Keeping Fit All the Way- Walter Camp 

When I started the experiment of the Senior Service Corps at New Haven, in the spring of 1917, all my men were over forty-five, and several of them had passed the seventy mark; yet all found increased health and efficiency from the prescribed regime. There was a distinct gain, not only in health, but in spirits and in temper. Nerves that had been at high tension relaxed to normal. Effort that had seemed exhaustive became pleasurable. The ordinary problems of business or finance, once so apt to be vexatious, lost their power to produce worry. In fact, these men had renewed their youth; they had altered the horizon-line of advancing age, across which only clouds of doubt and apprehension could be seen, to that of youth, radiant with the sunshine of hope and the promise of accomplishment.

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.