Muscles, however, will respond to other than this usual stimulus.  Thus an electrical current may have a similar effect.  Heat, also, may produce muscular contraction.  Mechanical means, such as a sharp blow or pinching, may irritate a muscle and cause it to contract.

We must remember that this property of contraction is inherent and belongs to the muscle itself.  This power of contraction is often independent of the brain.  Thus, on pricking the heart of a fish an hour after removal from its body, obvious contraction will occur.  In this case it is not the nerve force from the brain that supplies the energy for contraction.  The power of contraction is inherent in the muscle substance, and the stimulus by irritating the nerve ganglia of the heart simply affords the opportunity for its exercise.

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 left forearm on a table; grasp with the right hand the mass of flesh on the front of the upper arm.  Now gradually raise the forearm, keeping the elbow on the table.  Note that the muscle thickens as the hand rises.  This illustrates the contraction of the biceps, and is popularly called “trying your muscle” Reverse the act.  Keep the elbow in position, bring the forearm slowly to the table, and the biceps appears to become softer and smaller,—­it relaxes.