SKELETAL MUSCLE 



121 



surrounded by a small amount of granular protoplasm, migrate to the 

 periphery of the fiber and rest just beneath the connective tissue invest- 

 ment. Occasionally a nucleus is found which has not reached the surface. 

 Toward the end of the muscle fiber, the nuclei are numerous, and may 

 retain their central position. The growth of the fiber in length is supposed 

 to occur at the extremities. 



The central position of the nuclei in myoblasts in pig embryos was clearly de- 

 scribed by Schwann, in the second part of his treatise which established the cellular 

 structure of animals (1839). He believed, however, that the myoblasts were formed 

 by the coalescence of primary round cells arranged in a row. The gradual and nearly 

 complete transformation of the protoplasm into longitudinal fibrils was correctly 

 observed. Schwann found that the secondary cells, or mature fibers, were completely 

 enclosed in structureless membranes, which were clearly seen in shrunken fibers (Cf. 

 Fig. in). 



Every striated muscle fiber is completely invested by a membrane 

 named the sarcolemma (o-ap, flesh; Xe/x/xa, husk or shell). This term 



B S5-5 



Sarcoplasm. 



FIG. in. STRIATED 

 MUSCLE FIBER OF 

 FROG, TEASED APART 

 i N WATER. BEING 

 TORN AT x, AND 



SHOWING THE SARCO- 

 LEMMA AT S AND S 1 . 



Light band. Dark band. 



FIG. iia. -LONGITUDINAL SECTIONS OF STRIATED MUSCLE. 



A., Sketch to show the relation between the cells and fibers 



according to Baldwin, a., Fibrous membrane; b., nucleus 



of a muscle cell in vertical section; c., sarcolemma; d., 



myonbrils artificially separated. 



B., Part of a fiber from a straight muscle of the eye of a calf. 

 X 1000. The nucleus is seen in surface view; the sarco- 

 plasm contains chondrioconta. 



was introduced by Bowman (Phil. Trans., 1840) who described the mem- 

 brane as "a tubular sheath of the most exquisite delicacy, investing every 

 fasciculus ( or fiber) from end to end, and isolating its fibrillae from all the 

 surrounding structures." He confirms Schwann's statement that it is 

 not a fibrous structure derived from the surrounding connective tissue, and 

 he states that the nuclei of the muscle come to lie "at or near the surface 

 but within the sarcolemma." He adds, however, that he has seen similar 

 cells in the sarcolemma itself. Since Bowman's time there has been 

 prolonged discussion as to the nature of this membrane. The outer por- 

 tion, which may occasionally contain nuclei, appears to be of connective 

 tissue origin, and is comparable with a basement membrane. The inner 



