340

PRODUCTION OF ELECTRICITY BY ANIMALS.

Animal Electricity.

416. Almost all chemical changes are attended with some alteration in the electric state of the bodies concerned; and when we consider the number and variety of these changes in the living animal body, it is not surprising that disturbances of its electric equilibrium should be continually occurring. But these, when slight, can only be detected by very refined means of observation; and it is only when they become considerable, that they attract notice. Some individuals exhibit electric phenomena much more frequently and powerfully than others; and cases are occasionally recorded in the Human subject, in which there has been a most decided production of electricity, which manifested itself in sparks whenever the individual was insulated.

417. The sparks and crackling noise, however, which are occasionally observed on pulling off articles of dress that have been worn next the skin, especially in dry weather, are partly due to the friction of these materials with the surface and with each other; the production of electricity being greatly influenced by their nature. Thus, if a black and a white silk stocking be worn, one over the other, on the same leg, the manifestation of electricity when they are drawn off, especially after a dry frosty day, is most decided; but this would also be the case if they were simply rubbed together, without any connexion with the body.

418. In most animals with a soft fur, sparks may be produced by rubbing it, especially in dry weather; this is familiar to most persons in the case of the domestic Cat. But the electricity thus produced seems occasionally to accumulate in the animal, as in the Leyden jar, so as to produce a shock. If a cat be taken into the lap, in dry weather, and the left hand be applied to the breast, whilst with the right the back be stroked, at first only a few sparks are obtained from the hair; but after continuing to stroke for some time, a smart shock is received, which is often felt above the wrists of both the arms. The animal evidently itself experiences the shock, for it runs off with terror, and will seldom submit itself to another experiment.

419. But there are certain animals which are capable of producing and accumulating electricity in large quantities, by

ELECTRIC FISHES:-GYMNOTUS.

341

means of organs specially adapted for the purpose; and of discharging it at will, with considerable violence. It is remarkable that all these belong to the class of Fishes ;1 and that they should differ alike in their general conformation, and in their geographical distribution. Thus, the two species of Torpedo, belonging to the Ray tribe, are found on most of the coasts of the Atlantic and Mediterranean; sometimes so abundantly, as to be a staple article of food. The Gymnotus, or Electric Eel, is confined to the rivers of South America. The Malapterurus (commonly known as the Silurus) which approaches more nearly to the Salmon tribe, occurs in the Niger, the Senegal, and the Nile; and there are two other less known Fishes, said to possess electric properties, which inhabit the Indian seas.

420. Of all these, the Gymnotus (fig. 176) is the one which possesses the electric power in the most extraordinary degree. It is an eel-like fish, having nothing remarkable in its external appearance; its usual length is from six to eight feet, but it is said occasionally to attain

the length of twenty feet. This fish will attack and paralyse horses, as well as kill small animals; and the discharges of the larger individuals sometimes prove sufficient to deprive even Men of sense and motion. This power is employed by the fish to defend itself against its enemies; and even, it is said, to destroy its prey (which consists of other fishes) at some distance; the shock being conveyed by water,

Fig. 176.-GYMNOTUS.

as a lightning-conductor conveys to the earth the effects of the electric discharge of the clouds. The first shocks are usually feeble; but as the animal becomes more irritated, their power increases. After a considerable number of powerful discharges, the energy is exhausted, and is not recovered for some time; and this circumstance is taken advantage or in South America, both to obtain the fishes (which afford

1 Certain Insects and Mollusks have been said to possess electrical properties; but no special electric organ has been discovered in them.

342

ELECTRIC FISHES:-TORPEDO.

excellent food), and to make the rivers they infest passable to travellers. A number of wild horses are collected in the neighbourhood, and are driven into the water; the Gymnoti attack these, and speedily stun them, or even destroy their lives by repeated shocks; but their own powers of defence and injury are exhausted in the same degree, and they then become an easy prey to their captors.

421. The shock of the Torpedo (fig. 177) is less powerful; but it is sufficient to benumb the hand that touches it. From its proximity to European shores, this fish has been made the subject of observation and experiment more completely than the other; and some curious results have been attained. It seems essential to the proper

?

Fig. 177.-TORPEDO.

reception of the shock, that two parts of the body should be touched at the same time; and that these two should be in different electrical states. The most energetic discharge is procured from the Torpedo, by touching its back and belly simultaneously; the electricity of the back being positive, and that of the belly negative. When two parts of the same surface, at an equal distance from the electric organ, are touched, no effect is produced; but if one be further from it than the other, a discharge occurs. It has been found that, however much a Torpedo is irritated through a single point, no discharge takes place; but the fish makes an effort to bring the border of the other surface in contact with the offending body, through which a shock is then sent. This, indeed, is probably the usual manner in which its discharge is effected. If the fish be placed between two plates of metal, the edges of which are in contact, no shock is perceived by the hands placed upon them, since the metal is a better conductor than the human body; but if the plates be separated, and, while they are still in contact with the opposite sides of the body, the hands be applied to them, the discharge is at once rendered perceptible, and may be passed through a line formed by the moistened hands of two or more persons. In the same manner, also, a visible spark may be produced; but this is less easily obtained from the Torpedo than from the Gymnotus.-As to the uses of the electrical organs to the

STRUCTURE OF ELECTRIC APPARATUS.

343

Fishes which possess them, no definite information can be given. It is doubtful to what extent they are employed in obtaining food; since it is known that the Gymnotus eats very few of the fishes which it kills by its discharge; and that Torpedos kept

in captivity do not seem disposed to exercise their powers on small fishes placed in the water with them. The chief use of their electrical power appears to be, to serve as a means of defence against their enemies.

422. The electric organs of the Torpedo (fig. 178) are of flattened shape, and occupy the front and sides of the body, forming two large masses, which extend backwards and outwards from each side of the head. They are composed of two layers of membrane, the space between which is divided by vertical partitions into hexagonal cells, e, like those of a honey

c,

brain; me, spinal cord; o, eye and optic nerve; e, electric organs; np, pneumogastric nerve, supplying electric organs; nl, lateral nerve; n, spinal nerves.

comb, the ends of which are directed towards the two surfaces of the body. These cells-which are filled with a whitish soft pulp, somewhat resembling the substance of the brain, but containing more water—are again subdivided horizontally by little membranous partitions; and all these partitions are profusely supplied with vessels and nerves.-The electrical

344

STRUCTURE AND ACTIONS OF ELECTRIC ORGANS.

organs of the Gymnotus are essentially the same in structure, but differ in shape in accordance with the conformation of the animal; they occupy one-third of its whole bulk, and run nearly along its entire length, being arranged in two distinct pairs, one much larger than the other. In the Malapterurus

(fig. 179), there is not any electrical organ so definite as those just described; but the thick layer of dense areolar tissue which completely surrounds the body, appears to be subservient to this function; being composed of tendinous fibres interwoven together, and containing a gelatinous substance in its interstices, so as to bear a close analogy with the special organs of the Torpedo and Gymnotus.

423. In all these instances, the electrical organs are supplied with nerves of very great size, larger than any others in the same animals, and larger than any nerves in other animals of like bulk. These nerves arise from the top of the spinal cord, and seem analogous to the pneumogastric nerve (§ 458) of other animals. The influence of these nerves is essential to the action of the electric organs. If all the trunks on one side be cut, the power of the corresponding organ will be destroyed, but that of the other may remain uninjured. If the nerves be partially destroyed on either or both sides, the power is retained by the portions of the organs which are still connected with the brain by the trunks that remain. Even slices of the organ entirely separated from the body, except by a nervous fibre, may exhibit electrical properties. Discharges may be produced by irritating the part of the nervous centres from which the trunks proceed (so long, at least, as they are entire), or by irritating the trunks themselves. In all these respects, there is a strong analogy between the action of the nerves on the electric organs and on the muscles (Chap. XII.); and it may be safely affirmed that the Nervous force develops