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