There would be no sense of exhaustion if the building and the eliminating processes could be carried on with sufficient rapidity concurrently to make good all the expenditures of mental and bodily activities. Not only should we not need rest, but we should not even need sleep. The only occasion to stop, then, would be to take in more fuel (food), and if this could be taken while the body is in action, as fuel is fed to the steam boiler, there would be no necessity to stop. But apparently both the upbuilding and the elimination of waste normally lay behind the demands of even ordinary activity, so that a given muscle must have very frequent intervals of rest (every few seconds), and the organism, as a whole, must reduce activity to the minimum by sleep about one-third of the time. [Sidenote: Nature's devices to provide rest] As some of the muscles are used with practical continuity during the waking life, Nature resorts to some very cunning devices to provide the necessary rest. The tension upon the muscle of the eye is relaxed for an instant in the unconscious act of winking, but by reason of the persistence of visual sensation, this does not interfere with vision. Thus Nature has always used the principle involved in the moving picture. The heart must perform its work every instant, from the time before we are born until the end, but each muscle rests about one-third of every second--when it relaxes, and the chamber of the heart expands with the inrush of blood. [Sidenote: The body a pile of mysterious atomic masonry] Nature alone is the builder, and will do all that should be done if she only has the proper materials in proper proportions. We may well stand in awe and admiration of her mysterious atomic masonry, but let us lay no sacrilegious hand upon her work. LESSON XI COMMON DISORDERS--THEIR CAUSE AND CURE HEALTH AND DIS-EASE DEFINED Health is that condition of the human body in which the functions or activities work together in perfect harmony. Any serious interference with this condition we call dis-ease. Dis-ease, therefore, in its final analysis, is merely the expression of violated natural law. [Sidenote: Three fundamental laws of life] The harmonious working of the life-processes in the human body depends upon three things--(1) nutrition; (2) motion and (3) oxidation. Nutrition is the principal factor that controls the action of the living cells, for, if the body is kept up to its one hundred per cent of energy it will demand a certain amount of motion or exercise, and this will enforce the proper breathing (oxidation). We can see, therefore, that nutrition is the physical basis of all activities of life. By nutrition as here used I mean to include all chemical substances that may be supplied for the use of the body-cells, also the sum total of all chemical substances in solution in the circulating fluid or blood-plasma which bathes the body-cells. [Sidenote: The phenomenon of death caused by self-poisoning]