THE RELATION OF THE HEART-BEAT TO ELECTROLYTES. 181 



which accompany the contraction as well as the excitation processes in 

 muscle provide important evidence, and other facts might be adduced 

 in favour of the idea that an essential part in the mechanism of the 

 contractile process in muscle is played by surfaces or membranes 

 possessed of differential permeability towards ions. It is familiar that 

 such membranes form an integral part in the theories of excitation pro- 

 pounded by Nernst and elaborated by Lapicque and by Lucas and 

 Hill. 



The existence of membranes or surfaces in muscle is granted by the 

 histologist to almost any desired extent. In the case of skeletal 

 muscle indeed, it is scarcely an exaggeration to say that every histolo- 

 gist who has undertaken the investigation of its structure has described 

 and attached his name to some new membrane, line or band. 



It is clear then that we are not making any new or rash assumption 

 in stating that certain surfaces or mevihrancs form an integral 'part of 

 the muscle mechanism, and the ionic pcrmeahility of these membranes * is a 

 factor of importance in that mechanism. 



The possession by a membrane of differential ionic permeability is 

 no imaginary attribute : simple experiment shows it to be a very 

 usual property. This fact is brought out most clearly by experiments 

 on the influence of membranes on concentration cells. 



Direct experiment shows also that the ionic permeability of mem- 

 branes — and a great variety of them have been tested — is profoundly 

 influenced by certain features of the solutions in contact with them. 

 For a given membrane the factors likely to influence its ionic permea- 

 bility are — 



(1) The P-h of the solution with which it is treated. 



H 



(2) The presence in that solution of polyvalent ions. The simple 



trivalent positive ions are enormously more powerful than 

 the divalent. 



When a membrane such as a piece of peritoneal membrane, the lining 



of an egg-shell or a thin sheet of gelatine, is treated with a very small 



concentration of acid, or of one of the simple trivalent ions of which 



we have spoken, its ionic permeability is modified. Direct experiment 



shows that this altered condition is retained, it may be for minutes or 



hours, while the membrane is washed with a " neutral " solution free from 



the polyvalent ions — the persistence of the effect depends on the nature 



of the membrane, the concentration of the solution used to produce the 



* Here and in the following pages we employ the word membrane with the under- 

 standing that it need not necessarily mean anything more than the surface of separ- 

 ation between two phases in the system. 



