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Mode of Contraction of Voluntary Muscular Fibre. 118 
____ @leay 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.6a). * 
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 im 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 
distinetly in stretched fibrils from the lobster, examined very 
shortly after death, the clear space having scalloped edges 
(fig. 7 a). 
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 distinet 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 0). 
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 5). 
It will perhaps be considered trivial thus to describe the ap- 
pearance of the fibrille 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 striz of voluntary 
muscle.. 
In some kinds of muscular fibrille, it is a matter of great dif- 
ficulty to perceive any dark transverse line in the clear space: 
Ann. & Mag. Ne Hist. Ser. 2. Vol. iii. 8 
