192 



PHYSIOLOGY 



in diffusibility of the substances in solution, especially if the two 

 solutions be separated by a membrane. Very large differences may 

 be produced if this membrane be practically impermeable to one or 

 other of the dissolved substances. In the same way a sernipermeable 

 membrane, i.e. a membrane with different permeabilities for the 

 different ions of the two solutions, may suffice to bring the differences 

 OL potential of a concentration cell up to and beyond the extent which 

 is observed in living tissues. Supposing we have (Fig. 3.1) two solu- 

 tions, A and B, each containing an electrolyte, UV, in different con- 

 centrations separated by a membrane m. If u represents the velocity 



m 



A 



UV 



B 

 UV 



FIG. 31. 



of transmission of U through m, and v the velocity of V, then the 

 electromotive force of the cell is given by the formula 



U ~ v 



U -j- V 



0-0577. 



- Volt. 



If v is taken as very small, the membrane may be regarded as semi- 

 permeable for the corresponding ion V. Supposing we take potassium 

 chloride as the solution, we should have to make the concentration in 

 B eight times that in A, in order to get a current of a strength equal 

 to that obtained from the olfactory nerve of the pike, for example. 

 Macdonald has made such an assumption in order to explain the 

 normal nerve current. He suggests that the axis cylinder contains 

 an electrolyte which is equivalent to a 2-6 per cent, solution of potassium 

 chloride. It is unnecessary, however, to assume such great differences 

 of concentration if we regard the membrane as itself a solution of 

 electrolytes, as has been suggested by Cremer, or if we take different 

 substances on the two sides of the membrane. In the case of two 

 electrolytes, UjVj, U 2 V 2 (U being the cation in each case), separated 

 by a membrane with varying permeability for the different ions, the 

 electromotive force of the cell is given by the following formula : 



0-0577 log. 



10 



-f Vj_ 



