Transcriber’s Note: This book uses the convention that items printed in
the original text in italics are surrounded by underscores like _this_
and that items printed for emphasis in small-caps are rendered in all
caps.
EXPERIMENTS
AND
OBSERVATIONS
RELATIVE TO THE
INFLUENCE LATELY DISCOVERED BY
M. GALVANI,
AND COMMONLY CALLED
ANIMAL ELECTRICITY.
BY RICHARD FOWLER.
EDINBURGH:
PRINTED FOR T. DUNCAN, P. HILL, ROBERTSON &
BERRY, AND G. MUDIE; AND J. JOHNSON
ST PAUL’s CHURCH-YARD, LONDON.
1793.
------------------------------------------------------------------------
PREFACE.
The subject of the following experiments, has excited such general
curiosity, that every new fact respecting it, may afford some
gratification; and although the few which I have to offer, have not led
me to what many may think very important conclusions, they will not I
hope be found wholly undeserving of attention. The experiments were
begun, with the view of ascertaining if the influence discovered by M.
Galvani, be referrible to any known law of nature, or if it be itself a
new law.
Finding that it indicated, with tolerable accuracy, the presence of
very small degrees of the contractile power of muscles, without
appearing in the least to diminish that power, as electricity and most
other stimuli never fail to do; I thought it might be used with
advantage, as a test, in the investigation of some important subjects
in physiology; and I have accordingly employed it as such.
Every circumstance, observed in the course of these experiments, was
carefully noted down, at the instant it occurred, and the greater
number of these was made in the presence of gentlemen, whose accuracy I
had reason to hope would detect any fallacy, which might have escaped
myself. I have a particular pleasure, in expressing my obligation to Mr
George Hunter of York, for the very friendly assistance which he
afforded me, in almost every experiment, which rendered such assistance
necessary.
Edin. _May 28_. }
1793. }
------------------------------------------------------------------------
SECTION I.
_Are the Phenomena, exhibited by the Application
of certain different Metals
to Animals, referrible to Electricity?_
The whole train of circumstances, which preceded this discovery, had a
tendency to occasion the belief of its relation to electricity.
Some accidental appearances, certainly electrical, excited, by their
novelty, the attention of the Professor of Anatomy at Bologna, to the
investigation of the possible, but unknown, dependencies of the motions
of animals upon electricity; and the astonishing effects of that
influence upon the human body, particularly in paralytic diseases,
whether owing to derangements of the nerves, or of the muscles; the
experiments, which prove that the fluids of animals are better
conductors of electricity, than water is; and that, “if an electric
shock pass through a given part of a living animal, the same shock,
after the animal is dead, will be visibly transmitted over the surface
of the part, but not through it[1]:” the recollection, too, of that
singular power, which some animals possess, as the _torpedo_, the
_gymnotus electricus_, and the _silurus electricus_, of collecting and
discharging at pleasure the electrical fluid; but, above all, the
wonderful, but solitary, instance of an electrical shock received from
a mouse, under dissection, recently related by his countryman Cotugno;
were circumstances, which seem to have rendered the expectations of the
Professor not a little sanguine as to his success.
At length, after many ingenious and interesting experiments,
illustrative of the relations which subsist between artificial
electricity and the involuntary motions of animals, a happy accident
discovered to him the phenomena, which have since been the objects of
so much curious research, and which have given to immortality the name
of _Galvani_.
He one day observed, that some frogs, hooked by the spine of the back,
and suspended from the iron palisades, which surrounded his garden,
contracted frequently and involuntarily. Examining minutely into the
cause of these contractions, he found that he could produce them at
pleasure, by touching the animals with two different metals, at the
same time in contact with each other.
To a mind prepared by such observations and experiments as those which
had previously occupied M. Galvani, the resemblance which this new
discovery bore to the facts he had before observed, must have produced
conviction of the identity of their cause; and the experiments, which
have since been made both by himself, and Dr Valli, have given no small
degree of plausibility to the opinion. A repetition of some of these
experiments excited my doubts as to the legitimacy of the conclusions
which had been drawn from them, and induced me at length to proceed in
the following investigation.
My first object was to ascertain as well the various circumstances,
which are essentially requisite to the production of these new
phenomena, as those in which they can be rendered most obvious. After a
great variety of experiments, of which it would be unnecessary here to
relate more than the result, I found that I could not excite in an
animal the appearances described by Galvani with any substances
whatever, whether solid or fluid, except the metals: and that the
mutual contact of two different metals with each other, so far as I was
able to determine, was in every case necessary to the effect.
When metals are either calcined, or combined with acids, they are no
longer capable of exciting contraction. In estimating the comparative
powers of different metals as excitors, I found zinc[2] by far the most
efficacious, especially when in contact with gold, silver, molybdena,
steel, or copper, although these latter excite but feeble contractions
when in contact only with each other. Next to zinc, tin foil, and lead
appear to be the best excitors. But with zinc, and gold, silver, or
molybdena, I have frequently succeeded in exciting contractions in the
foot of a frog, upwards of a day after they had ceased to be excited,
by arming the nerve with tin foil, and using some other metal as a
conductor, in the way the experiment is commonly performed.
When the bulk of the metals is large, and the quantity of surface, of
an animal with which they are in contact, is considerable, I think, the
contractions are both stronger and more readily excited, than when the
reverse of this is the case. Thus I have almost always been able to
make a limb contract, by laying it upon a broad plate of zinc, and
employing a half crown piece for an excitor, long after a small piece
of zinc, and a silver probe, had failed to produce any effect.
I have said, that, in order to excite contractions, I believe it
necessary that two different metals, communicating with the part to be
excited, should be in contact with each other. Some few instances have
been observed, which seem to prove the contrary. In a lecture, so long
ago as October last, in which Dr Baillie of London mentioned Galvani’s
discovery, I think I remember him saying, that he had twice or thrice
produced contractions by the application of one metal only: and Dr
Valli, in his 9th letter upon this subject, speaks of his having done
the same with a pair of scissars, made of bad steel, and in a frog
recently killed. I think it not impossible that there may have been
some unnoticed fallacy in these instances. I happened one day to touch
the crural nerve of a frog, with a small gold tooth-pick slid from a
silver case, and the leg instantly contracted; I again touched it, and
it again contracted. At another time I observed contractions from
touching a nerve, with a silver cannula, and at another from placing
one in the folds of a silver chain. All these appeared at the time to
be so many decisive instances of contractions from the application of
one metal, till the following experiment seemed to afford a different
explanation. Having placed, one end of a silver probe upon the sciatic
nerve of a frog, lying in water some inches below the surface, I
observed that no contractions followed, neither did they, when I
touched the part of the probe above the surface with a piece of zinc.
But when I touched it at the surface, so that both the zinc and the
silver were in contact with the water, although the zinc was at the
same time many inches removed from the frog, contractions were produced
equally vigorous, as if both the metals had been in immediate contact
with the frog.
I was now no longer at a loss to account for the contractions produced
by the gold tooth pick, since the circumstances both of that case, and
of the experiment related, were probably the same, two metals in
contact with each other. The gold in immediate contact with a nerve;
and the silver case communicating with it, and the muscles through the
medium of the moisture, with which the whole was perhaps surrounded.
This led me to examine the chain, and the cannula. I found both the
links of the one, and the sides of the other, soldered with a different
metal[3]. So that, in these cases, two metals had at the same time been
in contact with a nerve, with moisture, and with each other.
However this may be with respect to the necessity of mutual contact,
between two different metals in order to excite contractions, I have
long ago found, that contractions may be excited in an animal, when no
more than one metal is in contact with it[4].
At the time I first observed this fact, I was making experiments to
ascertain whether it was possible to transmit the influence, which had
excited contractions in one leg, into another, removed to some distance
from it, and communicating with it, only by means of a single
conducting substance, (such as silver, for example). For this purpose,
I had separated from the trunk, and from each other, the hind legs of a
frog recently killed, and had detached their nerves as far as the knee.
I then laid them at some distance from each other, upon a plate of
glass, and included the nerve of one leg, and the foot of the other, in
the folded ends of a silver chain. With one hand I now raised from the
muscles, upon the end of a silver probe, the nerve of the leg, whose
foot was folded in the chain; and with a piece of zinc, in the other
hand, touched, at the same time, the nerve and the probe. This leg was
thrown into strong contractions; but none were excited in the other. I
then touched the chain, and nerve of the other, and, to my surprise,
both legs instantly contracted. I had observed, in the beginning of
November last, that it was not necessary for the metals to be in
contact with any thing but nerve, in order to excite contractions in
the muscles, to which it was distributed; and had mentioned this fact
immediately afterwards to the Medical Society of this place, as a
sufficient refutation of the theory, which Dr Valli had formed of
Galvani’s discovery. It would not, therefore, have surpassed my
expectations, had the influence, excited by the mutual contact of a
piece of zinc, and probe, with the nerve, passed through the medium of
the chain, from the leg, in which it first excited contractions, and
produced contractions in the distant leg. But, I now thought that I had
not only passed the influence from one leg to the other, but in one of
the legs in a direction contrary to the course of its nerves. The
removal of the leg, whose nerve communicated with the chain, convinced
me of my error: but, at the same time, discovered to me a fact of much
greater importance, than any with which I had hitherto been acquainted.
For now, upon touching the chain alone with the zinc, I found that the
leg, whose foot it still included, and whose nerve I held suspended
upon a probe, contracted as strongly as before. The influence of the
two metals, in contact with the nerve of the other leg, had not,
therefore, passed into, and excited this.
It had from the first been known, that contractions could be excited by
placing two different metals in contact, one with the nerve, the other
with the muscles, and making a communication between them: but, in this
experiment, the only metal in contact, either with the nerve, or
muscle, was silver. Neither had the influence passed through the chain,
and up the leg against the course of the nerve, in consequence of a
communication by means of moisture subsisting between the zinc, and the
foot, as well as between the silver chain, and the foot; for the
experiment succeeded equally well when the chain was removed, and the
foot laid upon a silver plate made perfectly dry. But when either the
zinc, or probe was held by another person not communicating with me; or
when either of them was insulated in a stick of sealing wax; no
contraction whatever took place. Neither, indeed, were contractions
excited in any part of the leg, except the foot, when the probe was
withdrawn from the nerve; and the foot, and silver, were both touched
with the zinc. It is then clear, that the influence, which, in the
former case, excited the whole leg to contraction, must have passed
through the medium of my body. It is not necessary that the silver
should be laid under the foot; all that is required, is, that it should
communicate with it by means of moisture; it may then be laid at almost
any distance from it[5].
The course of this influence, however, was still undetermined: it might
be from the muscles to the nerve: it might be from the nerve to the
muscles. To ascertain this, and to prove that the influence, which had
excited one limb to contraction, might pass on, through a foreign
medium, and excite contractions in another, I made the following
experiment.
The leg of a frog was disposed as in the former experiment. The probe,
suspending the nerve, was held by myself; the zinc excitor by another
person; and the leg of another frog formed the communication betwixt
us. So long as I had hold of the nerve, and the person assisting me
held the foot of this interposed leg, no contractions were excited in
it, by the influence, which passed through it and excited the other
leg. But when the person holding the zinc, held the nerve of the
interposed leg; and I held the foot, both legs contracted with equal
strength. From this experiment it is evident, that Galvani’s influence
had passed either from the muscles, or the zinc and silver; and in the
direct course of the nerves of both legs.
I was now in possession of an easy method of ascertaining the different
substances, which do, or which do not, afford a passage to this new
influence.
All the metals when pure appear to be excellent conductors; not quite
so good when in the ore; and, I think, least so when combined with
acids, forming metallic salts. They are however, in this state, by no
means bad conductors, even when so carefully dried, as to leave no
suspicion of the slightest degree of moisture adhering to their
surface. But, when the metals are calcined, their capacity as
conductors is quite destroyed: at least this was the case with the
calces of zinc, of bismuth, of iron, and of mercury; the only ones,
with which I have had an opportunity of making the experiment. I could
not observe that any contractions were excited through the medium of
stones, nor ever through barytes.
The different non-conductors of electricity are likewise, I find
non-conductors of this influence: even wood, charcoal, and linen, do
not conduct except when moist. But all the living vegetables I could
procure afforded it a ready passage: probably from the fluids which
they contain. While I held the probe which supported the nerve, I
touched the shoe of a gentleman, who applied the zinc to the silver
under the foot of the frog. Strong contractions were excited, but when
he took off his shoe, and we held it between our hands, no contractions
could be excited. In the first case, the influence had to pass through
no more than the thickness of the shoe: in the second, through its
whole length, which might not be all equally moist. This gentleman had
on thread stockings. When I touched the foot of another, who had on
cotton stockings, no contractions were excited. Cotton is a
non-conductor of electricity.
Oils of all kinds are so far from conducting, that if the fingers of
the person holding either the probe, or the zinc, have perspired much,
even this operates as a complete obstruction to the passage of the
influence: the instant the perspired matter has been wiped away, and
the fingers have been dipped in water, it again passes, and excites
contractions. When the intestines of a frog are removed, and its
abdomen is filled with oil, no contraction can be excited by placing
one metal upon its sciatic nerves, and bringing another in contact with
it, either above or below the surface of the oil.
There is something singular in this respect, with regard to mercury. If
the abdomen of a frog be filled with it, a piece of zinc passed through
it, so as to touch the sciatic nerves, excites contractions. But a
piece of silver, passed to them, excites none. Neither are any excited
by touching the silver, beneath the surface of the mercury, with a
piece of zinc. But I have before shewn, that, when water is used
instead of mercury, contractions may be in this way excited; yet
mercury is reckoned a much better conductor of electricity than water.
I have repeatedly passed this influence through a great length of thin
brass wire, and through the bodies of five persons communicating with
each other, by dipping their fingers in basins of water placed between
them; yet it did not appear to have lost any of its force, in this long
and diffused passage: for the contractions excited in the frog’s leg
were equally strong, as when it had passed only through one person.
Vitriolic acid, and alcohol appear still better conductors than water.
Wishing to ascertain whether it passed over the surface, or through the
substance of metals, I coated several rods of different metals with
sealing wax, leaving nothing but their ends, by which they were held,
uncovered. Contractions were excited as readily through the media of
these, as if they had not been coated. It seems to meet with no
obstruction in passing from link to link, of several chains, even when
no pressure, except that of their own weight, is used to bring them
into contact. I was led from this to hope, that I should be able to
make it pass through a very thin plate of air. I, therefore, coated a
stick of sealing wax, with a plate of tin-foil, and then made an almost
imperceptible division a-cross it with a sharp pen-knife. But even this
interruption of continuity in the conductor was sufficient effectually
to bar its passage.
The chains, through which it passed most readily, were of gold and
silver. It did not pass through a very long and fine brass chain,
unless as much force as could be used, without breaking the chain, was
employed to bring its links into close contact.
I next proceeded to examine if the capacity of different substances, as
conductors, or non-conductors, was at all affected by differences of
their temperature. But this was not the case with zinc, iron, water,
coal, or a common crucible, the only substances with which I tried the
experiment. A red hot iron, and boiling water, conducted equally as
well as iron and water that had not been heated: and neither the
crucible, nor the coal, became conductors from any addition of heat.
I at first thought that ice conducted; but as, on some trials, no
contractions were excited through its medium; and as it appeared
uniformly to conduct ill in proportion to the dryness of its surface; I
suspect that, if perfectly dry, it would not conduct at all. The
instant a part of its surface had been dissolved by the heat of the
room, contractions were excited with as much ease, as they usually are
through a bason of water. It would appear, therefore, that neither very
hot, nor very cold water disperse this influence, as has been asserted
by Dr Valli, nor do they seem in the least degree to diminish its power
of producing contractions[6].
It appears upon the whole to be necessary, that this influence should
pass to a part in a very condensed state, in order to excite
contractions: although there are some facts, which, without reflecting,
might lead one to suppose, that, passing even in a diffused state, it
would excite them. In making that experiment, in which the piece of
zinc under the foot of a frog is touched with zinc, while its crural
nerve is supported by a silver probe; no contraction takes place, if
the probe be either lowered, so as to come in contact with the muscles
of the thigh, or if it be made to touch the silver under the foot.
If again, two persons, one of whom holds the probe, the other the zinc,
communicate with each other by dipping their unemployed hands in a
bason of water; and the person using the zinc holds another leg of a
frog, suspended between his fingers by its nerve beneath the surface of
the water; no contraction will take place in this leg, when the silver
under the other is touched with zinc, at the same time that strong ones
are excited in that other. But, if its nerve be raised above the
surface of the water, it then contracts as vividly as the other. It
appears that in the last of these instances, at least the greater part
of the influence had diffused itself through the water, instead of
passing directly through the nerve, from the fingers of the person
holding it, and that in both it had passed into the legs, in too
diffused a state to excite them to contraction.
I have often likewise observed, that when the nerve of a nearly
exhausted leg of a frog had been laid upon a piece of zinc, and both
were touched with silver, the contractions excited were very distinct:
but when the zinc was placed in contact with the muscle, as well as
with the nerve, either no contractions could be excited, or such feeble
ones that they were scarcely perceptible.
Contractions, however, certainly may be excited in different parts of a
frog, without making any division of its skin, by laying the part of
the frog to be excited upon a plate of zinc, or tin-foil, and passing a
piece of silver over it, till all three are in contact with each
other[7]. Yet even here the influence does not pass into the part in so
diffused a state as it may at first appear to do. For the skin of these
animals is abundantly supplied with nerves, whose trunks communicate,
at different places, with those which supply the muscles. And the
contractions are always strongest, and most readily excited, when the
silver is passed over the course of any of the nerves, which go to the
muscles.
From the fact, which I have before mentioned, that a limb may be made
to contract, when the metals have apparently no communication with any
part of it except its nerve; it might reasonably be doubted, whether,
in any case, a communication between the muscles, as well as the nerve,
and the metals, were necessary, in order that contractions may be
excited.
Several considerations, however, induce me to believe, that such
communication is absolutely requisite. If the contact of two different
metals were alone sufficient to excite contractions; contractions
should always take place, whenever a good conductor is interposed
between the metals, and the nerve alone. But I have, in no instance,
observed this to be the case. In the experiment, where the crural nerve
must be supported upon a silver probe, it is necessary that the piece
of silver, with which the zinc is brought in contact, should
communicate either immediately, or through some good conducting medium,
with the muscles of the foot, or leg, before any contraction takes
place. And even in the experiment, where water forms the only
communication between the metals, and the origin of the sciatic nerves,
that same water, it must be observed, forms likewise a communication
between the metals and the muscles, to which these nerves are
distributed. But the fact, which appears to me most decisive of this
question, is the following: When a nerve, which for some time has been
detached from surrounding parts, is either carefully wiped quite dry
with a piece of fine muslin, or (lest this should be thought to injure
its structure,) suffered to remain suspended till its moisture has
evaporated; no contractions can be excited in the muscles, to which it
is distributed, by touching it alone with any two metals in contact
with each other. But, if it be again moistened with a few drops of
water, contractions instantly take place: and, in this way, by
alternately drying and moistening the nerve, contractions may, at
pleasure, be alternately suspended and renewed for a considerable time.
It may, indeed, be contended, that the moisture softened, and thus
restored electricity and free expansion to the dried cellular membrane
surrounding the fibres, of which the trunk of a nerve is composed; and
thus, by removing constraint, gave free play to their organization[8].
But from observing, that, in every other instance, where contractions
are produced by the mutual contact of the metals, a conducting
substance is interposed between them and the muscles, as well as
between them and the nerve; I think it would be unphilosophical not to
allow, that, in the instance in question, the moisture, adhering to the
surface of the nerve, formed that requisite communication between the
metals and the muscles.
I relate the following fact, in this place, because at the same time
that it gives further confirmation to the above opinion, it affords an
instance in which insulation diminished the effect of the metals. I had
one day laid the nearly exhausted leg of a frog upon my hand, with a
piece of zinc in contact with its nerve only; and, when I touched these
with a silver probe, tolerably strong contractions were excited, even
when the nerve appeared dry: but when both the leg and the metals, thus
disposed, were insulated by means of glass and sealing wax, the
contractions were scarcely perceptible. My hand, it would appear, had,
in these instances, supplied the place of the moisture in the other;
and been the conducting medium between the muscles and the metals.
This communication of the muscles with the nerve, through the medium of
the metals, had appeared to Dr Valli a circumstance so essential to the
production of Galvani’s phenomena, that (taking it for granted they
were occasioned by the action of the electrical fluid), it seems to
have suggested the hypothesis, which he has offered in order to account
for them.
Aware that no electrical phenomenon can possibly have place, except
between the opposite states of positive and negative electricity, or,
in other words, where there is a breach of equilibrium in the
distribution of the electrical fluid; he supposes it to be one office
of the nerves, to produce this breach of equilibrium, by continually
pumping (to use his own expression) the electrical fluid from the
internal parts of muscles, and in this way rendering them negative,
with respect to the external surface. The brain, he makes the common
receptacle for this fluid. The metals, he seems to consider in the
light of a conductor, interposed between the outside of muscles and
their nerves. And the rapid transmission of the fluid to restore the
equilibrium, as the cause of the contractions.
He presumes his hypothesis proved from the following considerations:
I. The interval which commonly takes place between the contractions;
which interval, according to him, is necessary for the restoration of
the breach of equilibrium.
II. From observing, that fishermen, in order to preserve their fish
from putridity, crush their brains; and thus, by interrupting the
medium between the external and internal surfaces of muscles, prevent
these repeated discharges of the electrical fluid, which, according to
Dr Valli, hastens their putridity.
III. From finding that in general, when the sciatic nerve on one side
of a living frog was divided, the other being left entire,
communicating with the brain, both armed and equally excited, the limb,
in which the nerve had been divided, preserved its power of contracting
longer than the other. From this well devised experiment, he concludes,
likewise, that animal electricity is the principle of life. That, on
the side where the nerve remained entire, it was withdrawn from the
muscles, and deposited in the brain. That, from the impossibility of
this taking place on the other side, where the nerve was divided, it
had continued in the limb, and enabled it to contract.
If it were indisputably true, as I once believed, that contractions
could be excited in a limb without the metals having any communication
with it, except through the medium of a nerve; this circumstance would
alone be a sufficient refutation of Dr Valli’s hypothesis: but, as I
have already shewn, that contractions were not in this way produced in
any experiment, which I have made, when no moisture, forming a
communication between the metals and the muscles, had been left
adhering to the surface of the nerve, it becomes necessary to have
recourse to less dubious arguments.
The Dr should have recollected that, in cases of a breach of
equilibrium in the distribution of the electrical fluid, all that is
required, in order to restore equality of distribution, is, the
interposition of a single conducting substance between the place in
which it abounds, and that in which there is a deficiency. Whereas, in
the phenomena, which he attempts to explain, two conducting substances
are necessary to the effect.
When a separated limb is placed under water, one would naturally
imagine, that from the perfect communication, which is then formed
between the external surfaces of muscles and their nerves, no breach of
equilibrium could possibly have place: yet we find Galvani’s phenomena
even more readily produced in this situation, than when both muscles
and nerves are free from surrounding moisture.
The following experiment was made with a view of rendering the
equilibrium of the electrical fluid, in different parts of frogs, as
perfect as possible.
The head of a frog having been separated from its body, the latter was
laid upon a plate of zinc, held by a person sitting in an insulated
chair, which communicated with the prime conductor of an electrical
machine. The machine was put in action, and both the person and the
frog were electrified positively. In these circumstances, no sparks
could be drawn from the frog, by the person holding it: nor could any
other electrical appearance take place between them. But, when a piece
of silver was passed over different parts of the frog, and, at the same
time, brought into contact with the zinc plate, contractions were
uniformly excited, differing not in the least, either in strength or
frequency, from those which are excited when no artificial electricity
is present. The result was precisely the same, when the frog and the
person holding it were negatively electrified. This experiment was
often repeated. The following experiment was made, in order to see if
the effect produced upon a frog, by the passage of artificial
electricity from any part of its body, would be increased by employing
two different metals as conductors.
A frog was laid, successively, upon a number of different metals,
insulated upon glass, and positively electrified by communicating with
the prime conductor of an electrical machine. The contractions produced
in the frog, thus disposed, by drawing sparks from it, with metals
different from those on which it was placed, were not in the least
stronger, than those occasioned by drawing similar sparks from it, with
conductors of the same metal.
In establishing a communication between two opposite electricities, as,
for example, between the two sides of a charged phial, it is matter of
indifference to which the conductor is first applied. But it is by no
means so, in the case of muscles and armed nerves. For, if one branch
of a conductor be applied to the tin-foil arming a nerve, before the
other branch has been applied to the muscles, it frequently fails to
excite contractions. If first applied to the muscles, this is very
seldom the case.
As for the intervals of rest which alternate with the contractions, and
which the Dr considers as employed by the nerves, in restoring the
breach of equilibrium between the internal surfaces of muscles, and
their external; these may possibly admit of a different explanation.
We find them alternating with contractions however excited. It is
difficult to conceive, that violent contractions should not derange in
some degree, however slight, the intimate organization of muscular
fibres: and some time must necessarily elapse before their elasticity
can have restored the organized particles, of which they are composed,
to that relative situation with respect to each other, which will fit
them for again contracting.
This explanation is drawn from observing the following facts. Hearts,
taken from the living thorax, and exposed to the action of a strong
stimulus, contract vividly for a time, and then cease to be effected by
any further application. If they be then removed from the stimulus, and
placed for a time either in cold water or in open air, they are
observed to regain their susceptibility of the action of stimuli, and
again contract. Mr Coleman, in his excellent dissertation on Suspended
Respiration, makes an observation, which I have often had opportunity
of verifying: that hearts distended with blood, and in which no
contraction can be produced, by scratching their surface with a pointed
instrument, contract spontaneously, if one of the large vessels, at
some distance from them, be cut so as to evacuate some of the blood.
The organization, in this case, is suffered to recover by the removal
of the stimulus, (distention) which had deranged it. Even, in the
living and entire animal, the heart does not renew its contractions, on
the first influx of blood. Some time must elapse, while it recovers
from the derangement occasioned by the preceding contraction.
I have repeatedly excited, by means of zinc and silver, contractions in
the leg of a frog, whose head had been divided from its body, upwards
of three days before. The receptacle, for the electrical fluid, was in
these cases removed. Now, either the nerves continued extracting it
from the internal parts of muscles, or they did not. If they did,
having no longer a receptacle, in which they could deposite their
electricity, they must have remained positively electrified; and thus,
being in the same state with the outer surface of the muscles, no
contraction should, according to the hypothesis, have been excited by
the application of the metals. But this is contrary to the fact.
If it be contended, on the other hand, that their pumping power had
ceased; then the first application of the metals, which produced a
contraction, having restored the equilibrium, which could not
afterwards be broken, must have precluded the possibility of further
contractions. But this too is contrary to fact.
This argument appears, to me, to do away all support, which the
hypothesis may seem to derive from the experiment, before quoted, of
applying the metals equally to both sciatic nerves, after one of them
had been divided; I may however remark, that the pain necessarily
excited by arming a nerve, whose communication with the brain was not
interrupted, would fully account for the more rapid exhaustion of the
muscles, to which it belonged, compared with such as had not been acted
upon by so strong an additional stimulus. As fact, however, is always
more satisfactory than argument, I shall relate the following
accidental experiment, in proof of the relevancy of the foregoing
observation.
Four days after I had divided the crural nerve of a female frog, full
of spawn, I found her dead; she had been observed alive the night
before. The application of the metals to the leg, whose nerve had not
been divided, did not excite the slightest contractions, but on
applying them to the leg, in which the nerve had been divided,
tolerably strong contractions were excitable, for more than twelve
hours after she was found. The spawning season had closed, upwards of a
week before this happened, and, as this frog had long been without a
male to assist her, it is probable, that her death had been occasioned
by the retention of her spawn, as it was found in a very dissolved
state. The pain, necessarily preceding such a death, could affect the
different parts of the animal, only through the medium of its nerves;
and hence the exemption of that part from its effects, to which the
communication, by nerve, had been interrupted.
The same observation will apply to that argument, which Dr Valli has
drawn, in support of his hypothesis, from the practice of fishermen. By
destroying the brain, they take away all sense of pain, and,
consequently, preclude that exhaustion which is so notorious for
disposing to putridity.
Should it, therefore, be ever proved, that the phenomena discovered by
Galvani are effects of the action of electricity, I cannot think Dr
Valli’s hypothesis will be deemed a satisfactory account of the manner
in which it produces them.
Strong, however, as is the analogy, which, in many particulars this
influence bears to electricity, considerable doubts must, I think,
still remain as to their identity.
The grounds of these doubts would best appear in an accurate and full
statement of the several points, both of resemblance and of difference
between this influence, electricity, and that power which distinguishes
the torpedo, the gymnotus, and the silurus; but, I can here promise no
more than a very imperfect and desultory sketch of these.
In order to accumulate artificial electricity, if I may be allowed the
use of such an expression, it seems necessary, that there should be
motion between two substances, an electric and a conductor. But,
neither motion nor electrics have any share in the production of that
influence which occasions the phenomena in question. The motion, here,
is the effect, and not the cause of the accumulation: and instead of
one conducting substance of any kind whatever, two metallic substances
seem indispensably requisite[9].
That influence, whatever it be, which is possessed by the torpedo, &c.
seems to depend entirely upon the will of these animals, both for its
production, and management, as appears not only from the retraction of
their eyes within their sockets, whenever they mean to give a shock,
but, likewise, from each shock being increased, diminished, or
withheld, as they are irritated or aware of some obstacle to its
transmission. But the will of an animal has no share in the production
of the phenomena discovered by Galvani.
In the scale of conductors of electricity, charcoal holds a higher
place than the fluids of animal bodies, and ice than the metallic
salts. But of the influence in question, I have found animal fluids,
and metallic salts, excellent conductors, at the same time that I have
never observed it pass through charcoal, or even dried wood. I have,
likewise, reason to believe that it does not pass through ice. Ice,
indeed, is but a very imperfect conductor of electricity, when free
from air bubbles, and when the experiments with it are made in a very
low degree of temperature. Yet we are told by Mr Achard, that it will
conduct electricity, even when Reaumur’s Thermometer stands at 6
degrees below 0.
But the temperature of the room, in which I made my experiments, was at
least 55 degrees above 0, by Fahrenheit’s scale. I may likewise remind
the reader of the experiment, in which the abdomen of a frog was filled
with mercury, and a rod of silver passed through it to the sciatic
nerves. A piece of zinc, touching both mercury and silver, excited no
contractions; whereas most vigorous ones were excited when water was
substituted for the mercury. A proof, as I take it, that water is a
much better conductor of this influence than mercury: but of
electricity, mercury is deemed a better conductor than water.
We are told by Mr Cavendish, that Mr Walsh found the shock of the
torpedo would not pass through a small brass chain: but the influence
discovered by Galvani, passes, without sensible diminution of its
effects, through a small brass chain of several inches in length, when
it is drawn so tight as to bring its links into close contact with each
other: and it passes through a gold chain when held between two
persons, and suffered to hang with a considerable bend. Yet, if we may
be allowed to judge of the comparative strength of the two influences,
by the effects which they produce upon animals, that of the torpedo
must certainly be allowed to be the strongest; and I see no other way
of accounting for its finding an insuperable obstacle to its
transmission, where the other finds scarcely any, except by supposing
that they are in reality different in their nature.
Dr Valli tells us, that he observed the hairs of a mouse, attached to
the nerves of frogs by the tin-foil, with which he surrounded them,
alternately attracted, and repelled by each other, whenever another
metal was so applied as to excite contractions in the frogs.
This experiment I have many times repeated, both in the manner
described by the Dr, and with every variation in the disposition of the
hairs which I could devise: but whether they were placed upon the
metals, the nerves, or the muscles, or upon all at the same time,
neither I, nor my friends who assisted me, have in any instance been
able to observe them agitated in the slightest degree.
I have made similar experiments upon a dog, and upon a large and lively
skate, by disposing, in the same way that I did the hairs of a mouse,
flakes of the finest flax, swansdown, and gold leaf; but although the
contractions produced in the skate, by the contact of the metals, were
so strong as to make the animal bound from the table, not the least
appearance of electricity was indicated.
I next suspended, from a stick of glass fixed in the ceiling of a close
room, some threads, five feet in length, of the flax which I used in
the former experiment; and approached some frogs, recently killed, and
insulated upon glass as near to them as was possible, without touching:
but the threads were in nowise affected by the contractions produced in
the frogs.
In this respect, therefore, this influence agrees with that of the
torpedo, &c. So far as I know, M. Volta’s instrument for collecting,
condensing, and rendering sensible, very small degree of electricity
has not been employed in the examination of either.
And indeed I am not sure, if, in examining the newly discovered
influence, by such a test, a sufficient quantity of electricity might
not be produced merely by the motion of the animals, subjected to the
experiment, to occasion some fallacy in the result. Certain, however,
it is, that although this influence did not affect the electrometer in
these experiments, it produces infinitely stronger effects upon an
animal, than any which can be produced by a quantity of electricity
sufficient to affect an electrometer to a very high degree. I have
frequently detached the crural nerves of frogs for some length; and
having supported them upon a rod of silver, have applied an excited
piece of glass, or sealing wax, to the whole length of this rod. The
coarsest electrometers have been effected by it, at considerable
distances: but I have never, in this way, been able to excite
contractions, unless by laying the rod upon the excited cylender of a
powerful electrical machine.
This new influence likewise resembles that of the torpedo, in producing
its effects almost equally well, when both it and the subject upon
which it acts are insulated from surrounding conductors. But an
experiment similar to that, which I have related, of insulating, and
positively electrifying, both the frog and the metals applied to it,
has never (so far as I am acquainted,) been tried with the torpedo.
Both these influences agree too, in not producing so strong an effect,
when the subject, upon which they act, is immersed in water, as when it
is in the open air. When the separated leg of a frog was held under
water, and formed part of the circuit through which this, to influence,
had to pass, in order to excite another leg; it never contracted,
although it did, and strongly, when held above the surface, as I have
already had occasion to notice. And we are told by Mr Walsh, that the
shock of the torpedo was four times stronger in air, than when given
under water.
This influence differs, both from that of the torpedo, &c. and from
electricity, in producing no sensation (in man at least,) at all
similar to that from an electrical shock.
With respect to the single instance related by M. Cotugno, it is
probable that both he himself, and all who have repeated experiments of
this nature, must have been long ago convinced, that he was deceived
into the belief of a shock, from the sensation produced by the
struggles of the animal he dissected.
That some kind of disagreeable sensation is occasioned by it, even in
frogs, independent of that which must necessarily arise from
irritation, and the contractions of their muscles, is evident from
their restlessness, and expressions of uneasiness. In other animals, as
I shall afterwards have occasion to shew, these expressions are still
less equivocal: and, in man, we can ascertain both their degree and
their kind. That they differ considerably from such as are produced by
electricity, will be proved when I come to speak of the effects of this
influence upon our senses.
But the most important, and characteristic difference, which I have yet
been able to discover, between this new influence and electricity,
consists in their effects upon the contractile power of animals and of
plants. The contractions of animals excited by electricity have a
tendency to destroy that power upon which contractions depend. But the
contractions excited, by the application of the metals, have, in all my
experiments, had the directly opposite effect. The more frequently
contractions have been, in this way, excited, the longer do they
continue excitable: and the longer are the parts, upon which such
experiments are made, preserved from putridity. An influence, capable
of exciting contractions, without occasioning exhaustion, was a thing I
so little expected to find, and so contrary to the character which had
been given of this, both by Galvani and by Dr Valli, that I, at first,
distrusted my own observation of the fact: but the number of
comparative experiments, which I had afterwards occasion to make,
though with views different from that of ascertaining the point in
question, convinced me that this influence, so far from destroying the
contractility of muscles, has a tendency to preserve it. Oxygene is, so
far as I know, the only stimulus in nature, whose effects are at all
analogous.
When a frog has been long dead, I have been sometimes more than a
quarter of an hour without being able to excite a single contraction by
the application of the metals: but after this, without at all varying
the means employed, contractions have appeared, and have become
gradually more and more vigorous.
It is said, (for I have never had an opportunity of making the
experiment,) that a stream of electricity passed through a sensitive
plant produces an almost immediate collapse of its leaves. But the
influence, discovered by Galvani, produced no such effect in the
following experiment. Having separated the leg of a frog from its body,
I freed its crural nerve from surrounding parts, and with one hand held
it supported upon the end of a probe. An assistant placed a piece of
silver under its foot, and held the zinc with which it was to be
touched. A sensitive plant formed the medium of communication between
us. He held the bottom of its stem between his fingers, while I held
the top: so that when the silver was touched by the zinc, the influence
passed up the plant, and through the whole of its stem. The frog’s leg
instantly contracted, and repeated its contractions every time the
silver and zinc were in contact: but the leaves of the plant did not
collapse; neither did they when any of its branches formed part of the
circuit.
I must, however, confess that the plant, upon which this experiment was
made, had been kept through the winter. With a young one the result
might possibly be different; but such an one I have not yet had it in
my power to procure.
The torpedo does not appear at all affected by the influence which
itself produces. Animals, in which Galvani’s phenomena are produced,
are strongly affected. From this circumstance, and still more from the
presence of metals being absolutely requisite to their production, some
may be induced to believe, that the influence, which causes them, is
something external to animals; and that it arises from the mutual
contact of the metals only. I must confess I was, for some time,
inclined to entertain this opinion; and its probability appeared to be
not a little increased by observing that its effects differed with the
metals employed, and were strongest when their surfaces were extended,
and applied horizontally to each other. I began, therefore, to suspect
that it might be some hitherto undiscovered property of metals; for
that it was not an electrical phenomenon, seemed still further proved
by the circumstance above related. It has been demonstrated, by the
very interesting discoveries of M. Volta, that, ‘wherever the capacity
of holding electricity is greater, there the intensity of electricity
is less’:--‘and that the capacity of a conductor is increased, when,
instead of remaining quite insulated, the conductor is presented to
another conductor not insulated; and this increase is more conspicuous,
according as the surfaces of those conductors are larger, and come
nearer to each other[10].’
When, therefore, a plate of silver, communicating with the leg of a
frog, was laid upon glass, and a plate of zinc was lowered horizontally
upon it, the capacities of both, for any electricity which they might
have contained, must have been so much increased, that no one will
suspect the contractions of the frog’s leg, to have been occasioned by
any discharge of the electrical fluid from them.
As little are we authorised to suppose, that the contractions were
produced in consequence of the metals attracting the electrical fluid
from the leg: for, since the leg was insulated, it is impossible that
it should have received a new supply of electricity, after having been
deprived by the metals of what it naturally possessed; and
consequently, after once or twice contracting, no further contractions
should have taken place: but this is contrary to the fact.
I have before shewn, that flakes of gold leaf, placed between the
metals, were not affected by their approach to each other; and that,
besides, a quantity of electricity, sufficient strongly to affect an
electrometer, was far too weak to excite contractions in the muscles of
a frog.
That this influence, however, whatever it be, is not derived from the
metals alone, but that animals at least contribute to its production,
as well as indicate its presence, is, I think, rendered highly
probable, by what I have already urged, relative to the necessity of a
communication between the metals, and the muscles, as well as between
the metals and the nerves.
I may likewise observe, that animals appear to have a much more
complete controul over its effects, than one would expect them to have
over an influence wholely external to them.
When living and entire frogs are placed upon a plate of zinc, or
tin-foil, and a piece of silver, or of gold, is passed over different
parts of their legs, and thighs, till it come into contact with the
plate; contractions are very seldom produced, and scarcely ever, if the
frogs be healthy and upon their guard. But the instant their sciatic
nerves are divided, the contractions produced are as free and vigorous,
as if the legs had been completely separated from the body. This
difference is not owing to the silver coming in contact with the wound,
necessarily made in order to divide the nerve; for I have always taken
care that it should not, and indeed when it did, no contractions were
produced, unless the nerve had been divided.
Taking off the head of an animal, or intercepting, in any way, the
influence of its will upon the muscles of the part excited, has
precisely the same effect. But the will is not able to controul the
effects of electricity, when the electricity is otherwise sufficiently
strong to excite muscles to contraction. I have repeatedly found that
even by the strongest voluntary contractions of the muscles of my arm,
I have not been able altogether to counteract the involuntary ones,
produced by electrical sparks, nor have I found that frogs could ever
counteract them.
On attending carefully to the state of the muscles of the legs of
living frogs, at the instant the metals were applied, I could perceive
by the touch, that, in many frogs, though by no means in all, their
muscles were perfectly soft and relaxed: a proof that they have other
means of counteracting the involuntary contractions, which the metals
have a tendency to produce, besides keeping their muscles in a state of
permanent and voluntary contraction.
-----
Footnote 1:
Cavallo.
Footnote 2:
On this metal Cronstedt has the following very curious remark: “It
seems to become electrical by friction, and then its smaller
particles are attracted by the loadstone; which effects are not yet
properly investigated.” Zinc is an ingredient of the best amalgam for
smearing the rubbers of electrical machines: But I have not been able
to render a bar of zinc electrical by friction, nor to find that its
smaller particles were in any state attracted by the loadstone,
unless they had been scraped off by means of an instrument of iron.
But, in this way, the dust of any metal is rendered susceptible of
the influence of the loadstone.
Footnote 3:
If further experiments should establish decidedly, that the mutual
contact of two different metals is absolutely necessary for the
productions of Galvani’s phenomena, may not this circumstance afford
an useful test of the purity of the precious metals? For instance,
contractions in an animal produced by the contact of a piece of gold
or silver, whose purity we wish to ascertain, with a piece of the
same metal known to be pure, would then prove incontestably the
presence of alloy.
Footnote 4:
In an able lecture, which Dr Monro lately delivered, chiefly upon
this subject, he demonstrated the possibility of exciting
contractions in the limb of a frog, without either of the metals he
employed being in contact with it; or having any other communication
with it than through the medium of some moist substance. In varying
this experiment, I find, that if a frog be divided in two parts, just
above the origin of the sciatic nerves, and put into a bason of
water, the hind legs may be thrown into strong contractions, by
bringing zinc, or tin-foil, and silver, in contact with each other,
at the distance of at least an inch from the divided spine, so long
as they are kept nearly in a right line with it. Water, in this case,
is the only communication between the metals and the origin of the
nerves.
Footnote 5:
The contractions produced seemed to be strong in proportion to the
extent of the surfaces of the metals in contact, strongest when a
large plate of zinc is laid horizontally upon a large plate of silver
or gold. If the zinc be suffered to remain in contact with the
silver, for a little time, the contractions of the leg cease. The
zinc may then be slid over the silver, till it even touch the leg
without renewing the contractions: but, in withdrawing the silver,
the leg contracts at the instant the silver parts from it!
Footnote 6:
‘L’eau trop échauffée, ou qui est en éboullition, disperse
l’électricité, de manière à en détruire les phenomènes.’
‘L’excès du froid prive l’eau même de la propriété de conduire le
fluide en question.’--_Dr Valli, Lettre 9me._
Footnote 7:
It was in this way, indeed, that I have always excited contractions,
when I have employed this new mode of influencing animals, as a test
of remaining life in any part of them.
They were constantly kept in fresh water, as the situation most
natural to them, during the whole of the time they were under
experiment; and their skins were suffered to remain as entire as
possible, since I found their muscles lost their contractile power,
in a few hours, and became rigid when exposed, deprived of their
skins, to the action of the water.
Footnote 8:
M. Fontana, in the first volume of his work on Poisons, mentions some
facts, which may, to some, appear to give considerable countenance to
this explanation. The microscopical eels found in dry and smutty
wheat; the seta equina or gordius of Linnaeus; and the wheal polypus,
all, when dry, become apparently dead: but again recover motion and
life when moistened with water. One of the latter was put, by M.
Fontana, upon a bit of glass, and exposed, during a whole summer, to
the noon-day sun. It became so dry that it was like a piece of
hardened glue. A few drops of water, however, did not fail to restore
it to life. Another was, in this way, recovered after a similar
exposure of a year and a half. Father Gumillo, a Jesuit, and the
Indians of Peru, are quoted by the same author, on the authority of
Bonguer, as speaking of ‘a large and venemous snake, which being dead
and dried in the open air, or in the smoke of a chimney, has the
property of coming again to life, on its being exposed, for some
days, to the sun, in a stagnant and corrupted water.’
But it would almost require the credulity of an Indian to credit the
testimony of the Jesuit.
Footnote 9:
Since what I had before written upon this subject went to the press,
I have been informed by a friend, that Dr Lind of Windsor has found,
that contractions may be excited in a frog by touching it with iron
alone. In a frog very recently killed, I have myself, sometimes,
excited contractions, by touching its nerves with iron and steel in
conjunction. But I can by no means consider this as a satisfactory
proof, that contractions may be excited by the contact of one metal
alone; since I have never been able to excite contractions with a
piece of iron, of the same quality throughout, applied to a frog
which had been so long dead as to leave no suspicion that the
contractions were occasioned by mechanical irritation. In Dr Valli’s
experiment, with scissars of bad steel, upon a frog recently killed,
these circumstances do not appear to have been sufficiently attended
to.
Footnote 10:
Phil. Trans. vol. 72. part i. Appen.
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