456 i^EPORT— 1900. 



section at either end and calculated into resistance per centimetre is the 

 standard of gross longitudinal resistance, and multiplied by the length of 

 short piece of nerve gives the value required. 



The determination of the differences of potential, or, as it is preferred 

 to call them, the available E.M.F.'s between pairs of points on nerve, and 

 calculations based upon these and upon the known resistances, have given 

 interesting information . 



(«) When the cross-section is connected to a point on the longitudinal 

 surface, the current found in the outer circuit passing from longitudinal 

 surface to cross-section can be found traversing the nerve from cross- 

 section to longitudinal surface, by the new differences of potential acquired 

 by intervening points on longi- 

 tudinal surface of nerve. A <ral- 

 vanometer circuit between two 

 such intervening points shows a 

 current reverse in direction to 

 that in the outer circuit, and 

 the difference of potential be- 

 tween two such points is exactly 

 that which would be the case if they are considered simply as points upon 

 a circuit, the only source of E.M.F. in which is the P.D. between the 

 extreme points, and with a certain fraction of the total resistance between 

 them ; which fraction is that given by the relation of the portion of the 

 gross longitudinal resistance of the nerve between the points to the 

 resistance of the primary external circuit and the resistance of the nerve 

 subtended by its points of application. 



If there is a pre-existing difference of potential between the two 

 intervening points (longitudinal current) before the primary external 

 circuit is formed, this ditierence remains and subtracts from the difference 

 of potential caused by the closure of this circuit. It is concluded from 

 this that the soui'ces giving rise to the two phenomena are separate, and 

 capable of existing simultaneously with an opposition of effects. 



Further, by tracing the differences of potentials round the circuit 

 formed by permanently joining tlie cross-section to a point on the longi- 

 tudinal surface it is possible to define the limits of the demarcation 

 source. 



In consequence of the behaviour of points possessing a difference 

 of potential, the cause of longitudinal currents, an examination has 

 been made of electrotonic currents, and it is found that quite simi- 

 larly the closure of a circuit placed for the observation of an electrotonic 

 current creates differences of potentials in intervening points of nerve due 

 to a passage in the nerve of a current in the reverse direction, which 

 reverse current may be observed by connecting up the intervening points 

 through a galvanometer ; and that in this case also the differences of 

 potential in intervening points can be calculated from a knowledge of that 

 portion of the gross longitudinal resistance which lies between them. 



In all these cases the currents observed — demarcation current, longi- 

 tudinal current, electrotoiiic current — can be demonstrated in this exact 

 manner to be new creations, the result of the connecting up of the observa- 

 tion circuit. The institution of the external circuit not only provides for 

 the first time a current through this external circuit, but also the traverse 

 of the same current in a return, and therefore reverse, direction through 

 tj)e nervp, 



