Because the animal appears only to excite it when irritated or otherwise disturbed. Then the fish is observed to twist its body, as if about to make a vigorous muscular exertion ; and a benubming sensation is instantly felt in the fingers, and even as far as the elbows. The fish is capable of making this benumbing effort many times in succession in the water, as well as in the air, when arrived at maturity, and even previous to the natural period of exclusion from the uterus of the mother. When caught in the net, it gives a shock to the hands of the incautious fisherman, who ventures to seize it. When concealed in the mud, it is capable of making its most violent efforts; and is able to benumb the limbs to such a degree, as to throw down the passenger who inadvertently places his foot upon the body.
We have already noticed the electrical organs of fishes in connexion with the Voltaic apparatus.* Their structure should be more specially mentioned here, although, to explain this fully would occupy more space than we can devote to the subject, great as is its naturo-philosophical interest. - The organs of the torpedo (of the ray tribe) are double, and occur in the fore part of the body, one on each side of the cranium, and extend back as far as the gill-openings, occupying the whole skin between the upper and under surfaces.
* See Popular Chemistry, pp. 40 and 47, PartV.
In the gymnotus electricus, (or electric eel) the organs occupy scarcely one half of the body of the animal, being two on each side, and extending along the sides and belly, from the head to near the tail. In the silurus electricus, found in African rivers, the structure of the organ is more simple than in the torpedo or gymnotus. It consists of a bed of very fine meshes, which cross each other in every direction, the whole being covered by a membrane, which is itself covered with a layer of fat. The nerves with which the organ is provided, proceed from the eighth pair; but are not so large in proportion as in the torpedo. - Abridged from Fleming's Philosophy of Zoology, vol. i.