The Demarcation Current of Mammalian Nerve. 311 



The calculated resistance per cm. is also considerably greater when 

 the resistance is measured from one point on the longitudinal surface 

 to another than in the case in which the resistance of a similar length 

 is taken from the cross section to a point on the longitudinal surface, 

 and greater still than the resistance of a similar length bounded by two 

 cross sections. 



When therefore a knowledge of the resistance is required as a basis 

 for calculations, a direct determination of the resistance is only of value 

 when the resistance is required to a current having the same path as 

 that used for the measurement of resistance. 



In any other case, as when the fraction of the longitudinal re- 

 sistance corresponding to a fraction of the length of the nerve is 

 required, it is better to use a calculated value than the value of the 

 directly determined resistance. This calculated value is best obtained 

 from the resistance per cm. of the longest available stretch of the 



nerve. 



***** 



2. If a point on the longitudinal surface is connected to the cross 

 section through a pair of non-polarisable electrodes and an outer wire 

 path, and a current is found to traverse the circuit so formed, then the 

 current can not only be found in the outer wire path by means of a 

 galvanometer placed in it ; but can also be followed in a return direc- 

 tion in the nerve, travelling in the opposite direction from the cross- 

 .section to the longitudinal surface, by means of the new differences of 

 potential, which the formation of the outer circuit immediately estab- 

 lishes in every intervening point of the nerve. 



This is true when any arbitrarily selected point on the longitudinal 

 surface is connected to the cross section. In each case the return current 

 through the nerve is found established as a new phenomenon, due to 

 the closure of the outer path, and is exactly the current due to the 

 action of a source of E.M.F. of the value determined as the potential 

 difference between the point on the longitudinal surface and the cross 

 section in a circuit of the resistance found. 



Experiment. Vagus Nerve of Cat. 



The nerve was excised and laid upon four non-polarisable electrodes, 

 A, B, C, D. The cross section was at A, the nerve stretched a small 

 distance beyond electrode D, and was suspended to the wall of the 

 moist chamber by a silk thread. 



The potential difference (so called) between points A and D was 

 determined as 0-00712 volt. 



The potential difference between the two intermediate points B and 

 C was zero, the points being equipotential. 



Points A and D were now permanently connected through the 



2 A 2 



