STRUCTURE OF STRIPED MUSCLE. 



585 



Muscular fibres consist, each, of them, of a tube or sheath 

 of delicate structureless membrane, called the sarcolemma, 

 enclosing a number of filaments or fibrils. They are 



Fig. 152.* 



Fig. i53-t 



cylindriform or prismatic, 



with five or more sides, 



according to the manner in 



which they are compressed 



by adjacent fibres. Their 



average diameter is about 



^-J^j- of an inch, and their length never exceeds an inch 



and a half. 



Each muscular fibre is thus constructed : Externally 

 is a fine, transparent, struc- 

 tureless membrane, called 

 the sarcolemma, which in 

 the form of a tubular in- 

 vesting sheath forms the 

 outer wall of the fibre, and 

 is filled by the contractile 

 material of which the fibre 

 is chiefly made up. Some- 

 times, from its comparative 

 toughness, the sarcolemma 

 will remain untorn, when 

 by extension the contained 

 part can be broken (fig. 

 152), and its presence is 

 in this way best demonstrated. The fibres, which are 

 cylindriform or prismatic, with an average diameter of 

 about ^-J-g- of an inch, are of a pale yellow colour, and 

 apparently marked by fine striae, which pass transversely 



* Fig. 152. Muscular fibre torn across; the sarcolemma still con- 

 necting the two parts of the fibre (after Todd and Bowman). 



f Fig. 153. A few muscular fibres, being part of a small fasciculus, 

 highly magnified, showing the transverse striae, a, end view of b, b, 

 fibres; c, a fibre split into its fibrils (after Sharpey). 



