alcohol.

68. Properties of Muscular Tissue. The peculiar property of living
muscular tissue is irritability, or the capacity of responding to a
stimulus. When a muscle is irritated it responds by contracting. By this
act the muscle does not diminish its bulk to any extent; it simply changes
its form. The ends of the muscle are drawn nearer each other and the
middle is thicker.

Muscles do not shorten themselves all at once, but the contraction passes
quickly over them in the form of a wave. They are usually stimulated by
nervous action. The delicate nerve fibrils which end in the fibers
communicate with the brain, the center of the will power. Hence, when the
brain commands, a nervous impulse, sent along the nerve fibers, becomes
the exciting stimulus which acts upon the muscles and makes them shorter,
harder, and more rigid.[10]

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.