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' Mode of Contraction of Voluntary Muscular Fibre. 115 
fibrillee, the surface of the membrane may have been thrown into 
delicate rugz by the elevation of the dark spaces above the clear 
ones, as may be often seen in stretched fibrils. 
I have seen appearances in the skate that would almost lead 
to the belief that this membrane was a fibril spread out laterally 
into a membrane; this would quite account for the strize on its 
surface. The subject requires more investigation. 
The form of the fibrillz I consider to be somewhat flattened or 
ribbon-shaped ; this can be easily seen when an isolated ies) 
becomes accidentally twisted. 
The conclusions which I would draw with regard to the struc- 
ture of muscular fibre from what I have myself observed, I shall 
now endeavour to give. — 
1. That (excluding the sarcolemma) an ultimate fibre of 
voluntary muscle is composed of two kinds of sarcous matter, 
arranged in a definite manner, having a tendency under certain 
circumstances to split up into fibrille (Pl. VII. fig. 9), very rarely 
into discs, and then generally after prolonged maceration in 
spirit. The fibrille are divided into dark and light spaces. 
2. That the dark sarcal element or space has some peculiarity 
in its molecular arrangement, differing from the clear sarcal ele- 
ment or space, which causes it to refract hght in a different way. 
That we are not entitled to say that it is composed of cells con- 
taining a fluid of greater density than that contained in the con- 
tiguous clear space ; in fact, that we are not able to say with any 
degree of certainty, that any portion-of a muscular fibril in the 
mature state is a cell contaiming fluid, as Mr. Erasmus Wilson 
believes. 
3. That the clear space can be distinctly seen to have a dark 
line crossing it transversely and dividing it into two equal parts, 
and that the dark space also presents a similar division caused by 
a line which is generally seen of a lighter shade than the other 
parts of the same space, and not a broad black band as is erro- 
“peer represented by Mr. Erasmus Wilson (fig. 6 a & fig. 5 a). 
hat no clear area exists at the edge of the fibrillee extend- 
oo ‘transversely outwards from the dark spaces, giving the 
fibrillee the appearance of a chain of nucleated cells, as is repre- 
sented by Dr. Sharpey and Dr. Carpenter (fig. 4). This conclu- 
sion I have been irresistibly led to by the following considera- 
~ tlons :— 
a. The fact that when two fibrille lie side by side, the edges 
of the black spaces are in accurate apposition. 
b. That if this lateral clear area really existed, the fibre would 
be spotted, or at least marked with longitudinal striz quite as 
distinct as the transverse ones, which in this case would not be 
well-marked (fig. 4a). 
Rx 
