SKELETAL MUSCLE 59 



For the sake of description, striated muscular tissue may be divided 

 into two classes, (a) skeletal, which comprises the whole of the striated mus- 

 cles of the body except (b) the heart. 



Skeletal Muscle. The muscle fibers of the skeletal muscles are 

 usually grouped in small parallel bundles, fasciculi. The fasciculi extend 

 through the muscle, converging to their tendinous insertions. Connective- 

 tissue sheaths, endomysium, surround the fasciculi and support the blood- 

 vessels, while a stronger sheath, the perimysium, encases the entire muscle. 



FIG. 73. A, Portion of a Medium-sized Human Muscle Fiber. B, Separated bundles of fibril 

 equally magnified; a, a, larger, and b, b, smaller collections; c, still smaller; d, d, the smallest which 

 could be detached, possibly representing a single series of sarcous element. X 800. (Sharpey.) 



The unit of muscular structure is the fiber. Each muscle fiber is a long 

 cylinder with fusiform ends. The fibers vary in diameter from 10 to 100 JJL, 

 while the length may reach as much as 40 mm. Each fiber is enclosed in 

 a distinct sheath, the sarcolemma. The sarcolemma is a transparent structure- 

 less sheath of great resistance which surrounds each fiber, figure 71. 



The substance of the fiber enclosed by the sarcolemma, the contractile 

 substance, contains a number of oval nuclei distributed along the length of 

 the fiber and lying just under or through the sarcolemma. Each nucleus is 

 accompanied by a small mass of granular protoplasm at its poles. The main 

 mass of the fiber is characterized by transverse light and dark bands, figure 

 73, from which the name striated muscle arises. 



Longitudinal striation is also apparent under certain modes of treat- 

 ment, figure 81. The muscle fibers can be split longitudinally into fibrils, 

 called sarcostyles, figures 73 and 74, each of which exhibits the characteristic 



