Wear and Tear - Silas Weir Mitchell

Each organ is to every other as an excreting organ. In other words, to insure perfect health, every tissue, bone, nerve, tendon, or muscle should take from the blood certain materials and return to it certain others. To do this every organ must or ought to have its period of activity and of rest, so as to keep the vital fluid in a proper state to nourish every other part. This process in perfect health is a system of mutual assurance, and is probably essential to a condition of entire vigor of both mind and body. (tell that to Charlotte Gilman!)

The Infant System - Samuel Wilderspin

Arguments to show the importance of exercise, might be drawn from every part of the animal economy. Without it, the circulation of the blood cannot be properly carried on, nor the different secretions duly performed; neither can the fluids be properly prepared, nor the solids rendered firm or strong. The action of the heart, the motion of the lungs, and all the vital functions, are greatly assisted by exercise. But to point out the manner in which these effects are produced, would lead us beyond the present subject.

An Iron Will - Orison Swett Marden

To prosper you must improve your brain power; and nothing helps the brain more than a healthy body. The race of to-day is only to be won by those who will study to keep their bodies in such good condition that their minds are able and ready to sustain that high pressure on memory and mind, which our present fierce competition engenders. It is health rather than strength that is now wanted. Health is essentially the requirement of our time to enable us to succeed in life. In all modern occupations—from the nursery to the school, from the school to the shop or world beyond—the brain and nerve strain go on, continuous, augmenting, and intensifying.