458 MOTION. 



To this kind of muscular fibre the term organic is often ap- 

 plied, from the fact that it enters especially into the construc- 

 tion of such parts as are concerned in what has been called 

 organic life (see note, p. 368). 



The muscles of animal life, or striped muscles, include the 

 whole class of voluntary muscles, the heart, and those muscles 



neither completely volun- 

 FlG - 158 - tary nor involuntary, which 



form part of the walls of the 

 pharynx, and exist in many 

 other parts of the body, as 

 the internal ear, urethra, 

 &c. All these muscles are 

 composed of fleshy bundles 

 called fasciculi, inclosed in 

 coverings of fibro-cellular 

 tissue, by which each is at 

 once connected with, and 



A small portion of muscle natural sue, igolated from thoge adjacent 

 consisting of larger and smaller fasciculi, /-n. -txo\ 17 r, 



seen in a transverse section, and the same to lt V* 1 *''. .''. . C 



magnified 5 diameters (after Sharpey). bundle is again divided into 



smaller ones, similarly en- 

 sheathed and similarly divisible ; and so on, through an un- 

 certain number of gradations, till one arrives at the primitive 

 fasciculi, or the muscular fibres peculiarly so called. 



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

 of delicate, structureless membrane, called the sarcolemma, 

 inclosing a number of filaments or fibrils. They are cylindri- 

 form or prismatic, with five or more sides, according to the 

 manner in which they are compressed by adjacent fibres. 

 Their average diameter is about -5^ of an inch, and their 

 length never exceeds an inch and a half. 



Each muscular fibre is thus constructed : Externally is a 

 fine, transparent, structureless membrane, called the sarco- 

 lemma, which in the form of a tubular investing sheath forms 

 the outer wall of the fibre, and is filled by 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. 159), and its presence is in this way best demonstrated. 

 The fibres, which are cylindriform or prismatic, with an aver- 

 age diameter of about ^Q of an inch, are of a pale yellow 

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

 versely round them, in slightly curved or wholly parallel 

 lines. Other, but generally more obscure striae, also pass lon- 

 gitudinally over the tubes, and indicate the direction of the 



