Brief reference will be made in this chapter only to such natural and systematic physical training as should enter into the life of every healthy person.

81.  Muscular Activity.  The body, as we have learned, is built up of certain elementary tissues which are combined to make bones, muscles, nerves, and other structures.  The tissues, in turn, are made up of countless minute cells, each of which has its birth, lives its brief moment to do its work in the animal economy, is separated from the tissue of which it was a part, and is in due time eliminated by the organs of excretion,—­the lungs, the skin, or the kidneys.  Thus there is a continuous process of growth, of decay, and removal, among the individual cells of each tissue.

[NOTE.  The Incessant Changes in Muscular Tissue.  “In every tiny block of muscle there is a part which is really alive, there are parts which are becoming alive, there are parts which have been alive, and are now dying or dead; there is an upward rush from the lifeless to the living, a downward rush from the living to the dead.  This is always going on, whether the muscle be quiet and at rest, or whether it be active and moving,—­some of the capital of living material is being spent, changed into dead waste; some of the new food is always being raised into living capital.  But when the muscle is called upon to do work, when it is put into movement, the expenditure is quickened, there is a run upon the living capital, the greater, the more urgent the call for action.”—­Professor Michael Foster.]

These ceaseless processes are greatly modified by the activity of the bodily functions.  Every movement of a muscle, for instance, involves change in its component cells.  And since the loss of every atom of the body is in direct relation to its activity, a second process is necessary to repair this constant waste; else the body would rapidly diminish in size and strength, and life itself would soon end.  This process of repair is accomplished, as we shall learn in Chapters VI. and VII., by the organs of nutrition, which convert the food into blood.

[Illustration:  Fig. 39.—­Showing how the Muscles of the Back may be developed by a Moderate Amount of Dumb-Bell Exercise at Home. (From a photograph.)]

82.  Effect of Exercise upon the Muscles.  Systematic exercise influences the growth and structure of the muscles of the body in a manner somewhat remarkable.  Muscular exercise makes muscular tissue; from the lack of it, muscles become soft and wasted.  Muscles properly exercised not only increase in size, both as a whole and in their individual structure, but are better enabled to get rid of material which tends to hamper their movements.  Thus muscular exercise helps to remove any needless accumulation of fat, as well as useless waste matters, which may exist in the tissues.  As fat forms no permanent structural part of the organism, its removal is, within limits, effected with no inconvenience.