496 



SCIENCE 



[N. S. Vol. XXXII. No. 824 



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 stim- 

 ulus occurs in the latent period and imme- 

 diately 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 resiilt 

 the tension of the terminal surfaces may be 

 increased, this resulting in the shortening 

 of the longitudinal axis of the dim band 

 and the displacement laterally of the con- 

 tents. This would imply that the energy 

 of muscle contraction comes primarily 

 from that set free in the combustion proc- 

 ess, and not indirectly as involved in the 

 former explanation. 



Whatever may be the cause of the altera- 

 tion in surface tension, there woidd seem 

 to be no question of the latter. The very 

 alteration in shape of the dim band in con- 

 traction makes it imperative to believe that 

 surface tension is concerned. The redis- 

 tribution 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 fiber 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 hexani- 

 trite of sodium, potassium and cobalt in the 

 cement substance between the membranes 

 of the fibers. In the smooth muscle fibers 

 in the walls of the arteries in the frog the 

 precipitate in the cement material is abun- 

 dant, and its disposition suggests that it 

 plays some part in the role of contraction. 

 Insid"} 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 chlo- 

 rides and phosphates indicate that the cy- 

 toplasm is almost wholly free from them, 

 and consequently there is very little inor- 

 ganic material inside of the fiber. Chlo- 

 rides and phosphates, but more particu- 

 larly the former, are abundant in the ce- 

 ment material, and their localization here 

 would seem to indicate that the potassium 

 of the same distribution is combined chiefly 

 as chloride. 



In smooth muscle fiber, then, the potas- 

 sium is distributed very differently from 

 what it is in striated fiber, 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 sub- 

 stance might be supposed to shift their 

 position under the influence of electrical 

 force so as to reach the interior of the mem- 

 branes of the fibers, in which case the sur- 

 face tension of the latter would be imme- 

 diately increased and the fiber 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 con- 

 traction in smooth muscle. 



It is of interest here to note that the 

 potassium ions have the highest ionic mo- 

 bility (transport number) of all the ele- 

 ments of the kationic class, except hydro- 

 gen, which are found to occur in connection 

 with living matter. Its value in this re- 

 spect 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 veloc- 

 ity of potassium ions would make the ele- 



