ELECTRO-PHYSIOLOGY 737 



nerve supplying each lateral half of the organ is distributed, so that 

 each half of the organ represents a battery of many cells arranged 

 in series. 



In Gymnotus the plates are vertical, and at right angles to the 

 long axis of the fish, and the nerves are distributed to their posterior 

 surface ; 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 hori- 

 zontal ; 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 th'j discharge is discontinuous ; an active fish may give as 

 many as 200 shocks per second. 



The electrical nerve of Malapterurus is very peculiar. It con- 

 sists 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 ene- 

 mies or their prey. Indeed, 

 Gotch has estimated the elec- 

 tromotive force of i cm. of the 

 organ of Torpedo at 5 volts. 

 Schonlein finds that the electro- 

 motive force of the whole organ 



may be equal to that of 31 



Daniell cells, or o'o8 volt for FIG. 293. DIAGRAM SHOWING DIREC- j 

 each plate, and it is one of TION OF SHOCK IN TORPEDO. 



the most interesting questions 



in the whol? of electro-physiology, how they are protected 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 

 surrounding it, and probably much greater. The central nervous 

 system and the great nerves must be struck by strong shocks, yet 

 the fish itself is not injured ; nay, more, the young in 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 extremely strong currents to 

 stimulate them ; and the electrical nerves are more easily excited 

 mechanically, as by ligaturing or pinching, than electrically. In 

 general, too, the shock is more readily called forth by reflex mechani- 

 cal 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 



47 



