that the waste of the system is not relieved. It may be that by reason of insufficient breathing of pure air, sufficient oxygen is not consumed to put the waste in condition to be eliminated. This poisonous carbon dioxid being hoarded, dulls the nerve sensation and the brain action and produces more or less of stupor. It may be because the circulation in some part of the body is clogged (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 in the fresh air, that the circulation may be quickened and the oxygen more freely carried to each part. 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 pores of the skin. Fifteen minutes of brisk exercise in one’s room is better than a five-mile walk, because if the exercises be intelligently selected, every organ and tissue is used, while walking exercises only about one-fourth of the muscles. If the circulation is clogged, the exercising must be kept up persistently, until the obstruction is removed and particular attention must be given to the supply of fresh air in the room. After sleeping in a room with impure air, one arises fatigued, because of insufficient oxygen to liberate the energy required for circulation and catabolism, and because the carbonic acid gas cannot be relieved without oxygen to cause combustion. As stated above, if the poisonous carbonic acid gas remains in the system, it deadens the nerve sensation and produces a semi-stupor.