Contraction is not, however, the natural state of a muscle. In time it is
tired, and begins to relax. Even the heart, the hardest-working muscle,
has short periods of rest between its beats. Muscles are highly elastic as
well as contractile. By this property muscle yields to a stretching force,
and returns to its original length if the stretching has not been
excessive.

[Illustration: Fig. 33.--Principal Muscles of the Body. (Anterior view.)]

69. The Object of Contraction. The object of contraction is obvious.
Like rubber bands, if one end of a muscle be fixed and the other attached
to some object which is free to move, the contraction of the muscle will
bring the movable body nearer to the fixed point. A weight fastened to the
free end of a muscle may be lifted when the muscle contracts. Thus by
their contraction muscles are able to do their work. They even
contract more vigorously when resistance is opposed to them than when it
is not. With increased weight there is an increased amount of work to be
done. The greater resistance calls forth a greater action of the muscle.
This is true up to a certain point, but when the limit has been passed,
the muscle quickly fails to respond.

Again, muscles work best with a certain degree of rapidity provided the
irritations do not follow each other too rapidly. If, however, the
contractions are too rapid, the muscles become exhausted and fatigue
results. When the feeling of fatigue passes away with rest, the muscle
recovers its power. While we are resting, the blood is pouring in fresh
supplies of building material.

Experiment 19. _To show how muscles relax and contract_. Lay your