The _Muscles_ are those organs of the body by which motion is produced, and are commonly known as _flesh_. A muscle is composed of _fascieuli_, or bundles of fibers, parallel to one another. They are soft, varying in size, of a reddish color, and inclosed in a cellular, membranous sheath. Each _fasciculus_ contains a number of small fibers, which, when subjected to a microscopic examination, are found to consist of _fibrillae_, or little fibers; each of these fibrillae in turn being invested with a delicate sheath. The fibers terminate in a glistening, white _tendon_, or hard cord, which is attached to the bone. So firmly are they united, that the bone will break before the tendon can be released. When the tendon is spread out, so as to resemble a membrane, it is called _fascia_. Being of various extent and thickness, it is distributed over the body, as a covering and protection for the more delicate parts, and aids also in motion, by firmly uniting the muscular fibers. The spaces between the muscles are frequently filled with fat, which gives roundness and beauty to the limbs. The muscles are of various forms; some are longitudinal, each extremity terminating in a tendon, which gives them a _fusiform_ or spindle-shaped appearance; others are either fan-shaped, flat, or cylindrical. [Illustration: Fig. 20. 1. A spindle-shaped muscle, with tendinous terminations. 2. Fan-shaped muscle. 3. Penniform muscle. 4. Bipenniform muscle.] [Illustration: Fig. 21. Striped muscular fibre showing cleavage in opposite directions. 1. Longitudinal cleavage. 2. Transverse cleavage. 3. Transverse section of disc. 4. Disc nearly detached. 5. Detached disc, showing the sarcous elements. 6. Fibrillæ. 7,8. Separated fibrillae highly magnified.]