DEVELOPMENT OF THE MUSCLES 



373 



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, w r hich 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 



'IQ. 373. Transverse section of human striped muscle fibers. 

 X 255. 



FIG. 374. Striped muscle fibers from tongue of 

 cat. X 250. 



divide or anastomose; but occasionally, especially in the tongue and facial mus- 

 cles, 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 sarco- 

 lemma 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 some- 

 times 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 



JAnat. Rec., 1916, 11. 



