ervous system.
The nervous system supplies a stimulus called the _nervous impulse_, which
reaches the muscles by the nerves, causing them to contract. By means of
nervous impulses, all of the muscles (both voluntary and involuntary) are
made to contract as the needs of the body for motion require.

*Energy Transformation in the Muscle.*--The muscle serves as a kind of
engine, doing work by the transformation of potential into kinetic energy.
Evidences of this are found in the changes that accompany contraction.
Careful study shows that during any period of contraction oxygen and food
materials are consumed, waste products, such as carbon dioxide, are
produced, and heat is liberated. Furthermore, the _blood supply to the
muscle_ is such that the materials for providing energy may be carried
rapidly to it and the products of oxidation as rapidly removed. Blood
vessels penetrate the muscles in all directions and the capillaries lie
very near the individual cells (Fig. 113). Provision is made also, through
the nervous system, for _increasing_ the blood supply when the muscle is
at work. From these facts, as well as from the great force with which the
muscle contracts, one must conclude that the muscle is a _transformer of
energy_--that within its protoplasm, chemical changes take place whereby
the potential energy of oxygen and food is converted into the kinetic
energy of motion.

[Fig. 113]

Fig. 113--*Capillaries* of muscles.

*Plan of Using Muscular Force.*--Two difficulties have to be overcome in
the using of muscular force in the body. The first of these is due to the
fact that the muscles exert their force _only when they contract_. They
can pull but not push. Hence, in order to bring about the opposing
movements(85) of the body, each muscle must work against some force that
produces a result directly opposite to that which the muscle produces.
Some of the muscles (those of breathing) work against the elasticity of
certain parts of the body; others (those that hold the body in an upright
position), to some extent against gravity; and others (the non-striated
muscle in arteries), against pressure. But in most cases, _muscles work
against muscles_.

[Fig. 114]

Fig. 114--*The muscle pair* that operates the forearm. For names of these
muscles, see Fig. 119.

The striated, or skeletal, muscles are