October 6, 1910] 



NATURE 



449 



spicuous 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 deter- 

 mined, 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 distri- 

 bution 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 neighbourhood 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 penetrated 

 by the he-\anitrite 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 con- 

 cepts 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 the 

 presence of potassium, as described, in the dim band re- 

 garded as an individual part, and not as a composite 

 structure. 



Now, on applying the Gibbs-Thomson principle 

 enunciated above, this distribution 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 con- 

 tour 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 con- 

 sequent 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 com- 

 bustion process in the dim bands. It may be that elec- 

 trical 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 



NO. 2136, VOL. 84] 



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. In support of this explanation may be 

 cited the fact that the current of action in muscle precedes 

 in time the contraction itself — that is, the electrical 

 response of the stimulus occurs in the latent period and 

 immediately before the contraction begins. 



It may, however, be postulated, on the other hand, that 

 the chemical changes occur in those parts of the dim 

 band immediately adjacent to the light bands, and as a 

 result the tension of the terminal surfaces may be in- 

 creased, this resulting in the shortening of the longitudinal 

 axis of the dim band and the displacement laterally of 

 the contents. This would imply that the energy of muscle 

 contraction comes primarily from that set free in the 

 combustion process, and not indirectly as involved in the 

 former explanation. 



Whatever may be the cause of the alteration in surface 

 tension, there would seem to be no question of the latter. 

 The very alteration in shape of the dim band in contrac- 

 tion makes it imperative to believe that surface tension 

 is concerned. The redistribution of the potassium which 

 takes place as described in the contracting fibrils of the 

 wing muscles of the scavenger beetle can be explained in 

 no other way than through the alteration of surface 

 tension. 



In the smooth muscle fibre potassium is also present 

 and in close association throughout with the membrane. 

 When a fresh preparation of smooth muscle is treated so 

 as to demonstrate the presence of potassium, the latter 

 is shown in the form of a granular precipitate of hexa- 

 nitrite of sodium, potassium, and cobalt in the cement 

 substance between the membranes of the fibres. In the 

 smooth muscle fibres in the walls of the arteries in the 

 frog the precipitate in the cement material is abundant, 

 and its disposition suggests that it plays some part in 

 the role of contraction. Inside of the membrane potassium 

 occurs, but in very minute quantities, which, with the 

 cobalt sulphide method, gives a just perceptible dark shade 

 to the cytoplasm as a whole. Microchemical tests for the 

 chlorides and phosphates indicate that the cytoplasm is 

 almost wholly free from them, and consequently there is 

 verv little inorganic material inside of the fibre. 

 Chlorides and phosphates, but more particularly the 

 former, are abundant in the cement material, and their 

 localisation here would seem to indicate that the 

 potassium of the same distribution is combined chiefly as 

 chloride. 



In smooth muscle fibre, then, the potassium is dis- 

 tributed very differently from what it is in striated fibre, 

 and on first thought it seemed difficult to postulate that 

 the contraction could be due to alterations of surface 

 tension. This, however, would appear to be the most 

 feasible explanation, for the potassium salts in the cement 

 substance might be supposed to shift their position under 

 the influence "of electrical force so as to reach the interior 

 of the membranes of the fibres, in which case the surface 

 tension of the latter would be immediately increased, and 

 the fibre itself would in consequence at once begin to 

 contract. The slowness with which this shifting into, or 

 absorption by, the membrane of the potassium salts would 

 take place would also account for the long latent period 

 of contraction in smooth muscle. 



It is of interest here to note that the potassium ions 

 have the highest ionic mobility (transport number) of all 

 the elements of the kationic class, except hydrogen, which 

 are found to occur in connection with living matter. Its 

 value in this respect is half again as great as that of 

 sodium, one-eighth greater than that of calcium, and one- 

 seventh greater than that of magnesium. This high 

 migration velocity of potassium ions would make the 

 element of special service in rapid changes of surface 

 tension. , 



Loew has pointed out that potassium in the condensa- 

 tion processes of the synthesis of organic compounds has 

 a catalytic value different from that of sodium. For 

 example, ethyl aldehyde is condensed with potassium salts 

 to aldol, with sodium salts to crotonic aldehyde (Kopf and 

 Michael). Potassium is, but sodium is not, effective in 

 the condensation of carbon monoxide. When phenol is 



