582 THE PHYSIOLOGY OF ELECTRICAL ORGANS. 



- 6 per cent. NaCl to obtain a lowered state of excitability, in which the 

 initial response alone presents itself. Even then any circumstance 

 raising the excitability may develop the normal multiple effect, such, for 

 instance, as section of the nerve near the organ. Schonlein has ob- 

 served an interesting instance of the abolition of the multiple response, 

 when the excitability is sufficiently lowered ; this occurred in Torpedo 

 on exciting the nerve trunk by an ascending galvanic current, the 

 lowered excitability being in this case that of the anodic extrapolar 

 region of the nerve extending to the nerve-endings in the organ discs. 



The members of the whole multiple response become feebler as 

 they are successively evoked, and when the intensity of one sinks 

 below the threshold of exciting efficiency, the reduplication of the 

 effect ceases. The duration or number of members is thus very vari- 

 able, being increased by great intensity of initial effect and high organ 

 excitability. In the living Malapterurus such a rhythmical response 

 may last 5 sec, and comprise 150 individual members, but as a rule 

 the multiple effect only lasts 02 sec, and seldom has more than fifteen 

 members. 



Similar auto-excitation has been observed in motor nerve, when 

 rendered highly excitable through cold, 1 and in the sartorius muscle 

 when locally squeezed ; 2 in these instances, the causation of the 

 phenomenon is of the same character, the electrical change of the 

 active tissue exciting it to renewed activity, but in no structure is the 

 production of secondary, tertiary, etc., effects so marked as in the elec- 

 trical organ. In Malapterurus this is so pre-eminently the case, that 

 the efficiency of the organ, as a weapon for protection and attack, is 

 thus enormously increased. 



The effect of repetition of the stimulus. — It does not appear 

 possible by rapid repetition of the stimuli to evoke responses which 

 shall be superimposed upon their predecessors. Probably the tissue 

 change, which is the basis of the response, cannot be reproduced until the 

 electromotive effect, which is its objective sign, has more or less sub- 

 sided. The presence of the change would lie then associated with a 

 period of diminished excitability. It appears probable from recent 

 researches by Gotch and Burch, that nerve shows a similar incapacity 

 for the production of a second electromotive change whilst that of a 

 precursor is still in progress. 3 The potency of the organ discharge, when 

 wielded by the fish, is thus not increased, like that of a muscle, by 

 such summation of mechanical effect as exists in the fused tetanic con- 

 traction. It is modified by the magnitude of the initial effect, which 

 is itself dependent upon that of the exciting stimulus, by the extent 

 of the auto-excitation which causes a repetition of the effect, and by the 

 number of the successive central discharges which reach the organ. 



The gradual diminution in the magnitude of the response, when the 

 organ is excited many times at such intervals (say five times per sec), as 

 to insure its excitation, has been already alluded to. In Torpedo 

 preparations, Schonlein 4 found that a rheotome experiment, involving at 

 least 500 excitations, could be repeated about ten times when the tissue 



1 Hering, Sitzungsb. d. I: Akad. d. Wissensch., Wien, 1882 (3), Bd. lxxxv. S. 237. 



2 Kiilme, Ztschr. f. Biol., Mtinchen, N. F., Bel. viii. S. 203. 



3 " Proe. Physiol. Soc," January 1899, Joum. Physiol., Cambridge and London, vol. 

 xxiii. 



4 Schonlein, Ztschr. f. Biol., Miinchen, Bd. xxxiii. S. 408. 



