It is of the utmost importance that young children especially have all the sleep they need, and that is precisely all that they can be induced to take. The best rule for you, then, to follow, is to go to bed when you feel sleepy, and to get up when you wake rested. Every child under twelve should have at least ten hours of sleep, and every grown person eight, or better still, nine hours. Time spent in sound, refreshing sleep, is time well spent. If you cannot sleep well, it is a signal that something is wrong with your health, or your habits--a danger signal of great importance, which should be attended to at once. The best and only safe sleep-producer is exercise in the open air. DISORDERS OF MUSCLES AND BONES The Muscles and Bones Have Few Diseases. Considering how complex it is, and the never-ceasing strain upon it, this moving apparatus of ours, the nerve-bone-muscle-machine, is surprisingly free from disease. The muscles, though they form nearly half our bulk, have scarcely a single disease peculiar to them, or chiefly beginning in them, unless fatigue and its consequences might be so regarded. They may become weakened and wasted by either lack or excess of exercise, by under-feeding, or by loss of sleep; but most of their disturbances are due to poisons which have got into the blood pumped through them, or to paralysis or other injuries to the nerves that supply them. The muscles of an arm, for instance, which has been lashed to a splint, or shut tightly in a cast for a long time, waste away and shrink until the arm becomes, as we say, "just skin and bone"; and the same thing will happen if the nerve supplying a muscle, or a limb, is cut or paralyzed. The bones have more diseases than the muscles, but really comparatively few, considering their great number and size, and the constant strain to which they are subjected in supporting the body, and driving it forward and doing its work under the handling and leverage of the muscles. Most of their diseases are, like those of the muscles, the after-effects of general diseases, particularly the infections and fevers, which begin elsewhere in the body; and the best treatment of such bone diseases is the cure and removal of the disease that caused them. [Illustration: CALLUS FORMED AROUND A FRACTURE An aluminum splint holds the parts of the bone together.]