Henry Gray (18251861). Anatomy of the Human Body. 1918.
Striped or Voluntary Muscle.Striped or voluntary muscle is composed of bundles of fibers each enclosed in a delicate web called the perimysium in contradistinction to the sheath of areolar tissue which invests the entire muscle, the epimysium. The bundles are termed fasciculi; they are prismatic in shape, of different sizes in different muscles, and are for the most part placed parallel to one another, though they have a tendency to converge toward their tendinous attachments. Each fasciculus is made up of a strand of fibers, which also run parallel with each other, and are separated from one another by a delicate connective tissue derived from the perimysium and termed endomysium. This does not form the sheath of the fibers, but serves to support the bloodvessels and nerves ramifying between them.
A muscular fiber may be said to consist of a soft contractile substance, enclosed in a tubular sheath named by Bowman the sarcolemma. The fibers are cylindrical or prismatic in shape (Fig. 373), and are of no great length, not exceeding, as a rule, 40 mm. Huber
1 has recently found that the muscle fibers in the adductor muscle of the thigh of the rabbit vary greatly in length even in the same fasciculus. In a fasciculus 40 mm. in length the fibers varied from 30.4 mm. to 9 mm. in length. Their breadth varies in man from 0.01 to 0.1 mm. As a rule, the fibers do not divide or anastomose; but occasionally, especially in the tongue and facial muscles, they may be seen to divide into several branches. In the substance of the muscle, the fibers end by tapering extremities which are joined to the ends of other fibers by the sarcolemma. At the tendinous end of the muscle the sarcolemma appears to blend with a small bundle of fibers, into which the tendon becomes subdivided, while the muscular substance ends abruptly and can be readily made to retract from the point of junction. The areolar tissue between the fibers appears to be prolonged more or less into the tendon, so as to form a kind of sheath around the tendon bundles for a longer or shorter distance. When muscular fibers are attached to skin or mucous membranes, their fibers become continuous with those of the areolar tissue.
The sarcolemma, or tubular sheath of the fiber, is a transparent, elastic, and apparently homogeneous membrane of considerable toughness, so that it sometimes remains entire when the included substance is ruptured. On the internal surface of the sarcolemma in mammalia, and also in the substance of the fiber in frogs, elongated nuclei are seen, and in connection with these is a little granular protoplasm.
Upon examination of a voluntary muscular fiber by transmitted light, it is