To realize more definitely the work which the skin does, consider the fact that a square inch of skin is calculated to contain twenty-eight thousand pores. These pores, if healthy, are at all times purifying the blood by insensible perspiration, and in times of vigorous exercise make that perspiration very sensible. This sensible perspiration is essential to health, for the pores must occasionally be opened wide and flushed, in order to cleanse them thoroughly. Not only is this action of the skin in exercise increased by the increased flow of blood to the surface, but also by the mere motion of the muscles under the skin. This last effect might be called the direct effect of exercise on the skin. How close this connection is between the skin and muscles may be seen from the fact that "any part of the skin of the hand is in connection with, perhaps, two hundred muscles." This "fact, showing the exceedingly numerous and complicated communications between a given portion of the skin and the moving organs,"[1] makes it easy to conceive how the skin is stimulated to action directly by exercise.
Bodily exercise is essential to the healthy action of the brain and the nerves. There is no real conflict between brain-work and bodywork. Brain presupposes body; can not exist without it; is dependent upon it for support and nourishment. Brain can not communicate with the external world, nor with other brains, so far as we know, except through the medium of the body. Consider how brain-power is formed and grows in a child. Is not the first exertion of mental power, as well as the first sign of life, connected with motion? Back of the child's outstretched hand there is in the mind a desire for something and a will to obtain it. Each consciously directed muscular action has two effects, one on the muscle used, another on the directing brain. Can there be any doubt that this mutual action of brain and body contributes to the growth of each? Can there be any further doubt that, the more organs which the brain supervises, and the more muscles which it controls and directs, the more opportunity the brain has for growth? "Brain is evolved from the organization."[2] First, there is "growth, the force for which was supplied from a hundred sources"; and, secondly, there is "a power grown. . . . No perfect brain ever crowns an imperfectly developed body." This, then, up to a certain time of life, is Nature's method of forming brain-power, viz., by the conscious activity of the bodily powers. The fact that most people are right-sided as well as right-handed is registered in their brains; the left side of the brain, which supervises the right side of the body, being generally the larger.
In this growth of the brain, the whole nervous system is involved. The spinal cord (almost a continuation of the brain), and every nerve, which from each organ brings intelligence of want, and every nerve that flashes the order to supply that want, all are brought into action
