October 14, 1910] 



SCIENCE 



495 



singly refractive substance, while it is 

 abundantly present in the central portion 

 of each sarcomere of the contracted fibril — 

 that is, in the doubly refractive material. 

 I am not inclined to question the former 

 point, as I have not investigated the micro- 

 chemistry of the muscle in the crab and 

 lobster, and my only criticism would be 

 directed against placing too great reliance 

 on the results obtained in the case of frog 's 

 muscle. The latter is only very slowly 

 penetrated by the hexanitrite reagent, and, 

 apparently because of this, alterations in 

 the distribution of the salts occur and, as 

 I have observed, the potassium may be lim- 

 ited to the dim bands of one part of the 

 contracted fiber and may be found in the 

 light bands of another part of the same. 

 In the wing muscles of insects in the un- 

 contracted condition such disconcerting re- 

 sults are not so readily obtained, owing, it 

 would seem, to the readiness with which 

 the fibrils may be isolated and the almost 

 immediate penetration of them by the re- 

 agent. Here there is no doubt about the 

 occurrence of the element in the zones of 

 the dim band immediately adjacent to the 

 light bands. 



Whether the potassium in the resting 

 fiber is in the sarcoplasm or in the sarco- 

 style I would hesitate to say. It may be 

 as Macdonald claims, but I find it difficult 

 to applj' in microehemical studies of muscle 

 fiber the concepts of its more minute struc- 

 ture gained from merely stained prepara- 

 tions. Because of this difficulty I haye 

 refrained from using here, as localizing 

 designations, other expressions than "light 

 bands" and "dim bands." The latter un- 

 doubtedly include some sarcoplasm, but in 

 the case of the resting fiber I am certain 

 only of the presence of potassium, as de- 

 scribed, in the dim band regarded as an 

 individual part, and not as a composite 

 structure. 



Now, on applying the Gibbs-Thomson 

 principle enunciated above, this distribu- 

 tion would seem to indicate that in the dim 

 band of a fibril the surface tension is great- 

 est on its lateral walls, in consequence of 

 which the potassium salts are concentrated 

 in the vicinity of the remaining surfaces, 

 i. e., those limiting the light bands. This 

 explanation would seem to be confirmed by 

 the observations I made on the contracted 

 fibrils of the wing muscles of a scavenger 

 beetle. In these the potassium was found 

 uniformly distributed throughout each dim 

 band, which, instead of being cylindrical in 

 shape as in the resting element, is provided 

 with a convexly curved lateral wall, and 

 therefore with a smaller surface than the 

 mass of the dim band has when at rest. 

 This contour suggests that the surface ten- 

 sion on the lateral wall is lessened to an 

 amount below that of either terminal sur- 

 face, followed by a redistribution of the 

 potassium salt to restore the equilibrium, 

 thus disturbed. The consequent shorten- 

 ing of the dim bands of the fibrils would 

 account for the contraction of the muscle. 



How the surface tension of the lateral 

 wall of the dim band is lessened in contrac- 

 tion is a question which can only be an- 

 swered after much more is known of the 

 nature of the nerve impulse as it reaches 

 the muscle fibril, and of the part played by 

 the energy set free in the combustion proc- 

 ess in the dim bands. It may be that elec- 

 trical polarization, as a result of the arrival 

 of the nerve impulse, develops on the sur- 

 face of the lateral wall, and as a conse- 

 quence of which its surface tension is di- 

 minished. The energy so lost appears as 

 work, and it is replaced by energy, one may 

 suppose, derived from the combustion of 

 the material in the dim band. In this case 

 the disturbance of surface tension would be 

 primary, while the combustion process 

 would be secondary, in the order of time. 



