on the
contrary, the muscle that is little used decreases in size and power.

_Illustrations._ 1st. The blacksmith uses and rests the muscles of his
arm when striking upon the anvil. They not only increase in size, but
become very firm and hard.

2d. The student uses the muscles of the arm but little, in holding his
books and pen; they not only become small, but soft.

3d. Let the student leave his books, and wield an iron sledge, and the
muscles of his arm will increase in size and firmness. On the other
hand, let the blacksmith assume the student's vocation, and the
muscles of his arm will become soft and less firm.

174. _When the muscles are called into action, the flow of blood in
the arteries and veins is increased._ The increased flow of blood in
the arteries and veins, causes a more rapid deposition of the
particles of matter of which the muscles are composed. If the
exercise is adequate to the power of the system, the deposit of new
material will exceed in quantity the particles of matter removed, and
both the size and energy of the muscles are increased. But there is a
limit to the muscles becoming strong by labor. Sooner or later, man
will attain his growth or power; yet by judicious exercise, care, and
discreet management, the greatest power of the muscles may be
preserved until advanced age.

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173-211. _Give the hygiene of the muscles._ 173. What is necessary
that muscles may attain size and strength? Give a law of the muscular
system. Show this by practical illustrations. 174. Why do muscles
increase in size when exercised?

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175. _The muscles are lessened in size and diminished in power when
the exercise is continued so as to produce a feeling of exhaustion._
The loss of material, in this instance, will exceed the deposition of
the atoms of matter. This is seen in the attenuated frames of
over-tasked domestic animals, as the horse. The same truth is
illustrated by the laborious agriculturist, who, in consequence of too
severe toil while gathering the products of the field, frequently
diminishes his weight several pounds in a few weeks. Exercise, either
for pleasure or profit, may fatigue, yet it should never be protracted
to languor or exhaustion, if the individual desires "a green old
age."

176. _The same amount of exercise will not conduce to the health of
all individuals._ If riding or walking one mile causes slight fatigue,
this may b