=Gross Structure of Voluntary Muscles.=--A beefsteak is seen to be chiefly red, although parts of it are white or yellowish. The white or yellowish flesh is fat; the red, lean flesh is voluntary muscle. If a piece of beef is thoroughly boiled, it may be easily separated into _bundles the size of large cords_. These bundles may, by the use of needles, be picked apart and separated into _threadlike fibers_ (Fig. 40). [Illustration: FIG. 40.--MUSCLE BUNDLES bound together by connective tissue sheaths.] =Microscopic Structure of Muscles.=--These threadlike fibers may, under a magnifying glass, be separated into _fine strands called fibrils. These last are the true muscle cells_; they are shown by the microscope to be crossed by many dark lines (Fig. 48). Hence _voluntary muscles are called striated or striped muscles_. Prolonged boiling and patient picking with a needle are needed to dissect muscle, because the bundles are held together by thin, glistening sheets of connective tissue by which they are surrounded. This connective tissue surrounds and holds in place the separate fibers of each bundle (Fig. 40). [Illustration: FIG. 41.--TWO MUSCLE FIBERS OF HEART.] [Illustration: FIG. 42.--INVOLUNTARY MUSCLE CELLS (or fibers).] The fibrils of involuntary muscles are _spindle-shaped_ (see Fig. 42). There are no cross lines on the fibrils; hence involuntary muscles are called _smooth or unstriped muscles_. 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. [Illustration: FIG. 43.--(For blackboard.) BICEPS relaxed and contracted.]