Page:Popular Science Monthly Volume 29.djvu/346

332 by exercise, and all are nourished by the circulating blood. Think of the immense strain upon the bodily powers to keep the brain and nervous system properly nourished! It is calculated that the brain alone requires one fifth of the entire supply of blood in the body. The drain upon the bodily vigor of a brain-worker would be greater than this fraction represents, if it were not for the law of Treviranus, according to which an organ not only takes from the blood certain materials, but also supplies to it other materials. "Just as, on a larger scale, the carbonic acid exhaled by animals is taken up by vegetables, and a poison thus removed from the atmosphere in which the animal lives, so by one organic element of the body the blood is purified from the waste matter of a higher element, which would be poisonous to it." So that a tired brain and quivering nerves may not be more wearied by physical exercise, but may be refreshed by it. This refreshment may result from two processes: first, by withdrawing the excessive supply of blood from the before active organ; and, secondly, by purifying the blood so that it may be ready to properly nourish the brain. And the muscular system not only acts as a store-house of vitality for the brain, and a purifier of its supply of blood, but it covers the nervous system, acting as its stay and protection. "To be weak is to be miserable. . . . Susceptibility of nerve and feebleness of muscle generally go together." To correct one deficiency is usually to cure the other weakness.

To the young, physical exercise is essential to growth, both of body and mind. Youth is not only the time to cultivate good habits, but also the time to store up vitality. At that time many abnormal developments can be corrected by appropriate exercises. At that period, too, the healthy balance between brain and body can better be established. To children, exercise is specially needful for healthy nerves, since, as compared with the nervous system of an adult, the nervous system of a child is five times as large, in proportion to the size of the body. In them, therefore, "that parasite of the blood," the brain, demands that a greater amount of time should be given to waste and repair of tissue by means of exercise, and that a greater amount of proper food should furnish the supply of nourishment. Short intervals of study, long intervals of play or light work of body, and that in the open air, if possible, should be the rule for children. As they increase in years more time can be given to conscious cerebration. At some periods of growth, all the way from the age of twelve to eighteen, according to the individual, special watchfulness is required of parents and instructors to see that the functions of growing organs are not interfered with by excessive attention to brain-work. At this critical time no study would be safer than too much study.

After a good muscular system has been developed in childhood and youth, a comparatively small amount of time judiciously devoted to