Positive Electrical Variation in Isolated Nerve. 
25 
to electrotonic changes produced in the nerve by the stimulating current. 
Waller took the view that the second of these possibilities is the more 
probable. 
Since the invention by Einthoven of the string galvanometer, this 
instrument has been employed in the investigation of the electrical changes 
in nerve. 
Mg. 1 shows the negative variation produced by a period of tetanisation 
of 30 sec, followed by the positive after-variation. The leading off non- 
polarisable electrodes have been placed in my experiments, one on the 
Fig. 1. 
Eesponse obtained from freshly isolated sciatic nerves of frog after tetanisation for 
30 sec. Non-polarisable electrodes applied to longitudinal surface and cross-section, 1 cm. 
between electrodes. Kesistance of preparation and electrodes = 25,000 Ohms. Upper 
line = signal indicating cessation of tetanisation. Zero line is marked by shadow of 
bristle. A control curve is given, representing the introduction into galvanometer circuit 
of a potential difference of 10"^ Volts. Lowest line = time marker indicating seconds. 
The record, which reads from left to right, shows (1) the termination of an upward 
deflection, the negative variation or action current, upon which are superposed the 
individual responses to the induction shocks, (2) a downward deflection, the positive 
after- variation, and (3) persisting positive change. 
longitudinal surface of the nerve and the other on the cross-section. The 
stimulating electrodes are of platinum and lie at the central end of the 
nerve. They are of the form recommended by Engelmann. Where 
localised cooling has been used the nerve between the leading-off electrodes 
is passed through a slit in a cardboard screen and the slit is closed by 
moist kaolin clay. The nerve and electrodes are enclosed in a moist 
chamber. 
Like Waller, I have found that the electrical variation accompanying 
activity may be reversed, and this is so especially if the nerves have been 
preserved for a considerable time before experiment. 
