356 ANATOMY OF VERTEBRATES. 



An animal must be in communication with the Torpedo by 

 two distinct points, in order to receive the shock. 1 If an insulated 

 frog's leg, fig. 207, C, touches the Torpedo by the end of the nerve 

 only, no muscular contractions ensue on the discharge of the bat- 

 tery; but a second contact by a portion of muscle, or any other 

 part of the leg, immediately produces them. 2 



The dorsal surface of the electric organ is positive, the ven- 

 tral surface negative. The Torpedo has no power of otherwise 

 directing the electric currents ; but Matteucci found that wound- 

 ing the electric lobes of the brain sometimes reversed the direc- 



o 



tion. 3 These currents, besides their effects on the living body, 

 exercise all the other known powers of electricity ; they render 

 the needle magnetic, 4 decompose chemical compounds, and emit 

 the spark. 5 The discharge of strong currents is usually accom- 

 panied by visible contraction of parts of the body, usually by a 

 retraction of the eyes of the Torpedo, and one muscle, fig. 139, 

 o, is arranged so as to constrict part of the circumference of each 

 battery ; but such consentaneous muscular action, though it may 

 add to the force of the discharge, is not essential to its produc- 

 tion. The benumbing effect seems to be produced by the rapid 

 succession of shocks delivered by the recent and vigorous fish. 

 Matteucci ascertained that, during the discharge, the nerves of 

 the organ were not traversed by any electric current. Pacini, 6 

 from a minute comparison of the organs, deduces that the elec- 

 tricity in the Torpedo is produced by the dynamical conflict 

 between the two polarities inherent in two sorts or different 

 degrees of innervation, as it is evolved in the thermo-electric pile 

 by the conflict of two polarities inherent in two different degrees 

 of temperature ; whilst in the Gymnotus it is produced, as in 

 the voltaic pile, by the chemical conflict between the materials 

 of the elements excited by the nervous influence. 



Humboldt has given a lively narrative of the mode of capture 



1 When the Neapolitan fishermen pull their nets to shore, their first act usually is 

 to wash the captured fishes by dashing over them bucketfuls of sea-water ; and if ;i 

 Torpedo be amongst them it makes its presence instantly felt by the shock transmitted 

 to the arm discharging the bucket. If the fish be handled, the shock is too strong 

 and painful to he willingly encountered a second time, and the arm continues long be- 

 numbed. Each repetition of the discharge, however, enfeebles its force, and the 

 surface of the fish capable of communicating the shock progressively contracts, as 

 life departs, to the region of the organs themselves. When the fisherman dashes 

 the stream of water over the Torpedo, the electric current passes up from the 

 dorsal surface of the batteries against the stream to the man's hand, and the circle 

 is completed by the earth extending from the man's feet to the ventral surface of the 

 prone fish. 



2 I.XXVII. p. 148. 3 Ib. 4 LXXX1I. 5 LXXVII. 6 CCXVIII. 



