244 On the Structure of Striped Muscular Fibre. [Apr. 3, 



by the presence of the globular heads of the muscle-rods, which have a 

 different refractive index from that of the ground-substance. That 

 such an explanation is possible, is shown by the examination of minute 

 oil-globules imbedded in gelatine, which appear under the microscope as 

 dark spots with a bright surrounding, the juxtaposition of several such 

 dots giving the effect of a bright band. That the bright transverse bands 

 in muscle are similarly produced by the juxtaposition of the rod-heads 

 would appear from the following amongst other considerations : — 



1. Where the rod-heads are smaller the bright bands are correspond- 

 ingly narrower. 



2. Where the rod-heads have become merged into the shafts, so as no 

 longer to be seen as distinct objects, the bright transverse stripes 

 have also entirely disappeared. 



3. When in contraction the rod-heads enlarge and encroach on the 

 shaft, their bright borders accompany them and encroach on the dim 

 substance, so that at last all appearance of dimness becomes entirely 

 obliterated, the bright borders becoming blended in the middle. 



4. The part of the muscle-rod where the head joins the shaft is 

 rendered indistinct by the brightness around the rod-head ; whereas 

 if this brightness were inherent in the ground-substance, this part 

 of the rod would stand out all the darker by the contrast. 



5. The appearance of a transverse section is corroborated ; for in 

 this case the rod-heads are seen so close together that the optical 

 effect of any one would become merged into those of its neighbours : 

 consequently the whole of the intermediate substance would appear 

 bright ; and this is actually found to be the case. 



6. The fact that both the dim and the bright substance of resting 

 muscle appear doubly refracting would seem to indicate that they 

 are of the same nature. 



The author then proceeds to give the result of his investigations of the 

 appearance of muscle under polarized light. He finds that, as regards 

 muscle at rest when placed between crossed Mcols, the whole fibre 

 appears illuminated ; in the contracted state, on the other hand, the 

 appearance is presented of illuminated stripes with dark intervals. The 

 latter correspond with the lines formed by the juxtaposition of the 

 enlarged ends of the muscle-rods ; these consequently are singly 

 refracting (isotropous), and so, in all probability, are the shafts of the 

 muscle-rods ; they do not, however, stand out as black streaks, since 

 they are surrounded by doubly refracting (anisotropous) ground- 

 substance, and are illuminated by the light which has previously 

 traversed this. In the same way it may readily be understood why, 

 in the resting muscle also, the rods, although isotropous, do not appear 

 as such. The conclusion, then, that the author arrives at on this point is 

 that the whole of the muscular fibre is anisotropous, with the exception 

 of the muscle-rods. 



