A muscle is an organ made up of fibers held together by connective tissue and surrounded by a sheath of heavier connective tissue. The fibers are grouped parallel to each other in bundles called fasciculi. See Fig. 20. Even the thinner sheets of connective tissue entering the bundle are shown in some portions of the photograph. The connective tissue entering the fasciculi is the endomysium. The size of the bundles or fasciculi varies in different muscles and determines to a certain extent the grain of the meat.

The connective tissue surrounding the fasciculi, the perimysium, varies in thickness, being quite perceptible to the eye in some muscles, as in the outer muscle of prime ribs shown in Fig. 20. In some muscles it is scarcely discernible. The connective tissue enclosing each muscle is known as the epimysium or muscle sheath. The thicker, denser portions of the muscle sheath are often little changed during cooking and constitute tough spots in the meat.

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Fig. 20. - This photograph shows the bundles of muscle fibers, the fasciculi, and the surrounding connective tissue, the perimysium. Even the connective tissue entering the bundles of muscle fibers is shown in many places. This muscle is the narrow muscle along the outer edge of a beef roast. The photograph was taken after the meat was cooked. The muscle is pulled apart and pinned back. Enlarged about three times.

The spaces between the muscles, the fasciculi, or the cells are referred to as intermuscular, interfascicular, or intercellular; the area within the muscle, the fasciculus, or cell is called intramuscular, intrafascicular, or intracellular.

Classes of muscles. Muscles are divided into two groups, the skeletal, voluntary, or cross-striated and the smooth, plain, or long-striated. The smooth muscles occur in the walls of the hollow viscera, i.e., the intestinal tract, the arteries, veins, etc. Heart muscle is cross-striated but in many of its characteristics it falls between the two groups, voluntary and involuntary. The skeletal and heart muscles compose the muscle designated as flesh by the Food and Drug Administration.

Some muscles are exercised more than others. The muscles in the body of an animal are not exercised or used equally. Thus, depending upon the extent to which the muscles have been used, "tender" and "less-tender" cuts are obtained from the same animal. In general the less-tender cuts are obtained from the neck and legs, the tender cuts along the back.