T/ie Demarcation Current of Mammalian Nerve. .">!.'> 



3. Similar experiments show that the closure of a circuit for the 

 observation of a longitudinal current also gives rise to similar changes 

 of potential in the intervening stretch of nerve. 



Similar experiments show that the closure of a circuit for the obser- 

 vation of an elect rotonic current affects the potential of every interven- 

 ing point in the same way. 



So that if a path joining two points on a nerve is found to be 

 carrying a current, whether it be demarcation, longitudinal, or 

 electrotonic, this current can be traced in the longitudinal axis of 

 the nerve, making use of its gross longitudinal resistance, and not 

 interfered with by currents from any other of the possible sources of 

 E.M.F., discovered by the determination of pre-existing differences of 

 potential of intervening points. None of these sources are brought into 

 action so as to affect a current in the longitudinal axis of the nerve 

 until an additional outer circuit of non-polarisable electrodes and wire 

 is formed for them; they then at once are brought into action and 



add to or subtract from the original. 



***** 



4. Long stretches of mammalian nerve vagus, sciatic, and phrenic 

 of about 10 cm. long have been taken, and laid upon an ebonite scale. 

 The E.M.F.s available between either cross section, and the points 

 distant 1, 2, 3, 4, &c. cm. from one end, have been systematically 

 measured, and from the measurements curves obtained with the nerve 

 as abscissa and the ordinates the E.M.F. between the immediately 

 underlying point of longitudinal surface of the nerve and the cross 

 section. 



Whatever be the difference between the E.M.F.s due to the two 

 cross sections (in some cases a difference of height of maxima o 

 0-006 volt, the lower one, e.g., being 0*002 volt, the higher 0*008 volt), 

 and whatever be the peculiarities of the curves, they are, notwith- 

 standing the difference of level, parallel for the greater portion of their 

 extent. The curve due to the available force between one cross 

 section and the longitudinal surface repeats all the maxima and 

 minima of the curve due to the other cross section at a different 

 level, the relations between the maxima and minima being preserved 

 unaltered. 



Such a condition of affairs is most readily explained by the assump- 

 tion that the determination of E.M.F. between a point on the longi- 

 tudinal surface and the cross section is always the determination of 

 the algebraical sum of two opposing forces, one acting radially at the 

 point on the longitudinal surface, and one at the cross section acting 

 in the longitudinal axis of the nerve ; and that the radial force 

 remains the same at a point of the longitudinal surface, whereas there 

 may be and usually is a difference between the two longitudinal forces, 

 one at each cross section. 



***** 



