ed on, regardless of the will
of the individual, or of any outward circumstances. If it required an
effort of the will to control the action of the internal organs we could
not think of anything else. It would take all our time to attend to
living. Hence the care of such delicate and important machinery has wisely
been put beyond our control.

Thus, too, these muscles act instinctively without training; but the
voluntary need long and careful education. A babe can use the muscles of
swallowing on the first day of its life as well as it ever can. But as it
grows up, long and patient education of its voluntary muscles is needed to
achieve walking, writing, use of musical instruments, and many other acts
of daily life.

[Illustration: Fig. 32.--A Spindle Cell of Involuntary Muscle. (Highly
magnified.)]

Experiment 18. _To show the general appearance of the muscles._
Obtain the lower part of a sheep's or calf's leg, with the most of the
lean meat and the hoof left on. One or more of the muscles with their
bundles of fibers, fascia, and tendons; are readily made out with a
little careful dissection. The dissection should be made a few days
before it is wanted and the parts allowed to harden somewhat in dilute
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 pow