e, 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.

Muscular strength provides the joints with more powerful ligaments and
better developed bony parts. After long confinement to the bed from
disease, the joints have wasted ligaments, thin cartilages, and the bones
are of smaller proportions. Duly exercised muscles influence the size of
the bones upon which they act. Thus the bones of a well-developed man are
stronger, firmer, and larger than those of a feeble person.

He who has been physically well trained, has both a more complete and a
more intelligent use of his muscles. He has acquired the art of causing
his muscles to act in concert. Movements once difficult are now carried on
with ease. The power of coordination is increased, so that a desired end
is attained with the least amount of physical force and nervous energy. In
learning to row, play baseball, ride the bicycle, or in any other
exercises, the beginner makes his movements in a stiff and awkward manner.
He will use and waste more muscular force in playing one game of ball, or
in riding a mile on his wheel, than an expert would in doing ten times the
work. He has not yet learned to balance one set of muscles against their
antagonists.

[Illustrat