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