SKELETAL MUSCLE 



67 



short piece of hair: it may be an inch long but only 

 inch in diameter. It is * a cylinder and so long in 

 proportion to its thickness that our diagrams cannot 

 show both ends and still be 

 broad enough for pictorial clear- 

 ness. The fiber is a modified 

 cell. Instead of having a single 

 nucleus it has a considerable 

 number of nuclei at intervals 

 near its surface. While ' most 

 animal cells are practically naked 

 a muscle fiber has a well-defined 

 envelope, the sarcolemma. At 

 either end of the fiber the sarco- 

 lemma is continuous with the 

 connective tissue. So it comes 

 about that each fiber is a muscle 

 on a small scale; it consists of 

 an elongated body of living con- 

 tractile substance within a sheath 

 and attached at its ends to non- 

 contractile tissue through which 

 its pull may be applied. 



Skeletal muscle is often called 

 striped or striated. The reference 

 is to fine transverse markings 

 which one sees in the fibers when 

 they are highly magnified. The 

 designation smooth, fixed upon 

 the viscera] type of muscle dealt 

 with in the preceding chapter, is 

 used because these cross mark- 

 ings are absent from such cells. 

 The striations are not surface 

 marks upon the sarcolemma, but stand for an obscure 

 but highly specialized internal organization of the living 

 matter. Very different interpretations of their meaning 

 have been advanced by different investigators. 



FIG. 13. Portions of 

 four fibers of skeletal mus- 

 cle. If carried out to their 

 ends these would be several 

 feet long, for often their 

 length is as much as 500 

 times their diameter. Two 

 are crossed to suggest their 

 relative transparency. The 

 one at the right has been 

 injured and the sarcolem- 

 ma remains spanning a 

 break in the striated proto- 

 plasm. 



