also seen marked cross-wise with dark
stripes, and can be separated at each stripe into disks. These cross
markings account for the name _striped_ or _striated_ muscle.

The fibrillae, then, are bound together in a bundle to form a fiber, which
is enveloped in its own sheath, the sarcolemma. These fibers, in turn, are
further bound together to form larger bundles called fasciculi, and
these, too, are enclosed in a sheath of connective tissue. The muscle
itself is made up of a number of these fasciculi bound together by a
denser layer of connective tissue.

Experiment 17. _To show the gross structure of muscle._ Take a
small portion of a large muscle, as a strip of lean corned beef. Have it
boiled until its fibers can be easily separated. Pick the bundles and
fasciculi apart until the fibers are so fine as to be almost invisible
to the naked eye. Continue the experiment with the help of a hand
magnifying glass or a microscope.

67. The Involuntary Muscles. These muscles consist of ribbon-shaped
bands which surround hollow fleshy tubes or cavities. We might compare
them to India rubber rings on rolls of paper. As they are never attached
to bony levers, they have no need of tendons.

[Illustration: Fig. 31.--A, Muscular Fiber, showing Stripes, and Nuclei, b
and c. (Highly magnified.)]

The microscope shows these muscles to consist not of fibers, but of long
spindle-shaped cells, united to form sheets or bands. They have no
sarcolemma, stripes, or cross markings like those of the voluntary
muscles. Hence their name of _non-striated_, or _unstriped_, and _smooth_
muscles.

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 carri