Personal Narrative Of Travels To The Equinoctial Regions Of America During The Years 1799-1804 - Volume 2 - By Alexander Von Humboldt And Aime Bonpland.
- Page 101 of 406 - First - Home
On Substituting Flame For The Drop Of Water, The
Communication Is Interrupted, And Is Only Re-Established, As In The
Gymnotus, When The Two Points Immediately Touch Each Other In The
Interior Of The Flame.
We are, doubtless, very far from having discovered all the secrets of
the electrical action of fishes which is
Modified by the influence of
the brain and the nerves; but the experiments we have just described
are sufficient to prove that these fishes act by a concealed
electricity, and by electromotive organs of a peculiar construction,
which are recharged with extreme rapidity. Volta admits that the
discharges of the opposite electricities in the torpedos and the
gymnoti are made by their own skin, and that when we touch them with
one hand only, or by means of a metallic point, we feel the effect of
a lateral shock, the electrical current not being directed solely the
shortest way. When a Leyden jar is placed on a wet woollen cloth
(which is a bad conductor), and the jar is discharged in such a manner
that the cloth makes part of the chain, prepared frogs, placed at
different distances, indicate by their contractions that the current
spreads itself over the whole cloth in a thousand different ways.
According to this analogy, the most violent shock given by the
gymnotus at a distance would be but a feeble part of the stroke which
re-establishes the equilibrium in the interior of the fish.* (* The
heterogeneous poles of the double electrical organs must exist in each
organ. Mr. Todd has recently proved, by experiments made on torpedos
at the Cape of Good Hope, that the animal continues to give violent
shocks when one of these organs is extirpated. On the contrary, all
electrical action is stopped (and this point, as elucidated by
Galvani, is of the greatest importance) if injury be inflicted on the
brain, or if the nerves which supply the plates of the electrical
organs be divided. In the latter case, the nerves being cut, and the
brain left untouched, the torpedo continues to live, and perform every
muscular movement. A fish, exhausted by too numerous electrical
discharges, suffered much more than another fish deprived, by dividing
the nerves, of any communication between the brain and the
electromotive apparatus. Philosophical Transactions 1816.) As the
gymnotus directs its stroke wherever it pleases, it must also be
admitted that the discharge is not made by the whole skin at once, but
that the animal, excited perhaps by the motion of a fluid poured into
one part of the cellular membrane, establishes at will the
communication between its organs and some particular part of the skin.
It may be conceived that a lateral stroke, out of the direct current,
must become imperceptible under the two conditions of a very weak
discharge, or a very great obstacle presented by the nature and length
of the conductor. Notwithstanding these considerations, it appears to
me very surprising that shocks of the torpedo, strong in appearance,
are not propagated to the hand when a very thin plate of metal is
interposed between it and the fish.
Enter page number
PreviousNext
Page 101 of 406
Words from 52411 to 52932
of 211397