The heart fibers are exceptional; they are the only involuntary muscle fibers that are striped (Fig. 41).
Thought Questions. Classification of Some of the Muscles.—Copy
the following list and mark I for involuntary and V for voluntary
after the appropriate muscles.
Muscles for chewing. Muscles of gullet. Muscles of the heart. Muscles that move arms. Muscles for breathing. Muscles in the skin that cause the hair to stand on end. Muscles that move eyelids. Muscles that contract pupil of eye. Muscles for talking. Muscles that contract and expand the arteries (in blushing and turning pale). Muscles that move eyeball. Muscles that give expression to the face.
Tendons.—The connective tissue which binds the fibers of muscles into bundles, and forms sheaths for the bundles, extends beyond the ends of the muscles and unites to form tough, inelastic white cords called tendons. Some muscles
are without tendons, and are attached directly to bones.
Study the figures and find examples of this (see Figs.
44, 75). To realize the toughness of tendons, feel the
tendons under the bent knee or elbow, where they feel
almost as hard as wires. The tendons save space in places
where there is not room enough
for the muscles, and permit the
bulky muscles to be located where
they are out of the way. Wherever
the tendons would rise out of
position when a joint is bent, as
at the wrist and ankle, they are
bound down by a ligament.
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Fig. 43.—(For blackboard.) Biceps relaxed and contracted.
Arrangement of Voluntary Muscles.—Circular muscles, called sphincter muscles, are found around the mouth and eyes. Muscles that extend straight along the limb either bend it and are called flexors, or straighten it and are called extensors. Most of
