208 



RARDWICKE'S SCIENCE-GOSSIP. 



[Dec. I, I860. 



ELECTRIC EISHES. 



THERE are some remarkable instances of the 

 generation of electricity in living animals, to 

 whom the power seems principally to be given as a 

 means of defence. Of these animals, the Torpedo 

 Raia appears to have been noticed at a very 

 early period, since we find a description of its 

 properties in the writings of Pliny, Appian, and 

 others. It inhabits the Mediterranean and North 

 Seas; its weight, when fully grown, is about 

 eighteen or twenty pounds. 



Fig. 247. The Torpedn. 



Fig. 21-7 is a representation of a female Torpedo, 

 the skin b having beeu flayed from the under-surface 

 of the fish to show the electric organs a. The 

 mouth, having the form of a crescent, is shown at 

 d ; the branchial apertures, five in number, at E ; 

 g p {/ the place of the anterior transverse cartilages ; 

 // // the exterior margin of the great lateral fin ; i its 

 inner margin on the confines of the electrical organ; 

 / the abdomen ; m m the place of the posterior 

 transverse cartilage, which is single, united with the 

 spine, and sustains the smaller lateral fins n n on 

 each side ; o is the anus, and p the fin of the tail. 

 Each electrical organ is about 5 inches long and 

 about 3 inches broad at the anterior end, and h an 

 inch at the posterior extremity. Each organ con- 

 sists wholly of perpendicular columns reaching from 

 the upper to the under surface of the body, and 

 varying in their lengl lis according to the thickness 

 of the parts of the body where they are placed. 

 The longest column is about H inch, the shortest 

 about -4 of an inch, and their diameter about T 2 T of 

 an inch. The figures of the columns are irregular 

 hexagons or pentagons, and sometimes have the 

 appearance of being quadrangular or cylindrical. 

 The number of columns in the fish examined by 

 John Hunter was 470 in each organ ; but in a very 

 large fish, 1 i feet long and weighing 73 pounds, the 



number was 1,182 in each organ. The number of 

 partitions in a column 1 inch long was 150. 



The Torpedo must be irritated to cause it to give 

 a shock, in the delivery of which it moves its pec- 

 toral fins convulsively; the shock is felt on touching 

 the fish with a single finger, and it can give a long 

 series of shocks with great rapidity. When the 

 Torpedo is placed on a metallic plate, so that the 

 plate touches the inferior surface of the organs, the 

 hand that supports the plate never feels any shock ; 

 though another insulated person may excite the 

 animal, and the convulsive movement of the pectoral 

 fins may denote the strongest and most reiterated 

 discharges. Direct contact with the electrical organs 

 of the fish is indispensably necessary for the recep- 

 tion of the shock, but the Torpedo has not the 

 power of directing its electrical discharge through 

 any particular object. 



By passing the discharge from a Torpedo through 

 a spiral of copper wire enclosing a steel needle, the 

 needle becomes magnetized in such a manner as to 

 show the direction of the current to be from the 

 back to the under part of the belly. Heating and 

 chemical effects have likewise been obtained. Accord- 

 ing to the experiments of Matteucci : — 1. All the 

 dorsal parts of the electrical organ are positive to all 

 the ventral parts. 2. Those points of the organ on 

 the dorsal face which are above the nerves which 

 penetrate this organ are positive relatively to other 

 points of the same dorsal face. 3. Those points of 

 the organ on the ventral face are negative relatively 

 to other ^points of the same ventral face. 



The Chjmnotus. 



This electrical fish is a native of the warmer 

 regions of America and Africa. There are several 

 species of the Gymnotus, but only one is electrical. 

 In general aspect it very much resembles an eel— 

 the body is smooth, and without scales (a pecu- 

 liarity of all electrical fishes). The electric organs 

 consist of alternations of different substances, and 

 are most abuudantly supplied by nerves ; their too 

 frequent use is succeeded by debility and death. 

 The electric organs may be removed without injury 

 to the fish. 



Eig. 218 is a copy of Hunter's engraving of the 

 Gymnotus, and fig. 219 is a correct representation 

 of a fine specimen which was for some time in the pos- 

 session of the proprietors of the Royal Polytechnic 

 Institution. In Hunter's engraving the skin is 

 removed to show the structure of the fish ; a is the 

 lower surface of the head ; c the cavity of the 

 belly; b the anus; e the back where the skin 

 remains ; g g the fin along the lower edge of the 

 fish ; e e the lateral muscles of this fin removed and 

 laid back with the skin to expose the small organs ; 

 / part of the muscle left in its place ; // the large 

 electrical organ ; h h the small electrical organs ; 

 m m \m the substance which separates the two 



