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 The heart fibers are exceptional; they are the only involuntary muscle fibers that are striped (Fig. 41).

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.

—(For blackboard.) 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