The central nervous system is divided into three layers or levels of successively later function. First and lowest come the spinal cord, medulla and pons, which mediate the reflex and spontaneous movements of infancy, and control the vital organs of the body. Then come the sensory and motor areas of the brain, which mediate controlled movements. These at adolescence develop a greatly increased power of innervation of the augmented mass of muscular tissue, resulting in a remarkable increase of strength. Lifting power increases fastest at fifteen, sixteen, and seventeen, biceps power at fifteen or sixteen, wrist power from fourteen to seventeen, and particularly leg power from thirteen to fifteen and at eighteen. Adolescents of both sexes nearly double their eleven-year-old grip by sixteen. This is evidently the golden period for the wise and judicious physical trainer. An English observer found that systematic gymnastics, which, if applied at the right age, produce immediate and often surprising development of lung capacity, utterly fail with boys of twelve, because this nascent period has not yet come. While strength and energy are increasing so rapidly, accuracy of movement, on the contrary, increases very little during the early teens. Disproportionate, inharmonious, saltatory growth would seem to be the order of nature. The energy of life flows now in one direction, now in another, so that strength and skill, structure and function, would seem to have a complementary or alternative development. That kind of education, therefore, that calls for motor control and precision in early adolescence, can only result in precocity and arrested development. At this period, on the contrary, it is the larger, more basal muscles that call for exercise and development; which nature sufficiently indicates by the sports and occupations to which, when not overburdened with lessons, youth naturally turns, namely, swimming, rowing, sailing, skating, hunting, hill-climbing, dancing (though its present form, unhappily, is degenerate), games of contest and rivalry, and the making of useful things. The plays and occupations of adolescence, unlike those of boyhood and girlhood, are mainly social and co-operative, and, in the normal, develop manly and womanly virtues, as well as that muscle and will power that is of such fundamental importance for all future growth and development.
The third and highest brain-level consists of the higher centres of the cortex front and back, which have no direct connection with the lower centres of sense and movement,
