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XII
(B)
SECOND
LETTER
25
October 1792
SOURCES
| PRINTED |
HANDWRITTEN |
| Phil.
Tr.
P. 1. 1793, p. 27
Ant.
Coll. T. II, P. 1. p- 141 |
Cart.
Volt. J 16, L 8, N 26
|
|
REMARKS
TITLE from Phil. Tr.
DATE from Phil. Tr. (§ 8 of Memoir III on Animal
Electricity – letter to Aldini – V says this letter
was written in August).
______________________
J 16 consists
in various fragments written in French.
L 8 is a
complete Ms. with common additions (§ 52 and part of §
52) and passages from handwritten sources of No. VIII ( C
)
In Cart. Volt.
No. 26, there is a letter of acknowledgement, thanks and
congratulations from Sir
Joseph Banks,
the President of the London Royal Society, dated London.
27 November 1792
|
SECOND
LETTER
25
October 1792
(24.)
Besides, it is clear that my observations, relating to
the sciatic nerve and the leg, also apply to the brachial nerve
and the arm. They also apply to any other nerve and to the
muscles and limbs governed by those nerves.
(25.) These preparations are similar to those described by
Mr. GALVANI. They
prove that it is advantageous to expose the nerves and even
better to dissect them free. This condition, however, is not
absolutely necessary, because we obtain the same convulsions and
motions of the limbs when we simply uncover the muscles and
leave the nerves hidden under the muscles in their natural
position, as shown by all my other experiments reported
previously. (Section 21, 22, 23).
(26.) After performing these experiments on reptiles,
birds and small quadrupeds, I studied larger animals: rabbits,
dogs, lambs, bovines.
The effects were not
only similar to those described previously but even stronger and
more durable, because the vital heat persisted for a longer time
in those large animals and in their limbs. On the other hand I
must mention that in most cold-blooded animals and particularly
in frogs, viability persists for several hours in severed limbs.
This prolonged viability, which renders the limbs so sensitive
to weak electrical stimulations, lasts only a few minutes in
warm-blooded animals and usually disappears before the animal
heat has vanished.
(27.) After performing successful experiments in large and
small animals of all species, be they intact or skinned,
beheaded or variously dissected, and also on severed large limbs
(most
of the time without using the preparation called
for by Mr. GALVANI,
namely, without exposing the nerves)
I decided to go further, that is, to study small limbs, or even
single muscles or small fragments of a muscle. The new successes
I thus achieved led me to other discoveries, which I shall
present shortly, after describing some of my experiments.
(28.) EXPERIMENT E. In a frog, I severed a leg with the thigh
attached, or a leg alone, or even one half or one quarter of a
leg. By applying the tin foil to a part of the severed limb and
the silver lamina to a different part, as usual
and on
establishing contact between the two metal plates, I always
obtained some convulsions and movements. In some cases I
separated a single muscle, e.g. the gluteus or the gastrocnemius
or even a small fragment of those muscles, no larger than a
grain of barley. The
same effects were obtained, namely, application
of two plates elicited
brisk, spasmodic contractions of the entire muscle or fragment.
EXPERIMENT F. I repeated the same experiments on a whole leg,
part of it, or a muscle or a muscle fragment, in chickens and
other birds ;
in slices of the gluteus
muscle of a rabbit, lamb etc., and
I obtained the same results, as long as the flesh was warm.
(29.)
Thus, we elicit strong contractions in the muscles of
warm-blooded and cold-blooded animals, and in all resected
fragments of the muscles, This is achieved by directly applying
the metal plates to the muscle itself, without preparing or
exposing the nerves. On the other hand, we previously showed
that we can stimulate the muscles by applying the metal plates
to two neighboring parts of a nerve (Sections 19 and 20,
experiments A and B). I draw the conclusion that it is not
necessary to provoke a discharge of electric fluid between the
nerve and the muscle or to move the fluid from the inside to the
outside of the muscle through the nerve and the conducting
circuit, or vice versa, as Mr. GALVANI assumes. I also
conclude that our experiments do not suggest any similarity
between the muscle and the Leyden flask. Where is the similarity
with the flask, when the two metal plates, connected by a
conducting wire, are applied, in close proximity to each other,
on the surface of the same nerve (Experiments A and B) or on the
surface of two similar muscles, or on the same muscle? (Experiments
C, D, E, G). We must admit that any effort to claim a similarity
with the Leyden flask would be vain.
(30.) EXPERIMENT G. If we apply silver foil to
one thigh of a frog
and a tin foil to
the corresponding region on the other thigh, we elicit
contraction of the muscles and the ordinary movements of the
legs as soon as we connect the two plates with a conducting arc.
(31.) Is this the way, I wonder, to induce the discharge of
two Leyden flasks, by connecting two homologous surfaces? Let us
abandon these ideas of flasks and discharges and these
far-fetched explanation; let us simply state that here, and in
all similar experiments, we have a transport of electric fluid
from one side to the other, provided the two sides are properly
covered with metal plates. The transport is brought about not by
a relative excess of fluid, which is difficult to imagine
between two similar parts (of the body), but by the different
nature of the plates, which must be made of different metals, as
I explained previously (Sections 20 and 21. Experiments B and C)
and in subsequent writings.
Actually
(32.) if two muscles, or two sites on the same muscle are
similarly "armed", that is, they are covered with
plates of the same metal, having similar tempering and hardness,
flexibility or stiffness, smoothness or roughness, and the
plates are applied in the same way, we can connect them with a
conducting arc
but will obtain no convulsion or movement.
(33.) I recognize that it is not easy to understand how and
why, by applying two different metals to similar parts of an
animal, and even to two neighbouring points on the same muscle,
we disturb the balance of the electric fluid and cause it to
flow continuously from one site to another; and why the flow
starts as soon as we connect the two plates with a conducting
object and lasts as long as the connection is maintained. Anyhow,
whether or not we are able to understand their cause, these
facts are proven by the experiments I described and will be
confirmed by many others. At the end of the description, I will
try to provide some explanations. These are facts that we must
add to our present knowledge of electricity, even though they
may seem extraordinary and difficult to reconcile with the
established laws. What I have
discovered is a new,
very peculiar law, which does not relate to animal electricity,
but to common electricity. In effect, the flow of electric fluid
is not short-lived, like a discharge, but continuous, and
persists as long as the connection between the two plates is
maintained, whether the plates are applied to living or dead
animal tissues, or to other non-metallic conducting objects,
provided they are good conductors, like water or moist bodies.
However, before I proceed to describe
new experiments that definitely prove my contentions, I must
briefly dwell upon those I have already reported (Section
20-32).
(34.) These experiments apparently suggest that we can elicit
strong contractions in any muscle of any animal, provided they
are viable, by simply applying two plates made of different
metals. Such a conclusion, however, is too general and my
experience taught me that some restrictions must be applied,
relating to both the classes and genera of animals and to
the different muscles of a given animal.
(35.) With regard to the different classes of animals: while
it is true that all the quadrupeds, birds, fish, reptilians and
amphibians which I tested do exhibit the phenomena I described,
it remains true that worms and many insects do not. I vainly
tried with earthworms, leeches, slugs, snails, oysters,
caterpillars. I was unable to elicit any movements, even by
means of small or medium-sized sparks or with discharges of
artificial electricity. I proceeded as follows:
EXPERIMENT I. I applied the tin foil and the silver plate to
various external and internal parts of slugs, leeches,
earthworms, to the best of my ability. I established
communication between the metal plates, either by bringing them
closer and closer to each other until they touched, or by
connecting them through another metal object. however, by
performing all these manoeuvres,
I never obtained any movement in any part of their body.
EXPERIMENT L. I made flask discharges to pass through their
body, both insulated and non-insulated. The discharges were
strong enough to elicit a medium size spark and to give me a
little shock; no movements or convulsions appeared.
(36.) Shall we conclude that the most imperfect animals, the
whole class of worms and many insects don’t have the same
sensitivity and irritability, the same electrical mobility, if I
may use this expression, which is observed in more perfect
animals? I don’t want to draw this general conclusion from my
experiments, which I performed, so far, on a small number of
worms and insects. On the other
hand, I must say that
my experiments were successful, without great difficulty, in
crayfish, scarabs, grasshoppers, butterflies and flies. It will
be useful here to explain how I handle these animals, which are
difficult to experiment with, due to their small size or because
they are covered with scales.
EXPERIMENT M. After beheading the fly, butterfly,
scarab
etc., I cut their chest wall, over its entire length, with a
penknife or small scissors and I introduce a piece of tin foil
into the wound, near the neck (so-called
silver paper is very convenient for this purpose). Then I deeply
introduce, at a more caudal level, the edge of a silver plate or
a small coin. When I push the latter object until it touches the
tin foil, the legs start bending and wriggling and the other
parts of the body also start shaking. It is amusing to elicit
the song of a cicada etc. in this way.
(37.)
Thus, it would be wrong to class the insects among
those animals that are devoid of the electric properties we are
discussing here. At most, if the caterpillars appear to lack
these properties, we can say that in the larva stage,
before they attain perfect
shape through metamorphosis, and before they acquire new organs
in the process, they are similar to worms in many respects,
including lack
of electrical sensitivity.
(38.)
Finally, if I may express my thoughts here, only those
animals that have well formed limbs, joints and muscles that
bring about the motion of the limbs, in particular the flexor
and elevator muscles, and the nerves which govern them, react
with a real, spasmodic contraction to weak discharges of
artificial electricity , or to a weak flow of electric fluid
produced by two different metal plates. The spasmodic
contractions provoke the motion and violent shaking of the limbs.
By contrast, worms and those insects that have no well-developed
limbs and joints, have no flexor muscles and exhibit only
vermicular movements are not affected by this kind of
electricity. This is a quite different animal organization, a
different mechanism of motion, that has been discovered and
explained for many species. These ideas of mine, which are still
somewhat vague, are based on experiments. Further experiments
will confirm or correct them.
(39.) With regard to different muscles in the same animal, I
can speak with more certainty. It is not correct that any muscle
will react with a contraction to the weak currents we use here.
An important distinction must be made regarding their function
in the animal organization. All muscles are not subject to the
rule of will, and capable of producing spontaneous movements. I
found that only those muscles that are governed by the will can
be stimulated and produce a movement as a result of a weak flow
of electric fluid produced by the contact with two different
metals. This is not the case for those muscles that are not
directly governed by the will, e.g., the muscles of the stomach,
bowels, etc, including the heart, which is usually so irritable.
The muscles of the diaphragm do react to the current (I
predicted this response before making the experiment) because
their motion is governed by the will.
EXPERIMENT N. It is surprising that a slice of good muscular
flesh, cut out, e.g., from the thigh of a lamb that was
slaughtered one half hour or one full hour before; it is
surprising, I say, that this piece of muscle, quite cold,
insensitive to any mechanical or chemical stimulation, is so
strongly affected by the electrical fluid transmitted from one
part to another, to the point of being seized by very strong
spasmodic contractions; whereas the heart, recently removed from
the same animal, when it is still warm and very irritable, is
not affected at all by the same treatment, namely, the
application of well fitting metal plates, connected by a
conducting body. Again, it is surprising that the heart beat,
when it is weak and slow, does not become more frequent; or,
when the heart is quiescent or dormant, it does not wake up
again, as it does after a very weak mechanical or chemical
stimulation.
(40.)
Thus, the electric fluid, which seems to be the proper
stimulus for the muscles governed by the will, is not an
effective stimulus for the heart and the other muscles whose
animal and vital movements are not controlled by the will. But
what would you say if I were to show that the electric fluid is
not even the direct, effective cause of the movements of the
"voluntary" muscles, that is, the muscles controlled
by the will? In these muscles, the electric fluid is only an
indirect cause, because it directly affects only the nerves.
This is what I learned from many experiments, which forced me to
abandon several attractive and much broader concepts. I was
attracted by the idea, which was also advanced by Mr. GALVANI,
that when the electric fluid hits the muscles with sufficient
strength, it acts as a stimulus which arouses their irritability;
that all muscular movements result from the flow of electric
fluid into the muscles, whether we use the artificial
electricity or the natural animal electricity; that the
movements that occur naturally in the living animals, or at
least the voluntary movements, were brought about by the same
cause, namely, the direct action of the electric fluid on the
muscles. However, I repeat, I had to abandon, with regret, all
these attractive ideas that seemed to provide a wonderful
explanation for our observations. Yes, we must assign certain
limits to the action of electricity in animals, which should be
conceived as being capable of directly exciting the nerves, as I
previously stated and will now demonstrate [1]
(41.) To begin with, the fact that electricity acts on nerves,
and that the nerves, thus stimulated, excite all the attached
muscles, even without the current reaching the muscles, has been
proved by Experiments A. and B., and also by an experiment by
Mr. GALVANI, which was the first experiment of all, and
originated the subsequent experiments, according to his own
narration. In these experiments of the Professor from Bologna,
and in my own experiments which I just mentioned ,
it is clear that the current crosses only a portion of the
crural nerve and
does not flow through any muscle of the leg. Nevertheless the
muscles of the leg, which are controlled by that nerve, undergo
convulsive contractions.
(42.)
Moreover, I maintain that even when the electric
current (here, I mean weak artificial discharges, or those
currents that are generated by applying two plates of different
metals) hits and penetrates the muscles that respond by
movement, it does so by stimulating their nerves, not by direct
excitation of the muscles themselves. This is shown by my
Experiments C. and D. (Sect. 21 and 23), where the tin foil and
the silver plate are directly applied to the muscles of the
animal, whether whole or dismembered. Here, the muscles that
show the most violent contractions are not those covered by the
two metal plates, but those that are controlled by a main nerve
trunk, which happens to be close to one of the plates. Thus, in
a frog, when we apply the tin foil on the lumbar region, where
the crural nerves are located at moderate depth, the leg
muscles are seized by strong convulsions, more than any other
muscle, including those that touch the other plate, that is, the
silver plate. I already showed this behaviour
in quadrupeds, dogs, lambs etc., in relation to
the sciatic nerve (experiment D) and I want to add that the leg
does not fail to be shaken when the nerve is not too deeply
hidden under the skin and other tissues, provided we apply one
of the plates to the proper location. This happens even when the
other plate is not located near the gluteus muscle or any
other leg
muscle, if it is not too far away. This is the reason:
EXPERIMENT O.: After beheading a lizard and exposing its
dorsal muscles by removing the skin, I apply a piece of tin foil
to the truncated upper end of the torso, in such a way that the
foil goes a little beyond the truncated area and reaches the
shoulders; then, I place a silver coin over the middle of the
spine. Finally, I slide the coin until it touches the foil.
Immediately, the legs move, the tail bends tortuously and the
entire body bends and jumps from right to left and vice
versa. Isn't this due to the upper part of the spinal cord,
the main origin of the nerves, being irritated?
(43.) By means of a similar operation [2]
we can
obtain approximately the same result in mice, birds etc. However,
in these animals we must remove not only the skin and other
integuments, but also some flesh, because their back is more
fleshy and the main spinal nerves and the spinal cord are hidden
by this flesh and by the bones of the spine. It is easy to
understand that the flow of electric fluid, produced by the two
plates, can penetrate those parts of the body that are covered
by the two plates, but only to some extent. In particular, it
cannot reach the spinal cord, and the main branches of the
nerves, which go into the limbs, if the bones, flesh and other integuments
are very thick. Thus, we can easily understand why
in large animals (dogs, lambs etc.) we do
not succeed in
eliciting the movement of all limbs by applying the plates to
the back, even after stripping the flesh off. The large branches
of the nerves remain too deeply hidden and buried in the tissues.
Only a few branches or ramifications lie just under the plates and
these branches reach only the adjacent, superficial tissues. As
a consequence, we generally see only some shallow palpitations
and contractions in a few muscles. If, occasionally, an entire
limb is set in motion, this happens because the nerve that goes
into that limb and controls its movement is not deeply buried,
but is covered only by a thin layer of tissue, that separates it
from the metal plates. This was the case in Experiments D and
following trials (Section 23 etc.), where we obtained large
movements in a dog's or lamb's leg by just applying one of the
plates near the sciatic nerve; the thinner the layer of tissue
surrounding the nerve, the stronger the movements of the leg.
(44.)
Thus, to elicit full
movements of the limbs in large animals, and not only superficial
contractions and palpitations in
a few muscles, it is necessary to know the position and
direction of the nerves and
to remove the common integuments, the fat,
etc.,
and also to reduce the thickness of the tissues that cover and
surround the nerves, before applying the metal plate. It may
even be impossible to elicit those movements and convulsions in
all the limbs simultaneously; whereas this is easily obtained in
small animals, just by removing the skin and part of the other
integuments, as previously shown (Sect. 42, Experiments O and
P). This is not even necessary in frogs, where we can leave the
skin in place because the skin, which is very thin and moist,
does not prevent the electric current from reaching the main
nerves and the spinal cord.
(45.)
Thus, to elicit movements
of the limbs, we must take into consideration both the direction
of the main nerves and
the position of the plates in relation to the muscles. Those
muscles that are located in between the plates, or close to one
of the plates, are more likely to produce spasmodic contractions,
and are often the only ones that produce those contractions.
This can happen, for instance, when the plates are not
positioned near a large nerve trunk, or when the nerves are
deeply located and surrounded by thick layers of tissue.
(46.) These observations and Experiments E, F (Sect 28) where
a single muscle or even a fragment of muscle, treated in the
usual way, does produce strong contractions , might suggest that
the electric fluid elicits the movements by directly irritating
the muscle fibres,
without any nervous intervention. Thus, the role of the nerves
would be neither primary nor strictly necessary, as I
claim. However, this argument has no strength, so long as we
don't demonstrate that those muscles and fragments do not
contain nerve fibres. If, on the contrary, such fibres
are present (and nervous branches must be present in every
fragment that responds to the stimuli, even, I would say, in
every muscle fibre),
then I can affirm that it is the thin nerve filaments, which are
distributed throughout the muscle, that are directly affected by
the electric fluid when it flows through the muscular substance.
The fluid acts on the sensitive nerves, and no further, and the
nerves act on the muscles. Thus, I can maintain that, most
likely, the electric fluid has no direct effect on muscular
contractions, except insofar as it stimulates the nerves.
Briefly, that the electric fluid is not the direct cause of the
muscular contraction. This statement, which seems highly
plausible in view of the findings that I described in the
previous pages, will be verified, in the most obvious way, by a
number of experiments which I performed on the tongue. These
experiments led me to new discoveries, which are both
interesting and curious.
(47.) Having succeeded in eliciting tonic convulsions and
strong movements in the muscles and limbs of both small and
large animals, without exposing any nerve, just by applying the
plates made of different metals to the exposed muscles, I
wondered whether we could not obtain similar results in humans.
I realized that this could be achieved in amputated limbs. But
how about intact, living human beings? This would have required
removing the skin, performing deep incisions, cutting out part
of the flesh in the regions where we planned to apply the metal
plates (as I showed to be necessary in large animals).
Fortunately, it come to my mind that the tongue offers to us an
exposed muscle, not covered by the thick integuments that cover
the rest of the body, a muscle that is very mobile, and
controlled by the will. Here is, I thought, a muscle that offers
all the necessary conditions for eliciting strong movements by
means of the metal plates. With these considerations in mind, I
performed the following experiment on my own tongue.
(48.) EXPERIMENT Q. After covering the tip of the tongue and
part of its upper surface, over a few twelfths of an inch, with tin
foil (so-called "silver
paper"
is most suitable for this purpose) I applied the convex part of
a silver spoon further inside on the flat part of the tongue. By
tilting the spoon I brought its tail into contact with the tin
foil. I expected to see my tongue quiver and, to this end, I
performed the experiment in front of a mirror. The expected
movements did not materialize. Instead, I experienced an
unexpected sensation, a strong, sour taste on the tip of the
tongue.
(49.) At first, I was surprised by this event. However, after
some reflection, I realized that the nerves that reach the tip
of the tongue have the role of conveying taste
sensations not of controlling motion in
that flexible organ. Thus, it was natural that the irritation
caused by the electric fluid should elicit a sensation of taste
and nothing else. To produce movements in the tongue, it would
be necessary to apply the metal plates near its root, where the
motor nerves are located. I then verified this hypothesis with
another experiment.
(50.) EXPERIMENT R. In a recently slaughtered lamb I cut the
tongue out, near its root, and I applied tin
foil near the cut end and
the silver spoon on its surface. I then proceeded to establish
a contact between the two metals, as usual. I had the
satisfaction of seeing the whole tongue twitch vigorously, lift
its tip, twist and bend
on both sides every time and as long as the contact was
maintained.
(51.) I repeated this experiment with the tongue of a calf,
which I placed on a silver dish, after applying the tin foil to
its root. The silver dish played the role of the second plate.
The result was the same. I repeated the experiment by using the
tongue of other
small animals, like mice, chickens, rabbits,
etc.,
and I obtained the same effect almost every time. I am saying
"almost" because sometimes the effect failed in small
animals. This may be due to the tin foil not being properly
applied to the right place, where the motor nerves enter the
tongue, or to the tongue being already cold. As I already
pointed out (Sect.
26), viability is rapidly lost in warm-blooded animals,
particularly in the tongue.
I
am, etc.
October 25, 1792
A. VOLTA
Revised by John Coggan, Oxford University
Notes
[1]
J
16 here reads: "Thus, we must assign narrow
limits to the action and influence of animal electricity. We
would have loved to share GALVANI's idea, that electricity is
the direct and effective cause of all muscular movements.
Unfortunately, we cannot maintain these ideas because my
experiments show that those muscles that are not controlled by
the will do not respond to the action of electricity . Therefore,
their movement must be due to a totally different cause. In
effect, a fully viable heart, which responds with strong and
frequent contractions to all sort of mechanical and chemical
stimuli, is not affected (by electrical stimuli) (
translator’s addition)
and maintains its rhythm without change, or remains still if it
had already lost its rhythmical activity. [Editors’note]
[2]
in LS we
read: "preparation" [Editors’
note]
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