When examined with a high magnifying power, the minute branches of the electrical nerves present a dotted appearance, showing as if the medullary substance were arranged within the sheath, not in a continuous line, but in a succession of small portions with a little space between each.* The second electrical nerve rises a little be hind the former. After leaving the cranium, it divides into two large branches, which, with the exception of a few twigs which go to the gills, are wholly distributed in the middle third of the electrical organs, in the same manner as the first pair.
The third electrical nerve arises from the brain close to the second, from which, however, it is separated by a thin cartilaginous plate. The greater portion of it goes to the electrical organ, and is distributed through its posterior third. It also supplies part of the gills, the gullet, the sto mach, and the tail. Dr. Davy says it appeared to him that the branch of this nerve which goes to the stomach is the principal nerve of that organ : it is spread over its great arch.t The same observer also points out as deserving of particular attention, a very large plexus of nerves formed by a union of the anterior and posterior cervical nerves, of the former of which there are seventeen on either side, and only fourteen of the latter. This plexus presents itself as a single trunk just below the transverse cartilage that divides the thoracic from the abdominal cavity. It sends a recurrent branch to the muscles and skin of the under surface of the thorax; but the larger portion is distributed upon the pectoral fin and the neighbouring parts. The motive and sentient powers of the muscles and integuments connected with the electrical organs seem to depend on this plexus.
The only other peculiarity of structure in the torpedo which can be supposed to be in any way connected with its electrical power, is in the system of mucous ducts, which is much more fully developed in it than in any other ray with which we are acquainted. It consists of numerous groups of glands arranged chiefly around the electrical organs; and of tubes con nected with these, having strong and dense coats, filled with a thick mucus secreted by the glands. The tubes open chiefly on the dorsal surface of the skin, and pour out the mucus, which, probably, serves as a medium of com munication between the electrical organs ; being, apparently, a better conductor of electricity than either the naked skin or salt water.' With regard to the development of the elec trical organs, it appears that, in the earliest stages of fatal growth, they cannot be seen. In a foetus of about seven-tenths of an inch in length, Dr. Davy found neither electrical Organs nor fins. In another, more than one inch long, the organs were beginning to appear, and the roots of the electrical nerves were visible, although the brain could not be seen. In this stage, the external branchial filaments were about six-tenths of an inch in length, and pre organs were distinctly formed, and the branchial filaments still long. These filaments Dr. Davy
supposes to be destined to absorb matter for the formation of the electrical organs, and, perhaps, the gills and adjoining mucous glands. They are most numerous and of greatest length while the electrical organs are forming, appearing just before these organs begin to be developed, and being removed when they are tolerably com plete.—In no other allied fishes is there the same " elaborate apparatus of filaments ;" where they do exist, they are less numerous and very much shorter.
2. The electrical organs in the Gymnotus.— This fish has a general resemblance in form to the common eel. Its electrical organs occupy nearly one-third of its whole bulk. They are formed by two series of tendinous membranes ; one of which consists•of horizontal plates, run ning from the abdominal cavity 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 filled 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 the Gymnotus dissected by'John llun ter,* which was about two feet four inches long, the large 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 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 diseharging surface is only about fifty-eight feet square.