Mode of Contraction of Voluntary Muscular Fibre. 113 



clear area may be observed in favourable specimens to have a 

 distinct edge, and when the fibril has been in no way distorted 

 or stretched, to be continuous with the edge of the dark area. 

 Crossing the clear space at its centre, and at right angles to the 

 length of the fibril, will be seen a distinct dark line ; this line di- 

 vides the clear area into two equal parts or divisions, which are 

 necessarily quadrangular. The dark space in the same focus 

 presents a shape very similar to the clear one, though generally 

 of a more elongated form ; its whole surface is dark, with the 

 exception however of a clear line crossing it in the same manner 

 as the dark transverse line does the clear space, and dividing it 

 equally into two dark particles (fig. 6 a) . 



In some cases I have seen the dark spaces divided into three 

 by two clear cross-lines ; an appearance I think which cannot be 

 relied on, as the other dark spaces in the same fibrils presented 

 the space as double only, with the single clear transverse line. 



When the fibrils are stretched, the dark space often appears as 

 if somewhat elevated above the clear space ; I have seen this very 

 distinctly in stretched fibrils from the lobster, examined very 

 shortly after death, the clear space having scalloped edges 

 (fig. 7a). 



With regard to the term dark space, it must not be supposed 

 that it is really opake ; for under a superficial focus it also be- 

 comes clear, as I shall presently describe. I shall still retain the 

 term as expressive of what is observed when the fibril is seen 

 under the distinct focus. 



If the focus of the instrument be now adjusted for the more 

 superficial part of the fibril, or a little above it, a remarkable 

 change is observed ; the general appearance of the fibril is dimi- 

 nished in distinctness, and what was before the dark space now 

 appears clear (but not so translucent as the clear space in the 

 distinct focus), and is then seen to be crossed transversely by a 

 dark line (fig. 6 b) . 



The clear area or space undergoes a similar change of appear- 

 ance, becoming quite dark, but no line can be observed to cross 

 it. The focus under which this is observed, to avoid confusion 

 I shall call the superficial focus (fig. 6 b). 



It will perhaps be considered trivial thus to describe the ap- 

 pearance of the fibrillse under an indistinct focus : but that it 

 is not so, I hope afterwards to be able to prove ; for on the 

 change of appearances thus presented, I believe hangs the true 

 explanation of the cause of the transverse striae of voluntary 

 muscle. 



In some kinds of muscular fibrillse, it is a matter of great dif- 

 ficulty to perceive any dark transverse line in the clear space : 



Ann. ^ Mag. N. Hist. Ser. 2. Vol. iii. 8 



