become harder, and respond more readily to the commands of the will; but if the exercise is too prolonged, or excessive, the opposite effect is produced, and they begin to soften and waste. Deficiency of exercise is apt to lead to weakening of the heart's action, from a change of the muscular structure into fat (fatty degeneration).
It has been calculated that, for an individual weighing 150 lb., and in good health, the daily amount of exercise ought to be equal to that put forth in walking 8 miles on level ground. For females of average strength, a smaller amount of outdoor exercise, equivalent to 3 or 4 miles daily, would generally be appropriate. The modern girl, however, is almost as hardy and athletic as her brother, and can safely exceed this amount.
Exercise in Childhood.—The amount of exercise in childhood and youth should be carefully regulated. It is important not to restrict too much the movements of infants, and care must be taken that their clothing is not too tight to allow ample freedom of the limbs.
During childhood and youth, effort should be made to exercise every important muscle in the body, each in its turn, so as to secure for all the muscles a complete and symmetrical development, and consequently robust health.
Physical exercise, as well as mental exertion, should be regulated with especial care in young girls about the epoch of puberty, since the changes of the constitution at that critical period often render ordinary rules and habits useless or even injurious.
In advanced life the power, as well as the inclination for active exertion, alike fail, but moderate exercise should be encouraged as long as possible.
Sleep.—Sleep is intended to repair the expenditure of power in the system consequent upon mental or bodily fatigue, and its duration should therefore be proportional to the loss of vigour actually met with during the preceding period of daily activity. In early infancy, the active processes of growth and development going on in the budding organism require a correspondingly greater amount of repair, which is largely contributed to by frequent slumbers, occupying a majority of the twenty-four hours.
The necessity for sleep, which is quite imperative in the young child, becomes gradually less and less pressing until, after the age of 2 or 3 years is reached, repose during the night only is required. In the prime of manhood or womanhood, the proper period is more readily determined, and is much shorter than that suited to infancy. In advanced life the expenditure of physical and mental power is smaller, and less need of prolonged repose is felt by the system; although in extreme old age, or second childhood, the body often reverts to its infantile habits of frequent slumber.
Importance of the Habit of Sleep.—Habit, which so powerfully