ELECTRO-PHYSIOLOGY 649 



many of the African rivers, the Nile in particular, and known for 

 ages, was scarcely investigated till forty years ago. 



In all these fishes there is a special bilateral organ immediately 

 under the skin, called the electrical organ. It is in this that the shock 

 is developed. It consists of a series of plates arranged parallel to 

 each other. To one side of each plate a branch of the electrical nerve 

 supplying each lateral half of the organ is distributed. This side of 

 the plate during the shock becomes negative to the other (Pacini's 

 rule), so that each half of the organ represents a battery of many cells 

 arranged in series. The direction of the shock through the organ 

 depends on the side of the plate to which the nerve-supply goes, and 

 the arrangement of the plates with reference to the natural position 

 of the animal. 



Thus, in Gymnotus the plates are vertical, and at right angles to 

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

 terior surface; the shock accordingly passes in the animal from tail 

 to head. In Malapterurus, although the arrangement of the plates 

 is the same, the nerve-supply is to the anterior surface ; for Max 

 Schultze has shown that although the nerve appears to sink into the 

 posterior surface, it really passes through a hole in the plate, and 

 spreads out on its anterior face. 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 discontinuous ; 

 an active fish may give as many 

 as 200 shocks per second. 



The electrical nerve of Mala- 

 pterurus is very peculiar. It 

 consists of a single gigantic 

 nerve-fibre on each side, arising 

 from a giant nerve cell. The FIG. 219. DIAGRAM SHOWING DIREC- 

 fibre has an enormously thick TION OF SHOCK IN TORPEDO. 



sheath, the axis cylinder form- 

 ing 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 

 estimated the electromotive force of i cm. of the organ of Torpedo 

 at 5 volts, and Schonlein finds that the electromotive force of the 

 whole organ may be equal to that of 31 Daniell cells, or o'oS volt for 

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

 whole 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 



