Mode of Contraction uf Voluntary Muscular Fibre. 115 



fibrillse, the surface of the membrane may have been thrown into 

 delicate rugse 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 ahnost lead 

 to the belief that this membrane was a fibril spread out laterally 

 into a membrane; this would quite account for the stria? on its 

 surface. The subject requires more investigation. 



The form of the fibrillpe I consider to be somewhat flattened or 

 ribbon-shaped ; this can be easdy seen when an isolated fibril 

 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 fibrillae (PI. VII. fig. 9), very rarely 

 into discs, and then generally after prolonged maceration in 

 spirit. The fibrillse are divided into day-k 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 light 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 containing 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 pi'esents 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- 

 neously represented by Mr. Erasmus Wilson (fig. 6 « & fig. 5 a). 



4. That no clear area exists at the edge of the fibrillse extend- 

 ing transversely outwards from the dark spaces, giving the 

 fibrillse 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- 

 tions : — 



a. The fact that when two fibrillse 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 stride quite as 

 distinct as the transverse ones, which in this case would not be 

 well-marked (fig. 4 a). 



8* 



