io8 Physico-Chemical Basis of Striatcd Musclc C ontraction [Sept. 



muscle rods is purely hypothetical. The surface tension theory 

 requires that changes in surface energy take place, and from what 

 follows it is apparent that these changes must be great — greater, in 

 fact, than the probable actual change on the rod surface. For it 

 seems hardly possible that such great changes in concentration and 

 in surface tension could take place. The values for the surface 

 tensions of pure acetic acid and concentrated salt Solution have been 

 taken from the literature ; whether such limiting values are ever 

 reached in living muscle is, for the present, purely hypothetical. 



If it be assumed that, during the chemical changes taking place 

 in a working muscle, the inorganic ions in the rod-surface film 

 rapidly change their concentrations, the film might be regarded as 

 an electrical double layer or Helmholtz double layer. Without a 

 doubt, changes in surface tension would result from the changes in 

 ion concentration. The more ions in one of these layers covering 

 a rod, the more they repel one another and the lower is the surface 

 tension, and vice-versa. But as has been pointed out before,^^ it is 

 not certain that such a double layer really exists between living 

 particles and their surrounding medium. And even if there were 

 such a layer, the total change in surface tension in such a layer is 

 hardly significant for the present purpose. The small Variation in 

 surface tension when the Variation is caused only by ions was prob- 

 ably overlooked by Robertson^"* and others who advocated a capil- 

 lary electric theory of muscle contraction. It is really a special 

 case of surface tension in which the variations in surface tension 

 are caused by the mutual repulsion of the ions in each of the layers. 

 But insofar as small amounts of certain organic substances, such 

 as fatty acids, can affect (depress) the surface tension of water 

 very much more than even improbably large amounts of inorganic 

 salts, the surface tension theory is given the benefit of the greatest 

 possibilities by assuming the changes in concentration from pure 

 acetic acid (23 dynes/cm.) to saturated sodium chlorid Solution 

 (85 dynes/cm.). This is as large a difference as can be assumed 

 from the experimental data on the surface tensions of Solutions. 



"Berg, W. N. : New York Med. Journal, 1907, July 20 and 27; and Ion, 

 1910, 2, 161-188. 



"Robertson, T. Brailsford: Trans. Royal Soc. South Australia, 1905, 29; 

 and Quarterly Jottr. Exper. PhysioL, 1909, 2, 303-316. 



