ELECTRIC RESPONSE 



\ r 



(b) 



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 

 of current of rest, 

 or a negative varia- 

 tion (e.g. muscles 

 and nerves), yet, in 

 certain cases, there 

 is an increase, or 

 positive variation. 

 This is seen in the re- 

 sponse of the retina 

 to light. Again, a 

 tissue which nor- 

 mally gives a nega- 

 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. 



A B 



-< Current of rest 



> Current, of action. 



FIG. 4. ELECTRIC BECORDER 



(a) M muscle ; A uninjured, B injured ends. E E' 



non-polarising electrodes connecting A and 

 B with galvanometer G. Stimulus produces 

 ' negative variation ' of current of rest. Index 

 connected with galvanometer needle records 

 curve on travelling paper (in practice, moving 

 galvanometer spot of light traces curve on 

 photographic plate). Rising part of curve 

 shows effect of stimulus ; descending part, 

 recovery. 



(b) O is the zero position of the galvanometer; 



injury produces a deflection A B ; stimulus 

 diminishes this deflection to C ; C D is the 

 recovery. 



