38 PROGRESS OF MICROSCOPICAL SCIENCE. 



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 aroimd 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 : conse- 

 quently 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 Nicols, 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 re- 

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

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

 they are surrounded by doubly refracting (anisotropous) ground-sub- 

 stance, and are illuminated by the light which has previously traversed 

 this. In the same way it may readily be understood why, in the rest- 

 ing 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. Various observers are then quoted in suj^port of the 

 accuracy of the description given ; and the probability is pointed out, 

 and supi^orted by an observation of Prof. Briicke, that in all cases in 

 which alternating disks of isotropous and anisotropous substances are 

 observed, the muscular fibre is in a state of contraction (although not 

 necessarily shortened) — that is to say that the anisotropous substance 

 has become accumulated between the shafts of the rods, the isotropous 

 disks being due to the rod-heads, between which there is no perceptible 

 amount of anisotropous substance left remaining. The author 

 concludes the paper by offering a conjecture as to the nature of the 

 substances which, according to his description, compose the proper 

 substance of muscle, and as to the probable mode in which the con- 

 traction is effected. He is inclined to regard the intermediate 

 ground-substance as the true contractile part, and thinks that it may 

 be allied in nature to ordinary protoplasm, the rods, on the other 

 hand, being elastic structures, and merely serving to restore the fibre 

 to its original length. 



