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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).

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).

Fig. 41.—Two Muscle Fibers of Heart.

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.