OF ELECTRICITY IN ANIMALS. 



51 



tween zero and the extreme point which it would have occupied 

 if the current had been continuous. 



In the muscles in which the shock is protracted, as in 

 the tortoise, a very prolonged change in the electrical state is 

 produced ; and therefore these muscles can by each of their 

 shocks cause a deflection of the magnetic needle. It is the 

 same with the movements of the heart ; each of these appears 

 to be only a shock of the cardiac muscle, and yet it deflects the 

 magnetic needle in the same manner as tetanus of an ordinary 

 muscle. This fact, that a negative variation is equally seen 

 in a muscle which is contracted voluntarily, is of the greatest 

 importance. It confirms the theory which assimilates con- 

 traction with tetanus, that is to say, with a discontinuous or 

 vibratory action. 



One point which has been long under discussion relative 

 to the manifestations of muscular electricity, is whether the 

 negative variation is caused by a change of direction in the 

 muscular current, or by a trausitory suppression of this 

 current. The latter hypothesis has been rendered extremely 

 probable by the numerous experiments in which the needle 

 of the galvanometer has never been seen to retrograde beyond 

 the zero point. Thus the phenomenon of negative variation 

 seems to prove the principle which we laid down at the com- 

 mencement of this article, that force is manifested in the 

 muscles in a difierent manner during activity and repose, aod 

 that the manifestation under the form of mechanical work is 

 substituted for that under the form of electricity. 



Electric fishes. — Animal electricity appears in a much more 

 striking form in the discharges produced by certain fishes. 

 In this case the special organs have for their object the pro- 

 duction of electricity; nevertheless, by their structure, their 

 chemical composition, and their dependence on the nervous 

 system, these organs remind us of the conditions of the mus • 

 cular apparatus. 



The number of species provided with electrical organs 

 which was formerly restricted to five,* has been remarkably 



* The five species formerly known were the Kaya torpedo, the Gym- 

 notus electricus, the Silurus electricus, the Tetraodon electricus, and the 

 Trichiurus electricus. 



