PROPAGATION OF PHYSIOLOGICAL CHANGE 495 



is accessible, its character well known ; this is the external 

 solution or lymph, which surrounds the fibres and bathes their 

 longitudinal surface. Modifications in this external solution 

 produce modifications in the electromotive force of the concen- 

 tration cell. Such modifications have been carefully studied, 

 and it has been shown that they are quantitatively consistent 

 with this assumption made as to the nature of the source of 

 E.M.F. The external solution being varied, the internal solution 

 is found to behave as a solution of electrolytes of such constancy 

 that the data obtained from one nerve by one variation of 

 the external solution are capable of accurate contrast with the 

 data obtained from another nerve with another variation. The 

 facts are so definite, indeed, that it is possible in this way to 

 determine the concentration of a solution, by placing it upon 

 the surface of a nerve instead of the normal external solution 

 and observing the alteration produced in the E.M.F. of this 

 concentration-cell. Here, then, is another road along which 

 to obtain a knowledge of the internal solution of electrolytes. 

 Given one condition it can obviously be made a high road 

 to their quantitative determination. If the specific nature of 

 these electrolytes were known, then that strength of their 

 solution which would reduce this E.M.F. to zero would coincide 

 with the strength of the internal solution. In the experimental 

 work done in this subject this point has been realised. There 

 is evidence, recently strongly reinforced by the use of micro- 

 chemical methods introduced by Macallum, that the electrolyte 

 concerned is potassium chloride. Doubtless this point, its 

 importance once realised, will be settled by such direct deter- 

 minations of the salts in nerve as is now being undertaken by 

 Alcock. It is significant of the times and of the ideas which 

 have been introduced in competition with such concepts as 

 " biogen molecules," that so much effort is being expended upon 

 the decision of this question. Assuming this internal electrolyte 

 to be potassium chloride, then it has been shown that the 

 solution present within the nerve-fibre is of such an extra- 

 ordinary concentration that it is impossible to think of such 

 a concentration as normally present there. Here then we have 

 a handsome paradox, but one for which there is possibly a clear 

 explanation. 



When nerve-fibres are injured as by a cross-section, they 

 are excited. If it is assumed that the excitation is a fatal one, 



