ELECTRIC RESPONSE II 
under the action of stimulus, the current of rest 
undergoes a transitory diminution, and how on the 
cessation of stimulus there is gradual recovery of the 
tissue, as exhibited in the return of the galvanometer 
needle to its original position. 
Two types of 
response — positive 
and negative. — It 
may here be added 
that though stimu- 
lus in general pro- 
duces a diminution (b) 
of current of rest, B D 
or a negative varia- 
tion (e.g. muscles o— ¢ 

and nerves), yet, M — <— Current of rese 
. —> Current of action 
certain cases, there 
Fic. 4.—Enecrric RECORDER 
1 increas 
eee oe, OF (a) M muscle; A uninjured, B injured ends. E B’ 
4t4] rariati non-polarising electrodes connecting A and 
positive variation. B with galvanometer G. Stimulus produces 
ey 4 a: ‘negative variation’ of current of rest. Index 
This is seen in the re connected with galvanometer needle records 
‘ $ curve on travelling paper (in practice, movin 
sponse of the retina galvanometer spot tot light traces curve on 
: aks photographic plate). Rising part of curve 
to hight. Again, a one effect of stimulus; descending part, 
4 : } recovery. 
tissue which nor- (b) O is the zero position of the galvanometer ; 
° injury produces a deflection A B; stimulus 
mally gives a nega- diminishes this deflection to C; CD is the 
recovery. 
tive variation may 
undergo molecular changes, after which it gives a positive 
variation. Thus Dr. Waller finds that whereas fresh 
nerve always gives negative variation, stale nerve some- 
times gives positive; and that retina, which when 
fresh gives positive, when stale, exhibits negative 
variation. 
