Mr. J. S. MacdoimM. 



resistance of a pair of non-polarisable electrodes and a wire joining 

 them, forming in all a circuit of resistance 135,000 ohms. 



Nerve AD ............... = 128,000 ohms. 



Electrodes ............... = 7000 ohms, 



The length of nerve AD = 4-3 cm. 



piece BC =1*8 cm. 



The " calculated value " of resistance B, C = 54,000 ohms. 



The value directly determined = 63,000 ohms. 



After closure of circuit A, D, the intermediate point B, nearest to 

 the cross section, became " + " to the more distant intermediate 

 point C, which was " - ". 



The value of this difference was 0-0028 volt. 



If in this experiment it is assumed that, (1) the only source of E.M.F. 

 is that found and measured as the potential difference between A ami 

 D, 0-00712 volt : and (2) that the path of the current is the simple one 

 of nerve electrodes and wire, i.e., the path through the nerve is simple, 

 and not divided into two sets of resistances carrying a current in 

 opposite directions (circuit completed in nerve itself) : then a difference 

 of potential should be found between points B and C of this path 



T 

 ' It E ' 



= _ 0-00284 volt, 



and this is practically the value actually found. 



In many similar experiments which have been performed, this 

 agreement of value found and value calculated has been found to hold 

 good within a small limit of error, entirely owing to an alteration of 

 the E.M.F. due to the cross section and to the lapse of time taken 

 to perform the experiment. 



When there is a pre-existing difference of potential between the 

 points B and C, this difference subtracts from the newly-acquired value 

 due to the closure of the circuit A, D, and the value actually found 

 is the algebraical sum of the pre-existing and the newly-acquired 

 difference. 



Since a pre-existing difference between B and C is the source of the 

 " longitudinal current," the last point in the above statement is con- 

 sidered of importance, as tending to show that the source of the 

 demarcation current and that of the longitudinal current can be 

 so separable as to oppose one another in a conveniently arranged 

 circuit. 



