1888-89.] Mr G. N. Stewart on Electrotonic Variation. 243 
Experiment 7. 
Distances — a, 7\ mm.; b, 7\ mm.; ^,10 mm. 
Polarising Current. 
Stimulation 
Effect. 
2 D f 
- 40 
B and C in contact. 
1 D Rh. 90 cm. t 
- 8 
3 D t 
- 28 
5 D t 
1 
r - 24 
I - 22 
1 D f 
- 9 
2 D f 
- 14 
5 D t 
- 3 
lDj 
1 
i -127 
| -114 
2D j 
i 
C + 28 
t + 22 
5 D l 
\ 
r +126 
1+119 
1 D 4, 
i 
> - 46 
\ - 49 
It was now desirable to compare the amount of the intra- and 
extrapolar stimulation effects ; and in order to avoid the uncertainty 
which must always exist when one tries to get quantitative com- 
parisons from experiments made on different days with different 
nerves, I determined to control the other observations by means 
of a set in which the intrapolar and extrapolar regions of the same 
nerve were led off alternately to the galvanometer. It was parti- 
cularly important to notice how the ratio between the amount of 
the two effects varied with varying density of polarising current 
when the latter was nearly strong enough to suppress the intrapolar 
effect altogether. 
The arrangement is shown in fig. 5 for the case where the two 
extrapolar regions compensate each other, and the two intrapolar 
regions are placed one in each coil of a differential galvanometer. 
A, B, C, D are, as before, the electrodes of one of the nerves ; 
A', B', C', D' those of the other. G, G' are the two coils of the 
differential galvanometer ; P is a Pohl’s commutator without cross 
wires, by means of which either AB or CD may be joined on to G : 
P' is a Pohl with cross wires, to alter the direction of the polarising 
current ; Com. is a compensator to complete the compensation when 
the extrapolar areas are led off; R is a rheostat to equalise the 
