1. The electrical organs in the torpedo.— The torpedo is a flat fish, possessing the same general appearance and structure as the rays, and classed along with them in zoological sys tems. The electrical organs occupy a large coverings are discovered investing the electrical organs. The outer one has longitudinal fibres, which are rather loosely adherent, and, around the margins of the organs, seem to inosculate with the skin. The inner fascia is of consider able density, forms the immediate tunic of the electric columns, and sends processes down between them to form their partitions. Through out their whole extent, the essential part of the electrical organs is formed by a whitish soft part of the broad expansions of the body, which in the other allied fishes are formed only by the lateral fins. They form two sepa rate masses, one on either side of the head and gills, extending outwardly to the cartilaginous margins of the great fins; and, posteriorly, to the cartilage which separates the thoracic from the abdominal cavity. Their form and the honey-comb embossments of their surfaces can be distinguished through the skin both of the dorsal and ventral aspects. The common inte guments being removed, two strong fascia] pulp, divided into numerous pentagonal prisms by the fascial processes just mentioned. These lie close together, parallel with one another, and perpendicularly between the dorsal and ventral surfaces of the fish, so that their extre mities arc separated from these surfaces only by their fascia! and the common integuments. When these are removed, the columns present something of the appearance of a honey-comb. The columns are longest next to the head and gills, and thence gradually diminish outwardly, until, on the external margin, they are only about one-sixth of the length of the internal ones. In a fish described by John !hinter,* of which the whole electrical organ was about five inches in length, the longest column was about one inch and a half, and the shortest about one-fourth of an inch in length. In the same fish the average diameter of each column was about two-tenths of an inch. In a fish from the Mediterranean, thirteen inches and a half in length, and about seven inches in breadth, (which, through the kindness of Dr. Allen Thomson, we have had an opportunity of examining in detail,) the length of the longest columns is one inch, and that of the shortest about three-tenths of an inch. Most of these columns are either irregular pentagons, or irregular hexagons ; a few are nearly tetra gonal. They are united to one another by short but strong fibres, and by a reticular expansion of tendinous threads spread through them. Their number varies considerably ac cording to the age of the fish. Hunter con jectured that a few new columns are added every year to the circumference of the organ. In one of the largest fish that has yet been paiticularly examined, which was four feet and a half in length, the number of columns in one electrical organ was 1182. Sir. Hunter found 470 in each organ in a fish of ordinary size. Mr. Ilunter described each column as being divided into numerous distinct compart ments by delicate membranous partitions, placed horizontally, at very short distances from each other. The interstices between them appeared to him to contain a fluid. lle found the partitions in several places adhering to one another by bloodvessels; and all, throughout their whole extent, attached to the inside of the column by a fine cellular, membrane. In a column of one inch in length, he reckoned 150 partitions, and it appeared to him that their number is the same within the same space in all the columns:I- Hence, he thought it likely that " the increase in the length of a column, during the growth of the animal, does not enlarge the distance between each partition in proportion to that growth, but that new partitions are formed and added to the extre mity of the column from the fascia." The partitions are covered with fine network of arteries, veins, and nerves. According to II unter, " they are very vascular." Ile described the numerous arterial branches which ramify on the walls of the columns as " sending in wards from the circumference all around, on each partition, small arteries which anastomose upon it, and passing also from one to the other, unite with the vessels of the adjacent parti tions." The partitions themselves are so deli cate as not to admit of being satisfactorily examined in the fresh fish : (all Hunter's obser vations were made upon fish that had been preserved in spirits, by which, doubtless, the delicate membranes were rendered more opaque, and therefore more easily visible.) In point of fact, Dr. Davy has never seen them in the course of the numerous dissections which he has made of the electrical organs in fish recently taken ; whereas, in specimens sent hither by him, preserved in spirits, Dr. Allen Thomson and the writer of this article have satisfactorily ascertained their existence and structure as described by hunter. Dr. Davy says, " when I have examined with a single lens, which magnifies more than 200 times, a column of the electrical organs, it has not exhibited any regular structure; it has appeared as a homo geneous mass, with a few fibres passing into it in irregular directions, which were probably nervous fibres."' However, after having im
mersed the organs in boiling water, Dr. Davy has occasionally seen something like a lami nated structure within the column. Itudolphi satisfied himself of the division of the columns by membranous partitions, and further, that each partition is supplied with a distinct nerve.-1 In a memoir on the comparative anatomy of the Torpedo, Gymnotus, and Silurus, Geoffroy described t the columns as being filled with a semifluid matter composed of gelatine and albumen.
A large quantity of fluid enters into the composition of the general mass of the elec trical organs. Dr. Davy has found that they lose more by drying than any other part of the fish—nearly 93 per cent.; while the soft parts in general, including the electrical organs, lose only 84.5 per cent.§ ile believes that the fluids of the organs hold various substances in solution, but the exact nature and proportions of them have not been ascertained. We are indebted to the same indefatigable observer for an ac count of the specific gravity of the electrical organs. lie found it to be very low compared with that of the truly muscular parts of the fish,—namely, 1.026, to water as 1.000, while that of a part of the abdominal muscles of the same full-grown fish was 1.058, and of the dorsal muscles 1.065. In a fish eight inches long, five inches across the widest part, and which weighed 2065 grains entire, the electric organs together weighed 302 grains, the liver only 105 grains.
No contraction has ever been seen in the electrical organs of living fish under the stimu lus of the strongest excitants, not even under that of galvanism ; so that, although what appear to be tendinous threads are spread amongst and over the columns, we have no reason to suppose that any muscular tissue enters into their composition. But, in all directions, they are exposed to the pressure of strong muscles, such as are plainly designed to compress them. Some of these are inserted into the marginal cartilages of the fins; and there is a set of very powerful ones, arranged in a cruci form manner on the ventral surface, so placed as to compress the electrical organs most strongly during their contraction. Dr. Davy remarks, " It is only necessary to compare these muscles as they exist in the torpedo with the same in any other species of ray to be convinced that they are adequate to, and designed for, the compression of the batteries." Some observers, as John Hunter, state that a large proportion of blood circulates through the electrical organs. Girardi found the torpedo much more full of blood than the other rays.* But Dr. Davy says, that there are very few vessels containing red blood in the organs them selves; although their teg-,umentary coverings and the adjoining mucous system are highly vascular. The arteries of the organs are branches from the arteries of the gills ; their veins run between the gills direct to the auricle. The temperature of the electrical organs is not at all higher than that of other parts of the fish. All anatomists who have examined the torpedo have had their attention much arrested by the great size of the nerves distributed to the electri cal organs. These consist of three principal trunks, all arising immediately from the cerebro spinal system. The two anterior trunks are re garded by Desmoulins and Majendie t as portions of the fifth pair of nerves, and the third as a branch of the eighth pair. But the first electrical nerve seems to have an origin altogether distinct from the root of what is unquestionably the main portion of the fifth pair, although it certainly is in very close proximity with it, and, in passing out of the cranium, the two nerves seem to be in some degree united for a short space. Immediately beyond this point of union, the electrical nerve sends a soft twig to a small cavity within the adjoining cartilage, (which Dr. Davy thinks is the ear,) and then divides into three small branches, and two large ones. One of the small branches goes to the gills, another to the neighbouring muscles, and the third to the mouth. The first of the large branches runs along the outer margin of the electrical organ, advancing first anteriorly, then going round to the posterior part of its circumference, and losing itself in the mucous glands of the tegu mentary system, without sending a single twig into the electrical organ itself. The other great branch is inferior to the former in position, but much more voluminous ; it enters the electrical organ, and is ramified through its anterior third part, passing between its columns, and giving off numerous twigs for the supply of the walls of the columns, and the partitions, on which it terminates ; some of which pass even into the gelatinous matter with which the columns are filled. This branch, from its very origin, has all its fibres separated, isolated, and parallel, held together only by cellular tissue, which also forms a kind of membranous sheath around the nerve. Just as it reaches the organ, it is divided horizontally into two portions, one of which runs near the upper surface, the other on the plaue between the lower and middle thirds of the thickness of the organ.