84 HANDBOOK OF PHYSIOLOGY. 



(3.75 cm). It is thus evident that the same fibre does not extend from 

 one end of a muscle to the other, and indeed it is known that in a fas- 

 ciculus fibrils are joined together by rounded or angular extremities in- 

 vested with their proper sheath the sarcolemma. 



Each muscular fibre then is thus constructed: Externally is a fine, 

 transparent, structureless membrane, the sarcolemma, which in the form 

 of a tubular investing sheath forms the outer wall of the fibre and which 

 contains the contractile material of which the fibre is chiefly made up. 

 Sometimes, from its comparative toughness, the sarcolemma will remain 

 untorn, when by extension the contained part can be broken (fig. 84), 

 and its presence is in this way best demonstrated. The fibres are of a 

 pale yellow color, and apparently marked by fine striae which pass trans- 

 versely round them, in slightly curved or Avholly parallel lines. The 



par KB 



Fig. 80. A. Portion of a medium-sized human muscular fibre. X 800. B. Separated bundles of 

 fibrils equally magnified; , , larger, and 6, b, smaller collections; c, still smaller ; d dlhe SnaUest 

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



sarcolemma is a transparent structureless elastic sheath of great resist- 

 ance which surrounds each fibre (fig. 84). There is still some doubt re- 

 garding the nature of the fibrils. 



A striated muscle fibre, when examined with a sufficiently high 

 power of the microscope, presents the following appearances, longitu- 

 dinally : 



^ (a.) Alternate dark and light parallel transverse stripes, to which 

 this variety of muscle owes its name, the depth of the stripes not 

 always being the same. 



(b.) With still higher powers of the microscope, the bright stripes 



