SKELETAL MUSCLE 89 



which are derived from mesodermal cells. Though not branching 

 and forming networks, as in the case of cardiac muscle, they are like 

 it in being syncytial. Distinct cellular limitations are not evident 

 and man\- nuclei are distributed along the length of each fiber. These 

 fibers do not break into semblances of cells u])on treatment with dis- 

 sociating fluids. The development of the elongate multinucleated 

 fibers is held by some to occur by repeated divisions of the nuclei 

 of myoblasts without accompanying division of the cytoplasm, 

 which increases in quantity and elongates. Others believe they 

 arise through fusion of the ends of adjacent myoblasts. Fibrils 

 appear first about the periphery and increase in number and dis- 



External perimysium 



{epimysium) 



Internal perimysium 



r> Fiber 



Fig. 48. — Diagram of a cross-seotion of a skeletal muscle. 



tribution during de\'elopment. The nuclei of the myoblasts at the 

 first appearance of fibrils are central, but later in development they 

 appear more peripherally located. 



The fibers are organized into skeletal muscles which are enclosed 

 in a relatively thick fibroelastic connective-tissue sheath. (Fig. 48.) 

 This is the external jjerimysium (epimysiimi) which continues inter- 

 nally as the internal perimysium to divide the muscle into bundles of 

 fibers, or fasciculi. (Fig. 49.) The individual fibers are surrounded 

 by a thin sheath of connective tissue, called the endomysium, 

 which contains fibrous and cellular elements of loose fibroelastic and 

 reticular connective tissues. (Fig. 50.) Fibrocytes and histiocytes 

 and undifferentiated mesenchymal cells in the connective-tissue 

 sheaths play a part in the repair of lesions of skeletal muscle which 

 does not exhibit marked powers of regeneration. The capillaries 



