Many diseases are favoured by deficient exercise, and can be averted by systematic exercises and the concomitant increased supply of pure air. It is often difficult to appraise the relative merits of exercise and pure air; but there can be no doubt that both are of extreme importance. The prevention of consumption, even in those with a strong hereditary tendency, is greatly helped by systematic exercises, especially those directed to the expansion of the chest cavity. In cases of consumption there is commonly a history of deficient exercise or an occupation involving a cramped position, as well as of living in an impure air. Various deformities are induced by defective exercise of particular groups of muscles. Thus drooping shoulders may be caused by shoulder-straps confining the action of the shoulder-muscles in the earlier years of life. Stooping is favoured by sitting in cramped positions in school, and by the use of desks not inclined at the proper angle. Lateral curvature of the spine is due to weakness of the muscles of the back, and is best treated in its earlier stages by gymnastic exercises specially directed to strengthening these muscles. The tendency to such curvatures is greatly increased in girls by the fact that their trunks are imprisoned in corsets as if in splints, and so exercise of the trunk muscles is reduced to a minimum. =Rules respecting Exercise.= 1. _The clothing during exercise should not be excessive_, and should not interfere with the free play of the limbs, nor with full expansion of the chest. Flannel is the best material to wear next the skin, as it absorbs perspiration without becoming non-porous. 2. _Avoid chill after exercise._ It is well, if there has been any perspiration during exercise, to strip and scrub the skin, particularly about the chest and arm-pits, with a rough towel. 3. _Exercise should be systematic and regular._ It is important to avoid sudden, violent, and competitive exercise. No severe exercise ought to be undertaken without a gradual training. 4. _The amount of exercise must be regulated by individual fitness._ A chain is no stronger than its weakest link. The muscles may be stronger than the heart or lungs, and the latter may be fatally injured by an amount of exercise which the muscles can well bear. Hence the importance of ascertaining the condition of the vital organs before entering on a course of training. Another important bearing of this rule is in relation to the exercise of growing boys and girls. When we remember that a boy at school will sometimes grow six to eight inches in a year, it is evident that much energy is being expended in this direction, and that _excessive_ gymnastic exercise can only do harm. Between the ages of fifteen and seventeen there is usually the greatest amount of physical development, and if there is great muscular strain at this period, growth is interfered with, and the power of resistance to disease may be seriously lowered.