STRIATED MUSCULAR FIBRE. 



substance of the fibre, when it is broken up, to present itself in 

 the form of delicate fibrillce, each of which is composed of a single 

 row of the primitive particles (Fig. 388). Sometimes, however, the 



Fig. 388. 



Striated Muscular Fibre, separating into fibrillas. 



lateral adhesion is the stronger, so that the Fibre tends to cleave 

 transversely into disks, each of which is composed of a layer of 

 the primitive particles arranged side by side. When the Fibrillar 

 are separately examined under a magnifying power of from 250 

 to 400 diameters, they are seen to present a cylindrical or slightly- 

 beaded form ; and they show the same alternation of light and 

 dark spaces, as when the fibrillar are united into fibres or into small 

 bundles. The dark and light spaces are nearly of equal length ; 

 but each light space is usually divided by a fine dark transverse 

 line, which, under a sufficient magnifying power, may be resolved 

 into a row of dark points. The number of these alternations in a 

 given length is extremely variable, and appears to depend in part 

 upon the state of contraction or relaxation of the fibre ; a converse 

 variation showing itself in the diameter of the fibrillar The ordi- 

 nary length of each space may be stated at about 1-17, 000th of an 

 inch, so that there would be eight or nine dark spaces, and as 

 many light, in the length of l-1000th of an inch ; but not un- 

 frequently there are double that number of alternations in the 

 same length. The average diameter of the Fibrillar seems to be 

 tolerably uniform in different animals, being for the most part 

 about 1-10, 000th of an inch : it has been observed, however, as 

 high as l-5000th of an inch, and as low as 1,20,000th, even when 

 the fibre was not put upon the stretch. In the ' anterior adductor ' 

 muscles, which draw together the valves of the shells of Tere- 

 bratulce, the Fibrillse (Fig. 388), which are so easily separable that 

 they can scarcely be bound together by a proper Sarcolemma, have 

 a diameter of l-7500th of an inch. 



569. In the examination of Muscular Tissue, a small portion 

 may be cut-out with the curved scissors ; this should be torn up 



