There are thousands of miles of minute tubing in the human body--the arterioles, veins, capillaries and lymphatic vessels. They ramify through every portion of the body tissues, the first carrying the vitalized blood for nourishment of the parts, the second returning the impure blood, charged with the waste of the structures, the third being the intermediate stage between the first and second, while the fourth and last, the lymphatic vessels, collect the surplus nutrition and return it to the circulation. In addition the lymphatics assist in the conveyance of effete matter. Whenever disease germs are present in the system, they first manifest themselves in the lymph, but this fluid being densely populated with phagocytes (white blood corpuscles), the micro-organisms are speedily destroyed, if the body is in a healthy, vigorous condition.
In view of the vital character of the fluids, activity of motion is indispensable for the best performance of their separate functions and exercise supplies the desired stimulus. Whenever a muscle is contracted the blood is wholly or partially expelled from it proportionately to the force of the contraction, and in its escape it carries with it the waste material; but as soon as the muscle is relaxed fresh blood from the arterial supply re-enters the structure, bearing fresh nutrition.
By a wise provision of Nature, the amount of nutrition supplied is always in excess of the waste products removed; that is, all things being equal, so that the more exercise a part is subjected to the more nutrition it receives. This explains the unusual development of certain parts of the body which are called into excessive use in certain occupations. But this unsymmetrical development is a thing to be avoided, as it is usually productive of certain deformities, such as stoop shoulders and certain peculiarities of gait, which are plainly noticeable in men employed in certain avocations.
The reason for this is perfectly simple, and may be expressed in two words--unequal nutrition--for the muscles that are unduly exercised appropriate the nutriment that should be equally distributed, so that the neglected muscles become weakened and stiff. Hence, any system of exercises designated to develop the body should be so arranged as to call into play every muscle in the individual, thus insuring harmonious development in every direction.