ELECTRIC FISHES 813 



the shock passes in the animal from tail to head. In Malapterurus, 

 although the direction of the plates is the same, and the nerve-supply 

 is also to the posterior surface, the shock passes from head to tail. 



In Torpedo, the plates or septa dividing the vertical hexagonal prisms 

 of which each lateral half of the organ consists are horizontal ; the nerve- 

 supply is to the lower or ventral surface ; and the shock passes from belly 

 to back through the organ. In all electric fishes the discharge is dis- 

 continuous; an active fish may give as many as 200 shocks per second. 



The electrical nerve of Malapterurus is peculiar. It consists of a 

 single gigantic nerve-fibre on each side, arising from a giant nerve-cell. 

 The fibre has an enormously thick sheath, the axis-cylinder forming a 

 relatively small part of the whole ; and the branches which supply the 

 plates of the organ are divisions of this single axis-cylinder. 



The electromotive force of the shock of the Gymnotus may be very 

 considerable; and even Torpedo and Malapterurus are quite able to 

 kill other fish, their enemies or their prey. Indeed, Gotch has esti- 

 mated the electromotive force of I cm. of the organ of Torpedo at 

 5 volts. Schonlein finds that the electromotive force of the whole 

 organ may be equal to that of 31 Daniell cells, or 0-08 volt for each 

 plate, and it is one of the most^ interesting questions in the whole of 

 electro-physiology, how they are pro- 

 tected from their own currents . There 

 is no doubt that the current density 

 inside the fish must be at least as 

 great as in any part of the water sur- 

 rounding it, and probably much 

 greater. The central nervous system 

 and the great nerves must be struck 

 by strong shocks, yet the fish itself is Fig. 311. Diagram showing Direc- 

 not injured; nay. more, the young in tion of Shock in Torpedo, 



the uterus of the viviparous Torpedo 



are unharmed. The only explanation seems to be that the tissues of 

 electric fishes are far less excitable to electrical stimuli than the tissues of 

 other animals ; and this is found to be the case when their muscles or 

 nerves are tested with galvanic or induction currents. It requires ex- 

 tremely strong currents to stimulate them ; and the electrical nerves are 

 more easily excited mechanically, as by ligaturing or pinching, than elec- 

 trically. In general, too, the shock is more readily called forth by reflex 

 mechanical stimulation of the skin than by electrical stimulation. But 

 that the organ itself is excitable by electricity has been shown by Gotch. 

 He proved that in Torpedo a current passed in the normal direction 

 of the shock is strengthened, and a current passed in the opposite 

 direction weakened, by the development of an action current in the 

 direction of the shock. And, indeed, a single excitation of the electrical 

 nerve is followed by a series of electrical oscillations in the organ, which 

 gradually die away. The latent period of a single shock is about 

 Z%Q second. The skate must be included in the list of electric fishes. 

 Although its organ is relatively small, and its electromotive force rela- 

 tively feeble, yet it is in all respects a complete electrical organ. It 

 is situated on either side of the vertebral column in the tail. The 

 plates or discs are placed transversely and in vertical planes. The 

 nerves enter their anterior surfaces ; the shock passes in the organ from 

 anterior to posterior end. Gotch and Sanderson have estimated the 

 maximum electromotive force of a length of I cm. of the electrical 

 organ of the skate at about half a volt. 



Whether the electrical organ is the homologue of muscle or of nerve- 

 ending, or whether it is related to either, has been much discussed. 

 Our surest guide in a question of this sort is the study of development ; 



