92 



ANIMAL ELECTRICITY. 



ning from the abdominal carity towards the tail, 

 placed one above another with short distances 

 between them ; the other of perpendicular plates, 

 forming, along with the other series, small quad- 

 rangular cells, which are rilled with a semi-gela- 

 tinous transparent substance. This structure is 

 divided longitudinally into two pairs of distinct 

 organs, one considerably larger than the other. 

 The greater pair (k k, fig. 49) lies above the 

 other, and immediately beneath the long mus- 

 cles of the tail. They are separated from one 



another by part of these muscles, by the air- 

 bladder, and by a central membranous partition. 

 They occupy a large portion of the lower and 

 lateral parts of the body, and are covered exter- 

 nally only by the common integuments. The 

 smaller pair are covered also by the muscles of 

 the caudal fin. Both pairs of organs are some- 

 what angular in their transverse section, trun- 

 cated anteriorly, tapering towards the tail. In 

 theGymnotus dissected by'John Hunter,* which 

 was about two feet four inches long, the large 



The surface of the electrical organs of the Gymnotus, on the right side, after removal of the integuments. 



, the lower jaw. 6, the abdomen, c, anus, d, pectoral fin. e, dorsal surface of fish, ff, anal 

 fin. ag, skin turned back, h h, lateral muscles ot' the anal fin turned back with the skin, to expose 

 the small electrical organ. *, part of this muscle left in its place, k k, the large electrical organ. / I, 

 the small electrical ortcan. m m, the substance which divides the large organ from the small, n, a 

 space from which the partition is removed. 



Fig. 50. 



A transverse section of the Gymnotus. 



a, the surface of the side of the fish. 6, the anal fin. c c r 

 cut ends of the dorsal muscles, d, cavity of the air-bladder. 

 e, body of the spine, f, 'spinal marrow, g, aorta and vena 

 cava. h h, cut ends of the two large electrical organs, i r, 

 cut ends of the two small organs, k, partition between the 

 two organs. 



organ of one side was about one inch 

 and one quarter in breadth at its 

 thickest part, and in this space there 

 were thirty-four longitudinal septa. 

 (In a specimen examined by Dr. 

 Knox, there were thirty-one of these 

 septa.f) The smaller organ in the 

 same fish was about half an inch in 

 breadth, and contained fourteen septa, 

 which were slightly waved. The per- 

 pendicular or transverse membranes 

 are placed much more closely toge- 

 ther than those of the other series. 

 John Hunter and Dr. Knox counted 

 two hundred and forty of them in 

 an inch. They are of a softer texture 

 than the longitudinal plates. It ap- 

 pears probable (as Hunter suggested) 

 that these septa, longitudinal and 

 transverse, answer the same purpose 

 as the columns in the torpedo. La- 

 cepode calculated that the discharg- 

 ing surface of these organs in a fish 

 four feet in length is, at least, one 

 hundred and twenty-three square feet 

 in extent; while in a torpedo of ordi- 

 nary size, the discharging surface is 

 only about fifty-eight feet square. 



The nerves of the electrical organs 

 of the Gymnotus are derived from the 

 spinal marrow alone. They are very 

 large and numerous, and are divided 

 into very fine twigs on the cells of the 

 organs. Dr. Knox counted fifteen 

 nervous branches distributed to each 

 inch of the organ. He describes 

 them as being flattened like the ci- 

 liary nerves of Mammalia. Each 



* Phil. Trans. Ixv. 1775. 



t Edin. Journ. of Science, i. 96. 1824. 



