The involuntary muscles respond to irritation much less rapidly than do the voluntary.  The wave of contraction passes over them more slowly and more irregularly, one part contracting while another is relaxing.  This may readily be seen in the muscular action of the intestines, called vermicular motion.  It is the irregular and excessive contraction of the muscular walls of the bowels that produces the cramp-like pains of colic.

The smooth muscles are found in the tissues of the heart, lungs, blood-vessels, stomach, and intestines.  In the stomach their contraction produces the motion by which the food is churned about; in the arteries and veins they help supply the force by which the blood is driven along, and in the intestines that by which the partly digested food is mainly kept in motion.

Thus all the great vital functions are carried 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]