PRESIDENTIAL ADDRESS. 749 



distributed, and it was found to be the case in the wing muscles of certain of the 

 insecta — as, for example, the scavenger beetles— in which the bands are broad 

 and conspicuous enough to permit ready observation on this score. In these the 

 potassium salt was found to be localised in the zones of each dim band adjacent 

 to each light band. Subsequently Miss M. L. Menten, working in my labora- 

 tory and using the same microchemical method, found the potassium similarly 

 limited in its distribution in the muscle fibres of a number of other insects. She 

 determined, also, that the chlorides and phosphates have a like distribution in 

 these structures, and it is consequently probable that sodium, calcium, and 

 magnesium have the same localisation. 



Macdonald has also made investigations on the distribution of potassium in 

 the muscle fibre of the frog, crab, and lobster, using for this purpose the 

 hexanitrite reagent. He holds, as a result of his observations, that the element 

 in the uncontracted fibril is limited to the sarcoplasm in the immediate' neighbour- 

 hood of the 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 microchemistry 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 pene- 

 trated 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 limited to the dim bands of one part of the contracted fibre and may be 

 found in the light bands of another part of the same. In the wing muscles of 

 insects in the uncontracted condition such disconcerting results 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 reagent. 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 fibre is in the sarcoplasm or in the 

 sarcostyle I would hesitate to say. It may be as Macdonald claims, but I find 

 it difficult to apply in microchemical studies of muscle fibre the concepts of its 

 more minute structure gained from merely stained preparations. Because of 

 this difficulty I have refrained from using here, as localising designations, other 

 expressions than ' light bands ' and ' dim bands. ' The latter undoubtedly include 

 some sarcoplasm, but in the case of the resting fibre I am certain only of tb'- 

 presence of potassium, as described, 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 distri- 

 bution would seem to indicate that in the dim band of a fibril the surface tension 

 is greatest 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 tension on the lateral wall is lessened to an amount below that of either 

 terminal surface, followed by a redistribution of the potassium salt to restore the 

 equilibrium thus disturbed. The consequent shortening 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 

 contraction is a question which can only be answered 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 process in the dim bands. It 

 may be that electrical polarisation, as a result of the arrival of the nerve impulse, 

 develops on the surface of the lateral wall, and as a consequence of which its 

 surface tension is diminished. 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, whilo the combustion process would be secondary, in the order of 



1910, 3 o 



