body. [Footnote: If a small ivory ball be allowed to roll down the cheek
toward the lips, it will appear to increase in weight. In general, the
more sensitive parts of the body recognize smaller differences in weight,
and the right hand is more accurate than the left. We are very apt,
however, to judge of the weight of a body from previous conceptions. Thus,
shortly after Sir Humphrey Davy discovered the metal potassium, he placed
a piece of it in the hand of Dr. Pierson, who exclaimed: "Bless me! How
heavy it is!" Really, however, potassium is so light that it will float on
water like cork.] By balancing it in the hand. The muscular sense is
useful to us in many ways. It guides us in standing or moving. We gratify
it when we walk erect and with an elastic step, and by dancing, jumping,
skating, and gymnastic exercises.

NECESSITY OF EXERCISE.--The effect of exercise upon a muscle is very
marked. [Footnote: The greater size of the breast (pectoral muscle) of a
pigeon, as compared with that of a duck, shows how muscle increases with
use. The breast of a chicken is white because it is not used for flight,
and therefore gets little blood.] By use it grows larger, and becomes
hard, compact, and darker-colored; by disuse it decreases in size, and
becomes soft, flabby, and pale.

Violent exercise, however, is injurious, since we then tear down faster
than nature can build up. Feats of strength are not only hurtful, but
dangerous. Often the muscles are strained or ruptured, and blood vessels
burst in the effort to outdo one's companions. [Footnote: Instances have
been known of children falling dead from having carried to excess so
pleasant and healthful an amusement as jumping the rope, and of persons
rupturing the Tendon of Achilles in dancing. The competitive lifting of
heavy weights is unwise, sometimes fatal.] (See p. 278.)