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Title: Experiments and Observations - Relative to the Influence Lately Discovered - by M. Galvani and Commonly Called Animal Electricity
Author: Fowler, Richard
Language: English
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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.
------------------------------------------------------------------------
SECTION II.
_Has Magnetism any concern in the Phenomena discovered by Galvani?_
In answer to this question I have little to say, as the experiments
which it suggested, and which I had an opportunity of making, have been
but few.
I have repeatedly excited contractions, both with the natural and the
artificial loadstone, but I could never observe any difference between
them, and such as were excited by unmagnetised iron, or an ore
containing an equal quantity of iron with the natural loadstone.
When the separated leg of a frog was laid upon a plate of iron, and a
loadstone was brought in contact both with its nerve and the plate, no
contraction was excited. I have often brought frogs, in every state of
preparation, as nearly as possible to a very sensible magnetic needle,
but no variation in its direction was in any case produced by the
contractions of the frogs excited by the metals.
------------------------------------------------------------------------
SECTION III.
_What are the relations which subsist
between the influence discovered by Galvani,
and the muscles, the nervous, and the vascular systems, of animals?_
In proposing to myself a question of this very extensive nature, it
will hardly be imputed to me, that I ever entertained, for a moment,
the idle expectation of being able completely to solve it. It is
prefixed to the following experiments as the most commodious general
head under which I could arrange, not only what I had further to say,
upon the influence discovered by Galvani, but likewise upon the several
physiological subjects, in the examination of which this influence was
employed merely as a test.
OF THE MUSCLES.
As I am acquainted with no criterion by which we can assure ourselves
of the complete separation of muscular fibres from nerves, without
rendering them objects too minute for accurate experiment; it can never
be in our power, so far as I am able to judge, to satisfy ourselves, if
this new influence can act immediately upon the muscular fibre. A doubt
must always remain, whether nerve has not been present; and from this
doubt will arise another still more difficult to solve, whether the
influence produced or excited by the metals have passed through the
nerve to the muscles? or if it have merely acted as a stimulus to the
nerve, serving to rouse that unknown energy, by which nerves are known
in certain circumstances to excite muscles to contraction.
The following experiments, made upon animals considered by anatomists,
in general, as destitute of nerves, may to some appear decisive of this
question, but to myself, I confess, they are by no means so. In by far
the greater number of animals, we are precluded from the possibility of
discovering nerves by their minuteness; yet the actions of these
animals, not merely excited by mechanical irritation, but so obviously
directed to the attainment of an end, oblige us to infer their
existence even where our senses, aided by the best glasses, do not
enable us to detect them.
Having laid some earth worms upon a plate of zinc, I tried to excite
contractions in them, by passing a rod of silver over different parts
of their length, till it came in contact with the plate; but for a long
time without producing any effect. Application of the metals to a part
recently divided seemed to produce as little effect. At length, I
perceived one of them dart itself forwards, whenever the silver was
passed under its belly near to a part which had been divided and
rejoined. On repeating the experiment again, and with more care, I
found, (as in the frog,) that when the animal was perfectly lively, and
upon its guard, no contraction could in this way be excited; but that
when a part had been rendered more sensible by previous disease, recent
irritation, &c. or when the worm was taken unawares by hanging it over
a probe, and lowering both upon the plate at the same instant; a sudden
and involuntary motion seemed to dart through a great part of the
worm’s length from the part touched towards the head; a direction
contrary to that in which it takes place in other animals. I never
could produce the same effect upon leeches. On varying the experiment,
a most whimsical, but satisfactory phenomenon presented itself. I had
laid a leech upon a crown piece of silver, placed in the middle of a
large plate of zinc. The animal moved its mouth over the surface of the
silver without expressing the least uneasiness; but having stretched
beyond it and touched the zinc plate with its mouth, it instantly
recoiled, as if in the most acute pain, and continued thus alternately
touching and recoiling from the zinc, till it had the appearance of
being quite fatigued. When placed wholly upon the zinc, it seemed
perfectly at its ease; but, when at any time its mouth came in contact
with the silver lying upon the zinc, the same expression of pain was
exhibited as before.
With the earth worm, this experiment succeeded still more decisively.
The animal sprang from the zinc in writhing convulsions; if, when the
worm stretched itself forwards, one of its folds lit upon the zinc, it
expressed little uneasiness in comparison of what it shewed when the
point of its head touched the zinc.
These extraordinary effects were, however, considerably different from
those produced by the metals upon the limbs of frogs, and other
animals. They had not so much the appearance of involuntary,
instantaneous convulsions, as long continued expressions of pain and
disgust; such as are produced by applying zinc and silver to the tongue
of a child.
A strong presumptive proof, in my humble opinion, that these animals
are endowed with a most exquisite organ of sense, and, consequently,
that they are not, as has been supposed, destitute of a nervous system.
Doubtful, therefore, if this influence can ever act upon the muscular
fibre, except through the medium of nerves, I shall reserve what I have
to say upon particular muscles, till I have related some facts relative
to the nerves.
OF THE NERVES.
It appears from every experiment, which has been made in prosecution of
Galvani’s discovery, that the nerves are very essentially concerned, in
all the phenomena which it exhibits. It becomes, therefore, an object
of inquiry, highly interesting, to ascertain if all the nerves of the
body are equally subjects of this new influence, or if its effects are
confined to those appropriated to muscles of voluntary motion. With
this view, I surrounded with tin-foil the parvagum and intercostal
nerves of several cows and sheep, while the auricles of their hearts
were still contracting, and placed one end of a bent silver rod, at one
time upon the heart itself, at another upon adjacent muscles, and
sometimes upon the nerves; but all without producing the slightest
perceptible variations, in the contractions of the heart, or a renewal
of them when they had ceased.
I likewise included the caroted artery in the tin-foil; and, at another
time, inserted the foil in longitudinal incisions made in the nerves,
that it might be more immediately in contact with their substance; but
still no contractions followed. I had as little success when I made
similar experiments upon a dog, cats, rabbits, fowls, and frogs; yet,
in all these animals, I could in general excite vigorous contractions,
by arming the nerves of parts obedient to the will: I say in general,
for in rabbits I have sometimes failed altogether; especially when they
have been drowned in very cold water. Soon after making these
experiments, I perceived from one of Dr Valli’s letters, published in
the Journal de Physique, that he had made a similar one upon the heart
of a dog, and with the same result. The heart, through the medium of
its nerves, is not excitable, therefore, by the same means which are
found efficacious in exciting other muscles to contraction. I confess I
had not expected this result. It has been asserted indeed, by many
physiologists of the first name[11], that the heart can in nowise be
affected by the application of a stimulus to its nerves, or to the
brain; but many considerations excited my doubts upon this subject, and
some experiments which I made at this place, more than a year ago,
tended to confirm me in an opposite opinion. That both the frequency,
and the strength of the heart’s contractions are affected by passions
of the mind, is a fact known to every one; but what is much more to the
purpose, since we know so little either of mind or of its mode of
influencing the body, we know that many derangements of the brain, such
as apoplexy, hydrocephalus, phrenitis, &c. together with all kinds of
mechanical injuries, (and what are these, but so many stimuli
irritating the brain, and consequently the nerves sent to the heart?)
affect the motions of the heart most materially and obviously. The
contractions of the heart, so long as the brain remains entire, may be
affected by a thousand different substances thrown into the stomach;
but it appears from the experiment of Mr Kite, that this is by no means
the case, when the functions of the brain are suspended by hanging, or
drowning[12]. Dr Whytt’s experiment on this subject is one of the most
decisive with which I am acquainted. He found, that opium operates much
more slowly in destroying the heart’s motion in frogs, deprived of
their brain and spinal marrow, than it does when these animals ate
entire. Several of my own experiments, though not made expressly with
this view, gave the same result with those of Dr Whytt. M. Fontana
tells us, he has discovered the heart of the wheel polypus to be a
voluntary muscle. It was probably this discovery which led him to try
the effects of his will upon his own heart. For the success of his
experiment, we have the testimony of his friend Dr Gerardi, Professor
of Anatomy in the University of Parma, who, in a very learned little
Dissertation on the Origin of the Intercostal Nerve, published in the
Journal de Physique for September last, makes the following short
mention of it; ‘Je ne dois point oublier de vous dire que M. Fontana a
la faculté d’accélérer, ou de retarder à volonté son pouls, sans aucune
contraction sensible des muscles.’
The direct experiments, by which I was first led to adopt the opinion
that the heart might be affected by the mechanical irritation of its
nerves, were made upon very young cats and rabbits; some with the
assistance of my friend Dr Physick, now settled in Philadelphia; others
in presence of several other gentlemen studying at this university. It
appeared very decidedly from two or three of these experiments, that
the contractions of the heart were quickened by irritating the brain at
the origin of the spinal marrow. In others again, the result was by no
means so clear. But it should be recollected that the evidence of one
accurate, and positive experiment, is not in the least invalidated by
twenty unsuccessful ones, especially upon animals of warm blood; where
the irritability of their muscles is so very fleeting, and the result
liable to variation from so many, as yet, unknown causes. The
irritability of the arteries, for example, is now completely
established, yet Haller’s experiments led him to deny it. And even
those of the accurate Verschnir, to whom we are indebted for
unquestionably the best series of experiments upon this subject, failed
of success (as we are told by Dr Dennison, in an excellent Thesis
confirming their truth,) when repeated before some of the Faculty here.
Immediately, therefore, on discovering the superior powers of zinc, and
molybdena, in exciting contractions, I began again to repeat with these
metals the experiments on the nerves passing to the hearts of frogs;
but for a long time without satisfying either myself, or others,
whether any effect was really produced. At length, however, I was so
happy as to succeed completely. On the 18th of March last, in presence
of my friends, Mr Hunter and Mr Thomson, having dissected away the
pericardium from a frog’s heart, which had an hour before ceased
spontaneously to contract, I removed the muscles, and cellular membrane
covering its nerves, and large blood vessels. I then placed one end of
a rod of pure silver in contact with one side of these nerves, and
blood vessels, and one end of a rod of zinc on the other, both of them
at about the distance of the third part of an inch from the auricles of
the heart. On bringing the opposite ends of these rods in contact with
each other, the auricle first, and then the ventricle of the heart
immediately contracted, and repeated their contractions as often as the
ends of the metal rods were made to touch each other. When a stick of
glass, wax, or wood, was made use of in place of one of the metals, no
contraction took place. Contractions, however, were excited by
irritating the heart itself with the point of a sharp instrument. The
contractions were both more vigorous, and more constant when the metals
were placed in contact with the heart itself, than when touching only
its blood vessels and nerves. I have several times attempted to trace
some of the nerves, which may be seen near the large blood vessels of
the heart of a frog, into the heart itself, in order to arm them
separated from other parts; but, partly on account of their minuteness,
and partly on account of the weak state of my eyes, which does not
permit me to look intently at minute objects, I have never been able to
succeed.
Since making this last experiment, I have repeated it upwards of twenty
times. In order to its complete success, it is necessary that the
spontaneous contractions of the heart should nearly, if not altogether,
have ceased; and, when in this state, the experiment is rendered still
more satisfactory by removing the heart from the body of the frog, and
laying it upon a plate of zinc. We are then sure that its contractions
cannot have been excited, by any mechanical irritation, arising from
the contractions of the muscles of the thorax.
For want of sufficient leisure, and convenient opportunities, I have
neglected to make this experiment upon any animals of warm blood,
except cats and rabbits. A few days after I had discovered the
possibility of exciting the heart to contraction by means of zinc, and
silver applied to its nerves, I procured an ordinary sized cat, and
drowned it in water, as nearly as possible, of its own temperature.
Four minutes after immersion, it was taken out of the water and dryed.
Its thorax was immediately laid open, but no contractions were observed
in any part of its heart, except in the right auricle, and even these
were very slight. A plate of zinc was then placed in contact with the
parvagum, and intercostal nerves, on one side of the trachea, and a
half crown piece in contact with those of the other; both at the
distance of about one third of an inch from the auricles. Every time
the zinc and silver were brought into contact, complete contractions of
the right auricle, and sometimes slight ones of the left were produced,
but none in the ventricles. The contractions were observed to become
stronger, in proportion as the metals were approached to the heart, and
were strongest when one or both was in contact with the auricle. I
think the contractions were fully as strong when molybdena, as when
silver was used. No contractions could be excited, by arming any of the
nerves of voluntary muscles, in this cat.
The next experiment was made upon a female cat, far gone with young.
She was drowned in very cold water, and although her thorax was opened
the instant she had ceased to struggle, which was in less than four
minutes after immersion, her heart had ceased to contract; nor could
its contractions be renewed, either by the application of the metals in
the way described, in the last experiment, or by pricking or otherwise
irritating its surface: but the diaphragm, the intercostal muscles, the
fore legs, and the ears, continued to contract long and vigorously,
when the metals were as usual applied to their nerves. On cutting into
the uterus, however, and taking out one of the young, I found both
auricles and ventricles of its heart, contracting most vigorously,
though the mother had now been dead upwards of twenty minutes.
An opportunity, not to be neglected, now presented itself, of trying if
it were possible to transmit this influence from the mother to the
fœtus, through the medium of the umbilical chord. I therefore applied
the two metals in the manner I before described, 1st, to the uterus of
the mother, and to the cotyledans; afterwards to several different
parts of her; but neither uterus nor fœtus were in any instance
affected. As little was the fœtus affected, by arming the chord itself.
As the hearts of the kittens continued their spontaneous contractions,
for more than an hour after they were taken from the mother, I had
repeatedly the pleasure of observing, and pointing out to Mr Thomson,
and Mr Simpson, who obligingly lent me their assistance in these
experiments, the effects of the metals when in contact with the
parvagum, and entercostal nerves, both of quickening the repetition of
the hearts contractions, while they continued spontaneous, and of
exciting them anew when they had ceased to be so. This experiment,
repeated upon a kitten a few days after birth, succeeded, but not quite
in so satisfactory a manner as the foregoing, although the heart
continued contracting for more than an hour and an half after the
thorax was opened. Its contractions were quickened, and rendered
vibratory by the slightest mechanical touch of its surface; so that it
was difficult to determine the precise share which the application of
the metals had in their production.
When these had ceased, I did not find that I could revive them by the
application of the metals. In the hearts of some young rabbits, upon
which I tried this experiment, the contractions appeared to be still
more decidedly, occasioned by the application of the metals, than even
in the cats.
Having ascertained this important fact, that one muscle, not subjected
to the influence of the will, might be made to contract by the
application of zinc and silver to its nerves; I proceeded to examine
whether the same were the case with respect to all involuntary muscles.
I could not, however, observe that any contractions were produced in
the stomach or intestines, by placing the metals near the stomachic
flexus and semilunar ganglion in a cat. I next proceeded to examine the
effects of the metals upon the different organs of sense.
M. Volta’s discovery of the sensation produced upon the end of the
tongue, by coating its upper and under surfaces with different metals,
led me to compare this sensation with that produced by electricity. I
found a very considerable difference between them. Both, indeed, are
subacid, but as unlike to each other, as the taste of vinegar is to
that of diluted vitriolic acid. That occasioned by the metals is
accompanied with what is familiarly called the metallic taste; and
differs according to the metals employed. With the greater number of
metals it is scarcely perceptible. With zinc and gold, I think, it is
strongest; next so with zinc and silver, or molybdena, and insufferably
disagreeable with any of them.
The sensation is most distinct when the tongue is of its ordinary
temperature, and when the metals are of the same temperature with the
tongue. When either the tongue, or the metals, or both, are heated or
cooled, as far as can be borne without inconvenience, scarcely any
sensation is produced. That this difference in the effect is owing to
the alteration which has been produced in the state of the tongue, and
not to that in the temperature of the metals, is evident from
experiments which I have already related; from which it appears that
neither the conducting, nor the exciting powers of metals are affected
by differences of their temperature. But I have found it the uniform
result of many experiments, that both the life and irritability of the
most vigorous frogs is completely destroyed in a few minutes, by
placing them in water heated to 106 degrees of Fahrenheit’s scale.
Cold, however, though it appears to affect the sensibility of the
tongue nearly as much as heat, did not, in one or two instances in
which I tried it, affect the irritability of the muscles of a frog.
Some separated legs contracted equally well after they had lain upon a
piece of ice for some hours, as they did before they had been in that
situation.
Whatever has a tendency to blunt the sensibility of the tongue, as
laudanum, a strong solution of opium in water, distilled spirits,
acids, &c. diminishes the effect of the metals. Acids, I think,
diminish it least.
On placing different metals in the meatus auditorius externus of both
my ears, and establishing an insulated metallic communication between
them, I felt, or fancied that I felt, a disagreeable jirk of my head.
The metals used were a silver probe, a roll of tin-foil, and a common
brass conductor belonging to an electrical machine. On withdrawing them
from my ears, I experienced a feeling similar to that which one has
after emerging from under water. I was not sensible of having hurt my
ears by the experiment, nor had I any uneasy sensation after it; but,
on getting out of bed next morning, I perceived both my pillow and my
face stained with blood; and, on examining, found that it had come from
one of my ears. An hæmorrhagy from this part had never happened to me
before. From whatever cause this accident happened, (and it is highly
probable that it arose from some hurt unperceived at the time), I need
not say, that I have never repeated the experiment, and that I
certainly never shall.
I never could perceive, that the senses, either of touch or of smell,
were in the least affected by the metals; but the effect which they
produce upon the eye is very remarkable. Having laid a piece of
tin-foil upon the point of my tongue, I placed the rounded end of a
silver pencil-case, against the ball of my eye, in the inner canthus,
and suffered them to remain in these situations till the parts were so
far accustomed to them, that I could examine the sensations produced; I
then brought the metals into contact with each other, and, to my
surprise, perceived a pale flash of light diffuse itself over the whole
of my eye. My tongue was at the same time affected with a similar
sensation to that produced when both the metals are in contact with it.
On darkening the room, the flash became more distinct, and of a
stronger colour. This sensation is not the effect of pressure upon the
eye, as in Sir Isaac Newton’s experiment; for no pressure should be
used. All that is required, is, that the silver lie between the lids of
the eye, and in contact with any part of the ball. If the experiment be
made with zinc and gold, instead of tin-foil and silver, the flash is
incomparably more vivid. I had the disagreeable opportunity of trying
this experiment upon one of my eyes, in a state of inflammation; and,
in this case, found the flash much more strong than it was in the
uninflamed eye. I tried it likewise upon a patient, affected with
amaurosis; but the man was so stupid that I could not satisfy myself as
to the precise result.
Recollecting that fine nervous twigs pass from the ciliary or
ophthalmic ganglion, through the sclerotic coat of the eye, to the
choroid coat, and to the uvea; and that this ganglion is in great part
formed from a twig of the nasal branch, of the fifth pair of nerves, in
conjunction with a branch of the third, I proceeded to try if, by
insinuating a rod of silver, as far as possible, up my nose, and thus
arming this nasal branch, I could, by bringing the silver into contact
with a piece of zinc, placed upon my tongue, pass this new influence up
the course of the nerve, and thus produce the flash in the eye. The
experiment answered my most sanguine expectation. The flash, in this
way produced, is, I think, if any thing, stronger than when the ball of
the eye itself is armed. I now thought I had discovered a certain
method, by which I could ascertain the effect of Galvani’s influence,
upon a very important, involuntary muscle, the human iris. It occurred
to me that the ingenious physiologist Dr Whytt, had been able, through
the medium of the nasal branch of the fifth pair of nerves, to produce,
at pleasure, dilatations of the contracted pupil of a boy, in the last
stage of hydrocephalus, by applying aq. ammonia to his nostrils; and
this instance of the affection, of an involuntary muscle, through the
medium of its nerves, had, previously to making any experiments upon
the subject, always operated with me as a strong presumptive argument,
that the contractions of the heart might be influenced in a similar
manner.
I therefore desired some of my friends to observe my pupil, while I
repeated the experiment, which I have above described. When the
external light was strong, they found some difficulty in determining,
whether the pupil contracted or not; but when no more light was
admitted, than what was just sufficient for discerning the pupil, they
perceived a very distinct contraction, every time the metals were
brought into contact with each other. This experiment requires some
attention, in order that it may succeed satisfactorily; but although I
have repeated it a great number of times upon the eyes of others, it
has seldom failed, when made in a steady light, and when the silver has
been passed far enough up the nose.
The dilatation of the pupil, instead of its contraction, on the
application of a stimulus to its nerves, as in the case related by Dr
Whytt, is, I apprehend, not so uncommon a circumstance, as it may at
first be supposed. I have myself seen three instances of it in diseases
of the head. One of these was in an epileptic patient, whose pupils,
during the intervals of his fits, became suddenly dilated whenever his
eyes were exposed to a strong light.
My friend, Mr George Hunter of York, while one day amusing himself with
repeating some of these experiments, discovered that by placing one of
the metals as high up as possible between the gums and the upper lip,
and the other in a similar situation with respect to the under lip, a
flash was produced as vivid as that occasioned by passing one of the
metals up the nose, and placing the other upon the tongue. It differs,
however, from the flash produced in any other way, in the singular
circumstance of not being confined to the eye alone, but appearing
diffused over the whole of the face. On attending to the concomitant
sensations produced by this disposition of the metals, I perceived that
a sense of warmth, at the instant they were brought into contact,
diffused itself over the whole upper surface of the tongue, proceeding
from its root to the point. Dr Rutherford, to whom Mr Hunter had
communicated this experiment, remarked, on repeating it, that a flash
is produced not only at the instant the metals are brought into
contact, but likewise at the instant of their separation. While they
remain in contact, no flash is observed.
This fact is precisely analogous to one already mentioned of
contractions being produced in the leg of a frog, at the instant one of
the metals in contact with the other metal is withdrawn from the leg.
After this full detail of these curious phenomena, I hardly need
remark, that they demonstrate the free communication, which subsists
between the several branches of the fifth pair of nerves, and
consequently give strong support, if not absolute confirmation, to the
well known doctrine of nervous sympathy, or of the reciprocal
influence, which different parts exert upon each other, through the
medium of nerves.
If I might be allowed to hazard a conjecture, where we cannot have
recourse to demonstration, I should say that the flash, observed in the
above experiments, was the effect of contractions excited in
involuntary muscles by the application of a stimulus to their nerves;
or, in other words, that the effects of the application of the metals
to the nasal branch of the first division of the fifth pair of nerves,
had been propagated through the ciliary ganglion, along the ciliary
nerves, and to the choroid coat, whose vessels it had excited into
instantaneous action; and that their action again (as in the case of
action excited by pressure, or a blow upon the eye,) had by stimulating
the retina occasioned the sense of light.
This supposition is, I think, rendered probable by several
considerations. I have already shewn that this influence can excite
contractions in involuntary muscles, through the medium of their
nerves. And certainly no reason can be assigned, _a priori_, why it
should not act equally upon every description of involuntary muscles;
upon those which make a part of the minutest vessels in the body, as
well as upon the heart, or upon the iris.
That it excites to increased action the arteries of the tongue in the
experiment, in which a sense of warmth is produced along its surface by
the application of the metals to the lips, seems to be almost
demonstrated; for it would be difficult to point out the presence of
another cause competent to occasion the evolution of the heat, in this
case, besides the increased action of the arteries: and that this cause
is competent to the effect we know from numberless experiments, too
familiar to need being particularized here.
Whether the metals, however, do or do not affect the action of the
blood vessels, is a question which admits of solution by experiment.
The following, I confess, was not quite satisfactory, and I have not
yet found leisure and opportunity to repeat it with all the attention
it requires.
I inspected the foot of a living frog with a microscope of very high
powers. In fixing the foot so as to keep the web expanded, a
considerable degree of inflammation was excited, notwithstanding every
precaution to avoid it. The current of blood was seen distinctly in
several vessels, now flowing rapidly, now slowly, and now in a
direction contrary to that in which it was first observed, but with
equal rapidity. A thin plate of zinc was introduced between the fleshy
part of the foot and its supporter, and a silver probe was used as an
excitor. To me, the circulation appeared very decidedly to be quickened
several times when the metals were made to touch each other: but the
gentlemen who assisted me could observe no change. To prevent the
contractions in the muscles of the leg from producing any fallacy, the
crural artery should be laid bare, and insulated from surrounding
parts, by passing a thin plate of glass, or sealing wax, between it and
them.
That the flash is the effect of such an increased action of the
vessels, composing the choroid coat, might be somewhat more difficult
to prove. It is however known to every one, that a blow, and that
pressure upon the eye, are capable, as I have before observed, of
producing a similar effect. And the following case, which Bonetus
quotes from Hermannus Cummius, if it may be credited, affords an almost
positive proof, that vision depends upon the stimulus given to the
retina by the activity of blood vessels in some part of the eye.
‘Quando theologus, plaga dolorifica, a rupta instrumenti musici chorda
accepta, nocte subsequenti jam adulta, e somno evigilans, cuncta clare,
ac si de die esset, vidit, adeo, ut minimos picturarum et tapetum
tractus observare, characteresque ex libro legere posset. Oculo vero
læso clauso, tenebras densissimas adesse ille percepit, eodemque iterum
aperto, conclave illustratum visum est, lucem tamen candelæ allatae
solisque splendorem de die, ægre tulit oculus affectus, quod per
aliquot dies duravit, tandemque sensim remisit.’
Haller speaks of such cases as by no means uncommon, and quotes the
names of several authors, who have related similar ones.
The direction of this influence, when suffered to pursue its natural
course, appears to be the same with that of most other stimuli, i. e.
from the place at which it first affects a nerve, onwards to the part,
in which that nerve terminates. I have repeatedly caused electrical
sparks to be passed into my own ulnar nerve at its passage over the
inner condyle of the humerus, but both the sensations and the
contractions produced by them have been entirely confined to the hand
and fore arm.
It appears too, both from the experiments of Dr Monro, and of Dr
William Alexander of Halifax in Yorkshire, that when no communication
is left between the trunk and posterior extremities of a frog, except
by its sciatic nerves, a strong solution of opium, injected under the
skin of its posterior extremities, deprives them both of their
sensibility and of their contractile power; but that it does not in the
least affect the trunk of the body. If, on the contrary, it be applied
to the trunk, it exhausts both the trunk and the extremities.
M. Galvani is said to have observed the effects of the influence, which
he discovered, diffused over the whole body of a frog, when the metals
were applied to a nerve merely laid bare, without being either divided
or separated from surrounding parts. If we are allowed to infer this
diffusion of the influence from the restlessness expressed by the
animal, M. Galvani’s observation may be just. If from the contractions
produced, I suspect it is by no means so; since, in every experiment
which I have made upon the subject, the contractions have been confined
to those parts to which the nerve touched by the metals was distributed.
That this influence, however, may pass in a direction contrary to the
course of nerves, is evident from some of the experiments which I have
related relative to its effects upon the senses, but is still more
clearly demonstrated by the following.
If, after having divided at the pelvis a frog recently killed, the
sciatic nerves be freed from cellular membrane up to their origin from
the spine, and all the parts below this, except themselves, be cut
away, the muscles on each side of the spine, for some little way up,
may be brought into contraction by touching the nerves alone with the
two metals in contact. This experiment has not always succeeded with
me, and never unless the frog had been recently killed. So long as the
hind legs remain undivided from the nerves, it never succeeded; the
only contractions produced being in the legs.
OF THE BLOOD VESSELS.
We are told by Dr Valli, that no contractions are excited by arming the
blood vessels; but as he has not told us whether his experiments were
made upon them while the blood still continued to flow through them, or
after they had been deprived of their blood, I determined to make the
following experiment.
Having laid bare, and separated from surrounding parts and from each
other, the crural artery, and nerve, in the thigh of a full grown frog,
I cut out the whole of the nerve between the pelvis and the knee. I
then insinuated beneath the artery a thin plate of sealing wax, spread
upon paper, and broad enough to keep a large portion of the artery
completely apart from the rest of the thigh. The blood still continued
to flow, through the whole course of the artery, in an undiminished
stream. The artery, thus partially insulated, was touched with silver
and zinc, which were then brought into contact with each other; but no
contraction whatever was produced, in any muscle of the limb. This
experiment was frequently repeated upon several different frogs, both
in whom the nerve was, and in whom it was not, divided. The result was
uniformly the same. But vivid contractions were produced in the whole
limb, when an electrical spark, or even a full stream of the aura, was
passed into the artery.
It, however, by no means follows from this experiment, that the
sanguiferous system of animals bears no relation whatever to the
influence discovered by Galvani. I have already shewn, that the heart
may be affected by it, and have given reason to believe, that the
smallest arteries of the body are not exempted from its action. Should
it ever be proved to be an exclusive property of animals, it is not
impossible but that even its origin may be traced to their sanguiferous
system.
-----
Footnote 11:
I have not been at the pains to inform myself, who first was the
author of this doctrine; but its adoption by Caldani, by Haller, and
by Fontana, and by all upon the faith of experiment, was certainly
sufficient to give it currency, in opposition to that of Willis,
Lower, Kaau, Boerhaave, Laghi, and even of the ingenious Whytt.
Footnote 12:
Mem. Med. Soc. Lond. vol. iii.
------------------------------------------------------------------------
SECTION IV.
_An attempt to investigate the Source from which the respective
Powers of Nerves, and of Muscles, are derived._
As yet, the question whence the nerves and muscles of animals derive
their respective properties, remains in a state of doubt. By many, the
brain has been considered as the source not only of the several
energies exerted by nerves, whether appropriated to sensation, to the
excitement of muscles subservient to the will, or distributed to organs
exempted from its influence; but likewise of that unascertained power,
by which muscles contract on the application of a stimulus.
By others again, these several properties are supposed to be derived
from the arteries, which may either supply the materials and
construction of that exquisite and peculiar organization, which fits
nerves and muscles for performing their respective functions, or may
furnish, from the blood, some subtile principle, such as that believed
by M. Fontana, to exist there, or such as that we are now examining,
which differently modified in different parts, may be the latent cause
of all the phenomena exhibited by animals.
The advocates for the first opinion observe, that whenever the brain is
considerably injured, or its free communication, by means of nerves,
with moving parts is interrupted, a deprivation both of sense and
motion is the uniform consequence: and, further, that the several
organs, both of sense and of motion, appear to suffer detriment from
the over strained exertions of the brain in thinking, equal to that
which they experience from their own exertions.
The second opinion is countenanced by facts and observations not less
important. From experiments of Haller; some which are recorded in one
of the early volumes of the Philosophical Transactions, and others, it
appears that a paralysis of the posterior extremities of animals was
induced by tying their aorta.
Both Dr Monro and Dr Alexander of Halifax have remarked, that when all
the blood vessels, supplying the posterior extremities of frogs, had
been divided, and a solution of opium injected under the skin of these
extremities, they became, in less than half an hour, both motionless
and insensible; whereas, the fore part of the body was not observably
affected six hours afterwards; and, in Dr Monro’s experiments, the
frogs lived till the day following. Hence Dr Monro concludes, ‘that
concomitant arteries, somehow or other, tune the nerves, so as to fit
them to convey impression[13].’
On the other hand, where it is intended that nerves shall convey
impressions with great accuracy, as in all the senses, and very
remarkably in the part which some have amused themselves by considering
as a sixth organ of sense, the distribution of blood vessels is more
profuse than in almost any other equal part. It is likewise universally
true, that increase of vascular action in a part is always attended
with a proportional increase of sensibility there.
From the valuable experiments made by Mr Cruikshanks, and which have
since received the fullest confirmation from those repeated by M.
Fontana and others, it appears, that whatever may be the relation
between brain and nerves, the latter may certainly be regenerated after
excision, and have their functions fully restored. Now, in what manner
this can be accomplished, unless by the agency of arteries, would, I
imagine, be no easy task to point out.
The influence discovered by Galvani appeared to me an admirable test,
by which something decisive might be ascertained relative to these
important points in the physiology of animals, and as such I have
employed it in the following experiments.
Considering, therefore, the brain on the one hand, and the sanguiferous
system on the other, as the possible sources from which nerves and
muscles might derive their power, I began by comparing the effects
which result from interrupting their communication, first with the
brain, and then with the arteries. This mode of procedure seemed to
afford the best prospect of information with respect to every object
which I had in view, but particularly with regard to the relations
which this influence may bear to the several parts examined.
Before relating the experiments, I must observe that the comparison was
instituted between the effects of only partially interrupted
communication; since it must be obvious that a complete interruption,
either of nervous or of arterious communication between any part of an
animal, and the rest of its body, could not have been effected without
so far injuring the animal, as to render the result fallacious.
_Experiments in which the Sciatic Nerves of Frogs were divided._
EXPERIMENT I.
I divided the sciatic nerve, on one side only, in four large frogs. The
division was made at the very top of their thighs, and before the nerve
had given off the first large branch to the muscles of the thigh. This
nerve lies immediately underneath the large crural artery, to which it
is closely attached by a sheath of fine but very strong cellular
membrane. A small nerve, which supplies some of the muscles on the
under side of the thigh, was suffered to remain undivided. The legs,
whose nerves had been divided, became completely paralytic below the
knee, and very nearly so above it. These legs too, immediately after
the division of their nerves, contracted vigorously when laid upon
zinc, and excited by passing a rod of silver in contact with the under
part of the knee till it touched the zinc; but the other legs which
were suffered to remain in their natural state, in order that the
contractility of one leg might all along be compared with that of the
other, did not contract when the metals were similarly applied to them.
These frogs were all killed by cutting off their heads; the first, at
the end of two days after dividing the nerve; the second, at the end of
five days; the third, at the end of seven; and the fourth, at the end
of nine. Their legs were carefully examined, in the manner I have
described, four or five times every day after their heads had been
taken off, so long as any contractions could be excited; but I could
not perceive, in any one of these instances, that the contractile power
continued either longer or more vigorous in the legs, in which the
nerves were not divided than it did in those in which they were.
Both in these experiments, and in all my others, where a comparison was
instituted between the two legs of the same frog, I divided equal
portions of skin on both thighs, that there might be no unequal
exposure of the muscles to the water, which would have occasioned a
fallacy in the result.
EXPERIMENT II.
On the 31st of March last, I divided, in two, a frog, in one of whose
legs I had four months before excited inflammation, by laying bare the
crural artery and nerve. The inflammation had been so violent and
general, that the frog lost its cuticle in consequence of it, and, when
compared with a healthy frog, its resperation was observed to be
remarkably frequent. Three weeks after this, when the wound in its
thigh had perfectly skinned over, I laid it open again, and divided the
sciatic nerve. No general inflammation this time took place, nor did
the wound again skin over; but for about a month before it was killed,
a large ulcer had formed immediately over the division of the nerve,
but had not proceeded down to it. The limb, at the time I killed the
frog, was as destitute both of motion and of sensation, as at the first
instant the nerve was divided, but contractions were excited in it, by
touching the ulcer with zinc and silver. When the frog was dead,
however, the contractions were found much more feeble in this than in
the other leg.
The metals were now applied to the sciatic nerves within the abdomen.
Vigorous contractions were excited in the sound leg, but none in that
whose nerve had been divided. Hence it was plain, that no actual
regeneration had taken place. On examining the nerve accurately at the
part divided, I found the divided ends, which had receded considerably
from each other, connected by a transparent gelatinous substance. From
the upper end, which appeared elongated into a conical form, several
red streaks projected into the interposed substance. The lower end was
opaque, thickened, and rounded. No appearance of spiral bands could be
detected, either in the interposed substance, or in the part of the
nerve below the division, when these parts were examined with the
assistance of a microscope. This substance had attained sufficient
consistence to support the under part of the nerve, when the upper was
raised with a pair of forceps. The leg, in which the nerve had been
divided, continued to contract as long as the other, though much less
vigorously, and the part, from which I could longest excite
contractions, was the ulcer.
EXPERIMENT III.
On the 14th of April last, I killed two other frogs, by dividing their
hind extremities from their bodies. In one, the right sciatic nerve had
been divided more than six weeks previous to its death. In the other,
one of the sciatic nerves had been divided between three weeks and a
month.
The legs of these frogs, examined by the metals both before and after
their separation from the body, were found in a state very different
from those before spoken of. The contractions were scarcely
perceptible. The incisions made through the skin, in order to get at
their nerves, had closed completely in less than a week after they had
been made.
The appearance of the muscles in the legs, whose nerves had been
divided, was found to be precisely the same as in those where nothing
had been done; but, notwithstanding this circumstance, even strong
electrical sparks excited but very feeble contractions. On examining
the nerves, the ends of that which had been longest divided were found
connected by a substance not at all resembling nerve, but similar to
that found in the former experiment, and evidently proceeding from the
upper division. In the nerve which had not been so long divided, this
circumstance was still more apparent, as the substance had not extended
quite to the lower division. The cellular membrane surrounding these
upper divisions had the appearance of innumerable vessels finely
injected, and some red streaks were seen projecting, as if from the
nerve itself, into the gelatinous production. In the sound nerves, the
obliquely transverse lines of alternate opacity and transparency, or,
as Fontana has called them, the white spiral bands of nerves, were seen
distinctly at the first glance of the eye, and without the assistance
of a glass; but no appearance of these could be found in the parts of
the divided nerves below the division; these were uniformly opaque.
Their bulk, however, was not in the least diminished. The organization
of nerves long divided, therefore, undergoes a very evident alteration,
although it is by no means so clear that the same change happens in the
muscles, to which these nerves are distributed. Yet their
susceptibility to the action of electricity, as well as to that of this
new influence, was nearly lost. Some may consider this as an additional
argument, that stimuli act upon muscles only through the medium of
nerves.
I have before observed that muscles of frogs, from whom the skin has
been stripped, become in a short time hard when exposed to the action
of water. Wishing, therefore, to see if there would be any difference
between these legs, whose nerves had been divided, and others, in this
respect, I laid them in water, and examined them every ten minutes, but
both became hard nearly at the same time. Mr Allen, a gentleman well
versed in physiological pursuits, was with me when I examined the
alteration which had taken place in one of these nerves, in consequence
of its having remained long divided, and I had afterwards an
opportunity of shewing it to Dr Rutherford. In all the frogs, whose
nerves I have divided, I have observed that the divided extremities,
though placed in most exact contact from each other, had after a time
receded at least 1/12 of an inch from each other.
_Experiments in which the Crural Arteries of Frogs
were tied as near to the Trunks of their Bodies,
as where the Nerves had been divided in the former Experiments._
EXPERIMENT I.
Both crural arteries of a full grown frog having been laid bare, one of
them was tied. The leg, in which this was done, became instantly weaker
than the other, and rather dragged when the animal was put into water.
The frog, however, could still jump about with great agility. Four
hours after this operation, it was killed by crushing its brain. It
continued to move its legs spontaneously, when touched, during more
than two days after this, and contractions were excitable by the
application of the metals for two days longer. Sometimes it appeared
rather doubtful, which leg contracted most vigorously, but, in general,
the leg in which the artery remained free did so, and contractions
could be excited in it, more than an hour after every means to excite
them in the other leg had failed.
EXPERIMENT II.
Ligatures were passed round the crural arteries of two other frogs, and
one of them was suffered to live thirty six hours afterwards, before
its head was crushed: the other four days. In these, the disproportion
between the vigour and continuance of the contractions in the compared
legs, was so much greater than in the preceding experiment, as to leave
no doubt of the effects produced by tying an artery. The leg, whose
artery had remained tied four days, never contracted near so strongly
as its fellow, and contractions had ceased to be excitable in it,
upwards of twenty hours before they had ceased in the leg, whose artery
had not been tied.
From these experiments, it appears decidedly, that a much greater
detriment to that condition of a limb, upon which contraction depends,
is induced by interrupting its circulation, than by intercepting its
communication with the brain.
But still, as the effects arising from the interception of the
influence of the brain, and of the circulation, were not compared with
each other in the same but in different animals, whose age, relative
strength, &c. might possibly differ, I thought proper to repeat the
comparison, in the following manner.
_Experiments in which the Sciatic Nerve was divided on one side,
and the Crural Artery tied on the other._
EXPERIMENT I.
I divided the sciatic nerve of one leg, and tied the crural artery of
the other, in a large frog. Scarcely any blood was lost in doing
either. Two days after this, I strangled it. During the first 24 hours,
the leg, in which the nerve had been divided, appeared to contract with
most vigour; after this period, the difference between them became more
doubtful; but the contractions were at no time stronger in the leg,
whose artery was tied, than in that whose nerve was divided.
EXPERIMENT II.
The same operations were performed upon a large female frog full of
spawn. Four hours afterwards, she was observed covered by a male, who
had been treated in a similar manner. I mention this circumstance, as
it tends to prove that the pain occasioned by the operation was
probably not so great as to produce much fallacy.
On the day following, she had spawned, and on the sixth day from the
operations, she was strangled. When laid upon a plate of zinc, and
excited by means of a rod of silver, the contractions were found
extremely feeble in the leg whose artery had been tied, and ceased
altogether in about twenty-two hours after her death. In the leg, whose
nerve had been divided, they appeared as vigorous as they usually are
in legs to which no injury has been previously done, and continued
excitable upwards of two days after they had ceased to be so in the
other.
EXPERIMENT III.
Having tied the crural artery on one side, and divided the sciatic
nerve on the other, on three full grown male frogs, I strangled them
all on the sixth day following. My motive for killing the frogs,
subjected to such experiments, either in this manner or by crushing
their heads, will be obvious. It was of consequence to preserve their
circulation as entire as possible, and, at the same time, avoid the
continuance of pain, which by exhausting all the parts of the body,
whose communication with the brain was not interrupted, might
considerably have affected the result of the experiments.
The contractions excited by means of the metals, were, in all these
instances, likewise as much more strong and durable in the legs, whose
nerves had been divided, than what they were in the legs, whose
arteries had been tied, as what I had found them to be in the preceding
experiment.
Having thus found, that a diminution of the circulation of a part, was
accompanied with a proportionable diminution of the respective powers
of nerves and muscles in that part, I next proceeded to examine if an
increased circulation would be attended with a proportionable increase
of these powers. That this is actually the case, with respect to the
nerves, the few facts which I have related of the eye, in a state of
inflammation, have a tendency to prove; and we all know how much the
sensibility of every part of the body is increased, by an increase of
vascular action. That a similar relation subsists between an increased
action of the arteries, and the contractile power of muscles, is, I
think, proved by the following experiment.
_Experiments made with a view of ascertaining
some of the Effects of Inflammation._
EXPERIMENT I.
I have before said that if a living and entire frog be set upon a plate
of zinc, contractions can very seldom be produced in any part of its
body by passing a rod of silver over it, so that the silver, the frog,
and the zinc, may be all in contact with each other. But, I have found
in upwards of twenty experiments, that when inflammation had been
excited in one of the hind legs of a frog, by irritating it with a
brush, contractions uniformly took place in that leg when the metals
were applied to it, although none had been produced in it before it was
inflamed, nor could still be produced in the other leg which remained
in its natural state.
EXPERIMENT II.
Having previously excited inflammation, by means of a brush, in the
foot and leg of a healthy and large frog, I cut off its head. The
contractions excited by the metals in the inflamed leg were in vigorous
and instantaneous jirks; those in the sound leg more languid and
difficultly excited. Spontaneous motions continued at this time nearly
the same in both. Till the end of the second day, after this frog’s
head had been taken off, the contractions excited in the inflamed leg
continued uniformly, and beyond all comparison more vigorous than what
I could by any means excite in the sound leg. But, after this time, the
inflamed leg became hard as a piece of wood; probably in consequence of
the effusion to which the inflammation had given rise.
The event of five similar experiments was so nearly the same, that I
should be thought unnecessarily minute, were I to relate them in detail.
We are now perhaps prepared to account for the deficiency of
contractile power in those legs, whose sciatic nerves had been divided,
the one, between three weeks and a month, the other, six weeks,
compared with its continuance in the leg, whose nerve had been divided
upwards of three months. It appears, from the circumstances of those
experiments, that some of the arteries, appropriated to the supply of
the sciatic nerves of frogs, have the same course with the nerves
themselves; since the deposition of new matter could in all be traced
from the upper division of the nerves. It is obvious, therefore, that
the part of the nerves below the division, must have been deprived of
so considerable a portion of their usual arterial supply, as in time
would occasion some alteration in their structure, and consequently in
their powers. We accordingly find that such alteration of structure,
and such deficiency of power, had actually taken place. It is further
probable, that, in proportion as the supply from the arteries was
restored, the powers of that nerve, which had been three months
divided, had been likewise restored. This supposition is countenanced
by every instance in which nerves are reproduced; as we find the
functions of the parts in which they had been divided, are not
immediately, but gradually restored.
M. Fontana seems too hastily to have adopted the opinion, that the
sciatic nerves, when divided, are probably never reunited by truly
nervous structure, because no reunion took place during the very short
period which he suffered to elapse between their division, and their
subsequent examination. In the experiments, which I have related, the
progress towards reunion seems to have borne a very exact proportion to
the time the nerves had remained divided; and, in an experiment related
by Dr Monro, where the sciatic nerve of a frog had been divided a year
previous to the death of the animal, the reproduction was advanced so
far as to have the appearance of being perfect. Nor can I doubt, that
both the sensibility and the motion of the limb would have been
restored, had the animal been permitted to live a sufficient length of
time. The following fact renders the supposition at least extremely
probable.
In the first volume of the Edinburgh Medical Essays, the case of a
Captain of a man of war is related, who entirely lost the use of his
right arm, in consequence of a gun-shot wound received in his neck. The
circumstances of the case are such as leave no reason to doubt, that
the loss of the power of motion, in this gentleman’s arm, was owing to
the division of the cervical nerves proceeding to the arm: yet both the
full use, and strength of this arm, were restored, after a period of
about two years and a half. A proof perfectly satisfactory that an
actual regeneration of nerves had, in this case, taken place; and if in
this, one sees no reason why it should not equally take place in any
other part of the body.
It might be difficult to assign a satisfactory reason for the very
speedy reproduction of the intercostal, parvagum, and recurrent nerves,
when compared with the great length of time required for the
reproduction of others. May it not be owing to the very profuse manner
in which they are supplied with arteries, probably both in an
ascending, and in a descending direction; from above, by the superior,
and from below, by the inferior laryngeal arteries?
It appears upon the whole, therefore, tolerably certain, that the
sanguiferous system contributes more immediately than the brain to the
support of that condition of muscles and of nerves, upon which the
phenomena of contraction depend; since that condition is much more
injured by intercepting the influence of the former than of the latter.
Every experiment and observation, which has been made upon the subject
of nutrition, and of the reproduction of parts, clearly demonstrates
that nerves and muscles, in common with every other part of the body,
derive their structure from the arteries; and it is evident, that upon
this structure their several properties must in some measure depend.
But M. Galvani’s discovery of a subtile influence, which may be
transmitted apparently from one part of an animal to another through
foreign media, may reasonably give rise to a conjecture that the
phenomena exhibited by nerves and by muscles may perhaps depend more
immediately upon some such influence; and reasons exist, which might
induce some to suspect that even this is derived from the blood.
_Experiments suggested by some opinions of M. Fontana._
From the greatest number of experiments, perhaps, ever made by one
physiologist, M. Fontana has been led to conclude, that the venom of
the viper, opium, and several other poisons, which he examined, produce
no effects whatever, when applied immediately to nerves and muscles
alone, but that they destroy life, by exerting their influence upon
some subtile principle existing in the blood.
Independent of the experiments, published by M. Fontana, on this
subject, his opinion respecting the existence of such a principle may
be thought to receive no inconsiderable countenance, from the opinions
of Harvey and of Mr Hunter, concerning the life of the blood, and from
those experiments, by which Mr Hewson has demonstrated, that changes
are instantaneously produced upon the coagulability of the blood, by
passions of the mind, and whatever else affects the action of the heart
and arteries. An experiment made by Dr Alexander of Halifax, and
published at this place in the year 1790, in his excellent Thesis, ‘De
partibus corporis quae viribus opii parent,’ may at first appear a
sufficient refutation of M. Fontana’s opinion.
He found that thirty three drops of a strong solution of opium in
water, injected into the jugular vein of a large rabbit, destroyed it,
as in M. Fontana’s experiments, in four minutes and a half; whereas,
the same quantity injected into the crural vein in each leg of another
rabbit, with an interval of twenty six minutes between the two
injections, although it rendered the animal sleepy and stupid for a few
hours, did it no material or permanent injury. Hence, Dr Alexander
concludes, that the opium, injected into the jugular vein, did not
destroy the animal by acting upon the blood alone, since if it had, the
same effect, should have been produced, by introducing an equal
quantity into any other vein of the body; but a quantity double of
that, which had occasioned death when introduced into the jugular vein,
failed to occasion it when introduced into the crurals.
It is not, however, by one experiment, formidable as it must be allowed
to be, that the innumerable hosts brought to the contest by M. Fontana
ought to be combated. Besides, it might be objected even to this one,
that the opium was introduced into veins, from which it must have been
so much longer in passing to the arterial blood, than from the jugular
vein, and consequently so much more diluted, and perhaps too altered in
its nature before it got there, as might be sufficient to account for
the difference of result in the two cases compared.
The opportunity afforded by M. Galvani’s discovery, of putting the
truth of the opinion held by M. Fontana more fully to the test, and the
possibility which presented itself, that if any such principle, as he
supposes in the blood, should really be found to exist there, it might
prove to be identically the same with that discovered by M. Galvani,
induced me to make the following experiments.
EXPERIMENT I.
Having selected two frogs as nearly as possible of the same size and
vigour, I deprived one of its blood by opening, first, one of its
crural veins, then, a crural artery, and last of all, the heart. To
assure myself of the complete evacuation of its blood, I next injected
water into its heart, and immediately afterwards forty drops of a
strong aqueous solution of opium[14].
I then removed the sternum of the other frog, and having made an
opening into the ventricle of its heart, injected into it likewise
forty drops of the solution. Less blood was effused in doing this, than
one would at first expect; for the ventricle contracts so strongly,
immediately after the incision, as to prevent much blood from passing
out, unless the incision be made, as it was in the other frog,
purposely large.
The moment, at which each injection was made, was accurately noted, and
the time expended in evacuating the blood from the first frog, was
allowed for. The frog, from which the blood had been withdrawn, ceased
to contract, when irritated, very nearly an hour before the other, even
calculating not from the time of injection, but from the moment I began
to bleed it; nor could I by means of the metals excite contractions in
it, for upwards of a day before they had ceased to be excitable in the
other frog.
EXPERIMENT II.
As evacuating the blood from a living animal is rather a severe
operation, and might have occasioned some fallacy in the last
experiment, by subjecting the frog, in which this was done, to a
greater degree of pain, and consequently of exhaustion, than what the
other was subjected to, I crushed the brains of two other frogs before
I proceeded, as in the former experiment, to withdraw the blood from
one of them. Instead of forty, I injected no more than thirteen drops
of the strong solution of opium, into each of the hearts of these
frogs. The instant the injection had entered, both hearts became white,
and ceased from contracting. Forty eight hours after the injection of
the opium, the contractions excited by the metals in the frog, deprived
of its blood, had become very slight, particularly in the limb whose
vein and artery had been opened. The other frog still continued to
contract with so much vigour, as to raise its body from the plate of
zinc, upon which it was laid. Seventy two hours after the injection, no
contractions could be excited in the frog, from which the blood had
been withdrawn, except some very slight ones in the leg, whose artery
and vein had not been opened. The contractions in the legs of the other
frog, continued still so vigorous as to raise its body from the plate,
and some were produced even by mechanical irritation.
Ninety six hours after the opium had been injected, (both the frogs
having lain out of water all night,) that without blood was found quite
putrid. In the other, the contractions, produced by exciting the legs,
were sufficiently strong to move the feet: as the body, however, had
become putrid and offensive, it was thrown away.
EXPERIMENT III.
The heads of two other full grown and lively frogs, having been
crushed, their hearts were laid bare, and the blood was evacuated from
one of them, as in the former experiments. A small portion of the skull
of each then being removed, eight drops of the strong solution of opium
was injected upon their brains. At least half the quantity seemed to
return from the wound. Both frogs became instantaneously motionless
after the injection, but, in about an hour, were considerably recovered.
Spontaneous motions continued during more than fifty hours, in the legs
of that from which the blood had not been drawn, and contractions were
excitable by the metals, upwards of 24 hours after they had ceased to
be so, in that from which the blood had been drawn.
The following experiments may be deemed still more satisfactory, than
the preceding, from the circumstance of the comparison having been
instituted, between the effects of opium, upon different, but similar
parts of the same frog, differently circumstanced.
EXPERIMENT IV.
One of the crural arteries of a frog having been included in a tight
ligature, as near as possible to the body, I suffered four days to
elapse, and then injected through a perforation in its skull, eight
drops of the strong solution upon its brain, and in a direction towards
its spinal marrow. This frog continued most violently convulsed for
more than an hour, and, in two, was to all appearance dead. When laid
upon zinc, and excited with silver, the contractions were not at first
perceptibly stronger in one leg than in the other. After eight hours,
however, they were evidently most strong in the leg whose artery
remained free. After 21 hours, this difference became still more
decided. At the end of 34 hours, scarcely any contractions could be
excited in the leg whose artery had been tied; though they continued
vigorous in the other; and, at the end of 46 hours, they had ceased
altogether to be excitable, in the leg whose artery was tied. In the
other, they continued during several hours afterwards.
EXPERIMENT V.
Having tied one of the crural arteries of another frog, I filled its
stomach, immediately afterwards, with a saturated solution of opium in
water. The difference between the strength, and the continuance of the
contractions, excited by the metals, in the two legs of this frog, was
not so great as in the former; yet still the difference was
considerable in favour of that leg in which the artery remained free.
EXPERIMENT VI.
In two other frogs, in each of which a crural artery had been tied, and
the solution of opium (without regard being paid to quantity),
repeatedly injected underneath their skulls immediately after; the
contractions appeared to be very little weaker in the legs, whose
arteries were tied, than what they were in the legs in which they were
not tied, and they continued excitable during an equal length of time
in both.
EXPERIMENT VII.
Having tied the crural artery of another frog, I immediately filled
both its stomach and abdomen with a strong solution of opium. In an
hour after this, it was to appearance quite dead. At the end of eight
hours, the contractions, excited by the metals, had become very feeble
in the leg whose artery was tied, in comparison of what they were in
the other leg; and, at the end of twelve hours, no contractions could
be excited in any part of the frog, except in the leg whose artery
remained free. In this they continued excitable about an hour longer.
As it was possible, that the more speedy exhaustion of the legs, in
which the arteries were tied, might have been owing in some measure to
the pain, occasioned by that operation, I repeated the experiment with
the following variation.
EXPERIMENT VIII.
I first divided the sciatic nerves, in both legs of two frogs, and then
tied the crural artery in one leg of each. Eight drops of the solution
of opium were immediately afterwards injected upon their brains. But
the event of this experiment was precisely the same with the majority
of those before related. The contractions excited by the metals, in the
legs whose arteries were tied, were uniformly more feeble, and of
shorter duration, than those excited in the other legs: yet it is
evident, that, in all these experiments, the very reverse of this ought
to have taken place, had it been true, as M. Fontana has asserted, that
opium has no effect upon any part of the body, except through the
medium of the blood.
The experiments however, which I am now to relate, may perhaps appear
still more satisfactory.
EXPERIMENT IX.
Having laid equally bare both the sciatic nerves of a frog, at the
upper part of its thighs, I passed a ligature round one of them, and
drew it as tight as it was well possible, without dividing the nerve. I
then removed a portion of its skull, and with a small brush, kept it
constantly wet with laudanum during several hours. The frog soon became
convulsed; and, during ten or twelve hours, continued in that state of
exquisite sensibility, which opium never fails to produce in these
animals. It may here be worth remarking, that, while they are in this
state, the slightest touch of a feather, or even breathing upon them,
excites instantaneous convulsions. The leg whose nerve was tied,
remained paralytic during this time, but when it was laid upon zinc and
excited with silver, it contracted as strongly as the other. After
forty three hours, the contractions were very feeble in the leg whose
nerve was not tied, but still vigorous in the other. After fifty three
hours, no contractions could be excited in any part of the frog, except
in the leg whose nerve was tied. In this they were sufficiently strong
to move the foot, and continued so for more than an hour longer.
EXPERIMENT X.
One of the crural nerves of another frog having been tied in a similar
manner, eight drops of the strong solution of opium were injected upon
its brain. The animal instantly became motionless, but, in less than an
hour afterwards, was considerably recovered.
The contractions, excited by the metals, in the leg whose nerve was
free, soon became more feeble than those excited in the leg, whose
nerve had been tied. This disproportion, between them, continued
increasing during ninety six hours, after the opium had been injected,
when contractions could no longer be excited in the leg whose nerve
remained free. In that, in which the nerve had been tied, they
continued upwards of 4 hours afterwards.
EXPERIMENT XI.
Immediately after having divided the sciatic nerve, in one thigh only,
of three other frogs, I injected as much of the strong solution of
opium underneath their skulls, as could possibly be retained. The legs,
in which the nerves had been divided, continued contractile several
hours after the others had ceased to be so.
Hence, then, we see no reason for suspecting that the more speedy
cessation of contractions in those legs, in which the crural arteries
were tied, than in those on which no operation was performed, was owing
to the pain occasioned by such operation, since even the more painful
operations of tying or dividing the sciatic nerves, were attended with
no such effect.
Upon the whole, therefore, it appears, that the conclusion which M.
Fontana draws from his numerous experiments with opium, ‘That the
circulation of the blood and humours in the animal machine, is the
vehicle for opium, and that, without this circulation, it would have no
action on the living body,’ is the very reverse of that which I am
warranted to draw from the experiments I have just related; since the
parts, most affected by the action of opium, were not those in which
the circulation remained most entire, but those in which it had been
almost altogether interrupted; and since in two parts where the
circulation remained equal, and entire, the action of opium was
rendered unequal, by interrupting the communication of one of them, by
means of nerves, with the parts to which the opium was applied.
The existence, consequently, of any such principle in the blood, as
that supposed by M. Fontana to exist there, is rendered far too
problematical, even to allow me to expect that it can ever be proved:
far less that it may turn out to be the same with that discovered by M.
Galvani, or with that, whatever it may be, upon which the phenomena of
nerves and of muscles may depend.
-----
Footnote 13:
Essays Physical and Literary.
Footnote 14:
This solution, which is the same that I employed in all my subsequent
experiments, was of the same strength with that used by Dr Alexander
in the greater number of his, viz. an ounce of crude opium mixed in a
mortar with two ounces and a half of water, and filtered through
paper, after having remained twelve hours, in a close corked bottle,
near a chamber fire.
------------------------------------------------------------------------
APPENDIX.
I was unwilling to interrupt the narration of the preceding
experiments, by the mention of the following facts, which they afforded
me an opportunity of observing, as they were not immediately connected
with the objects on account of which the experiments were instituted;
and I have yet some few to relate, which, from the haste with which
these sheets were prepared for the press, I had omitted to insert in
their proper places.
1. In one of my first experiments, in which I had occasion to suffer a
frog to remain tolerably entire, so long as contractions could be
excited in any part of its body, I was surprised to find, on removing
its sternum, that its heart had ceased to contract, nor could be roused
by the application of any stimulus whatever, notwithstanding the
contractions in its hind legs, excited by the metals, were still
vigorous, and continued so for several hours afterwards. On paying
particular attention to this circumstance in another frog, upon whose
brain opium had been injected, I found that its legs continued
excitable, upwards of forty hours longer than its heart. This discovery
of the continuance of the contractile power, in the muscles of the
posterior extremities, so long after its disappearance in the heart, is
so contradictory to the opinion generally received upon this subject,
and long established among physiologists, that I can scarcely expect it
should be credited, by those who may not themselves have opportunities
of observing it. It is a fact, however, of which, in the course of
these experiments, I have had the most satisfactory and uniform proofs,
both in such frogs as have, and in such as have not, been under the
influence of opium. If a different opinion has hitherto been held by
experimentalists upon this subject, it should be recollected, that,
till the discovery made by Galvani, we had no means of ascertaining the
presence of the contractile power of muscles, which had not, at the
same time that they indicated its continuance, a tendency to destroy
it, and consequently to render it impossible for us to trace its
natural progress to extinction.
I have, more than once, observed the same circumstance in both cats and
rabbits.
2. Dr Alexander, in his excellent Thesis already quoted, tells us, that
the contractility of all the voluntary muscles of frogs was destroyed
in the course of a very few minutes, by injecting eight drops of a
strong solution of opium in water, (similar to that which I employed)
upon the surfaces of their brains. But that the contractions of their
hearts did not appear to be much, if at all, affected by this
treatment. In all the similar experiments, which I have made, the event
has been very different. I have not found it possible by any quantity,
either of aqueous or of spirituous solution of opium, injected upon the
brains of frogs, to produce that rapid extinction of the contractility
of their voluntary muscles, of which Dr Alexander speaks. They commonly
recovered in less than an hour, from the complete insensibility and
paralysis, first occasioned by the injection of the opium, and after
that time, their spontaneous motions almost always continued for
several hours longer, and, by the application of the metals,
contractions were excitable even for days. Their hearts, as I have
already said, uniformly lost their susceptibility of the action of
stimuli, long before their posterior extremities.
3. The arguments against the antiquated doctrine of transudation,
through parts of a living body, are already so numerous and
satisfactory, that it may be thought unnecessary to notice in this
place, a decisive one so far as relates to the skin of frogs, at least,
which may be deduced from the fact already mentioned; that so long as
the skin was suffered to remain upon the limbs of frogs, placing them
in water, very evidently preserved the contractility of their muscles,
whereas when the skin was taken off, the muscles became hard, and
incapable of contracting, in a very few hours. Had there been a
possibility of water soaking through the skin, this difference could
not possibly have had place.
4. In speaking of some of the relations, which subsist between the
influence discovered by Galvani and the nerves, I omitted mentioning
the following facts.
A very different effect is produced by applying the metals to the brain
or spinal marrow of frogs, from what is produced by applying them to
their nerves. In the latter case, I have observed, that every muscle,
to which a nerve below the part touched is distributed, is brought into
instant contraction. But no muscles are brought into contraction, when
the metals are applied to the brain or spinal marrow, except such as
derive their nerves from the part immediately in contact with the
metals. The influence does not stimulate or pass along the spinal
marrow, as it would along the trunk of a nerve, to affect all other
nerves branching off from it.
I first became acquainted with this fact, while making the following
experiment. Having laid bare the brain of a living frog, and put a stop
to its spontaneous motions, by gently pressing upon the brain, I
introduced a long slip of tin-foil doubled underneath a part of the
skull, which had not been removed, and placed a silver probe upon its
tongue. The only muscles which contracted, when the tin-foil was bent
over the nose of the frog, so as to come in contact with the probe,
were those which move the eyes, and the transparent membrane which
defends them, those of the tongue and of the throat. When the tin-foil
was twisted into a thin roll, and passed a little way down the spine,
the muscles of the upper extremities and of the thorax contracted; when
a little further, those of the back and of the abdomen contracted; and
when, introduced still further, to where the sciatic nerves leave the
spine, the posterior extremities were, for the first time, strongly
convulsed. I have repeated this experiment very frequently; and have
always found, that, as soon as the spontaneous motions of frogs had
ceased, the contractions, excited by the metals, were uniformly
progressive from the head downwards, corresponding exactly to the
progress of the metals down the spine. The experiment sometimes
succeeds when neither the brain nor the spinal marrow have been laid
bare, and when even the skin has not been divided, but, when the frog
is placed upon a plate of zinc, and one of the ends of a bent silver
wire is placed upon any part of its spine, while the other is made to
touch the plate.
---------------------
5. As it has not been till very lately, that I have been able to
procure an electropherus, I have as yet made but few experiments with
it; their result, however, is such as tends still more to confirm me in
the opinion, that the influence, discovered by Galvani, has no relation
whatever to electricity.
Having, first, so far freed the instrument, from the small quantity of
electricity collected, by wiping it, that none was indicated by a very
sensible electrometer of linen-yarn, suspended from the wooden part of
its handle; I placed it within a few inches of a glass stand, upon
which I had laid a plate of zinc, supporting a frog recently killed,
and with its sciatic nerves within the abdomen laid bare. A bar of zinc
formed the communication between the frog and the metal plate of the
electropherus. Contractions were then excited in the frog, by placing
one end of a bent silver wire, insulated in sealing wax, upon the
nerves of the frog, and the other end upon the bar of zinc. After
strong contractions had, in this way, been kept up for about half a
minute, I carefully removed the bar of zinc, by means of a stick of
wax, that there might be no possibility of the electricity escaping, if
any should have been collected. The metal plate was then raised from
the varnished surface. The electrometer attached to its handle was very
slightly affected; but a fine thread, presented to the plate, was
perceptibly attracted by it.
I had a strong suspicion, that the electricity, thus collected, had
been excited solely by the friction of the frog’s legs during
contraction, against the insulated plate of zinc upon which it lay; and
I soon found that my conjecture was just; for an equal quantity of
electricity was obtained from another frog similarly disposed, when
contractions were excited in it, by merely mechanical irritation.
The result was the same when these frogs were laid successively upon
the metal plate of the electropherus itself, and excited, the one in M.
Galvani’s method, the other by mechanical irritation only.
These experiments were very frequently repeated, but the quantity of
electricity collected was always greater where the contractions, or, in
other words, the friction had been most considerable, and did not, in
any instance, appear to depend on the means employed to excite the
contractions.
What still further proves, that the electricity, in this way collected,
had no dependence whatever, for its production, upon the application of
the metals to the frog, but had been merely the portion of electricity,
naturally possessed by the frog, in common with other conducting
substances, is, that when the electricity, which was collected from its
first contractions, had been drawn off from the plate, no more could
afterwards be collected, although the contractions, excited by the
metals, still continued as vigorous as ever.
6. When the electropherus was charged with electricity, as highly as it
was possible to charge it by friction, the contractions produced by the
insulated metals in a prepared frog, laid upon the metal plate of the
electropherus, were not at all affected by raising it from the
varnished surface. A proof that the phenomena in question are not
affected, either by the condensation or rarefaction of the electricity,
in either the animal or the metals, by which they are exhibited.
I have not found, that any quantity of electricity, which I could
accumulate in the metal plate of the electropherus, did ever, when
discharged into the nerve of a frog, excite contractions nearly so
strong as what are excited by the application of zinc and silver; nor
could I, at any time, collect a sufficient quantity of electricity,
from five insulated frogs, sufficient to excite contractions in a
single leg of a frog, though recently separated from its body, and
consequently excitable by stimuli of very weak powers.
---------------------
The politeness of the very learned Mr Robison, Professor of Natural
Philosophy in this University, enables me to lay before the public the
following communication; which, independent of its intrinsic merit,
affords an additional gratification, by evincing the great interest
taken in the subject, by one whose abilities and extensive knowledge so
well qualify him for giving it a full investigation.
TO MR FOWLER,
EDIN. _May 28. 1793._
SIR,
About a fortnight ago, my son told me of a curious experiment, with
a piece of zinc and a piece of silver applied to the tongue, by
which a strong irritation, resembling taste, was produced, and that
a luminous flash was excited, by applying one of them to the eye. I
immediately repeated them according to his directions, and my
curiosity was greatly excited to prosecute them in a variety of
circumstances. I understand, that these experiments have originated
from the curious discoveries made some time ago in Italy, of which
I was informed last winter. But I have been so much out of the
world for some years past, that I have had no opportunity of
knowing what was going forward.
Being informed, that you have been long engaged in experiments on
this subject, and are about to favour the public with an account of
them, I have taken the liberty of communicating to you, a few facts
which have occurred to me, some of which, perhaps, may be new to
you.
1. I find, that if a piece of zinc be applied to the tongue, and be
in contact with a piece of silver, which touches any part of the
lining of the mouth, nostrils, ear, urethra, or anus, the sensation
resembling taste is felt on the tongue. If the experiment be
inverted, by applying the silver to the tongue, the irritation
produced by the zinc is not sensible, except in the mouth and the
urethra, and is very slight. I find the irritation by the zinc
strongest when the contact is very slight, and confined to a narrow
space, and when the contact of the silver is very extensive, as
when the tongue is applied to the cavity of a silver spoon. When
the zinc touches in an extensive surface, the irritation produced
by a narrow contact of the silver is very distinct, especially on
the upper side of the tongue, and along its margin. This irritation
seems to be mere pungency, without any resemblance to taste, and it
leaves a lasting impression, like that made by caustic alcali.
2. If the zinc (finely polished) be applied to the ball of the eye,
the brightness of the flash seems to correspond with the surface of
contact, of the silver with the tongue, palate, fauces or cheek.
The same thing happens when the silver is applied to the eye.
3. When a rod of zinc, and one of silver are applied to the roof of
the mouth, as far back as possible, the irritations produced, by
bringing their outer ends into contact, are very strong, and that
by the zinc resembles taste, in the same manner as when applied to
the tongue.
4. I had been paring my toe nails with scizzars, and had cut off a
considerable portion of the thick skin, so that the blood began to
_ooze_ through, in the middle of the wound. I applied the zinc
there, and an extensive surface of silver to the tongue. Every time
I brought the metals into contact, I felt a very smart irritation
by the zinc at the wound.
5. I made a piece of zinc having a sharp point, projecting
laterally from its end. I applied this point to a hole in a tooth,
which has sometimes ached a little, and applied the silver in an
extensive surface to the inside of the cheek. When the metals were
brought into contact, I felt a very smart and painful twitch in the
tooth, perfectly resembling a twitch of the toothack. I thought
this twitch double, and that one of them happened before the metals
came into absolute contact. I am now almost convinced, that this is
the case, for when I make the silver rest on a dry tooth, without
touching the tongue or fauces, I have no twitch on bringing the
outer ends of the metals together: showing that there is not a
proper communication through a dry tooth. If, while the outer ends
remain in contact, I touch the silver with the tip of the tongue,
still no twitch is felt in the tooth. If I now separate the outer
ends of the metals, keeping the tongue applied to the silver, a
slight twitch is felt in the moment of separation, and a strong
double twitch when they are again brought into contact. N. B. This
twitch is prevented, by allowing the tongue or lip to touch any
part of the zinc.
6. I had a number of pieces of zinc made of the size of a shilling,
and made them up into a rouleau, with as many shillings. I find
that this alternation, in some circumstances, increases
considerably the irritation, and expect, on some such principle, to
produce a still greater increase. If the side of the rouleau be
applied to the tongue, so that all the pieces are touched by it,
the irritation is very strong and disagreeable. This explains what
I have often observed, the strong taste of soldered seams of metal.
I can now perceive seams in brass and copper vessels by the tongue,
which the eye cannot discover, and can distinguish the base
mixtures which abound in gold and silver trinkets.
If any of the above facts can add to the stock of knowledge you
have acquired on this subject, it will give me great satisfaction,
and I shall not fail to communicate any thing which may afterwards
occur. My indisposition hinders me from taking an active part in
the researches, to which this wonderful and important discovery
incites; but it is both my duty and my earnest wish, to contribute
my feeble assistance to every gentleman engaged in this interesting
pursuit.
I find that common silver thread makes a very good conductor, and
this to any distance.
Since writing the above, I have found a very easy way of producing
very sensible convulsions, (I think muscular) and corroborating my
opinion, that the communication (of this part of the whole effect)
takes place before contact.
Put a plate of zinc into one cheek, and a plate of silver, (a crown
piece) into the other, at a little distance from each. Apply the
cheeks to them as extensively as possible. Thrust in a rod of zinc
between the zinc and the cheek, and a rod of silver between the
silver and the other cheek. Bring their outer ends slowly into
contact, and a smart convulsive twitch will be felt in the parts of
the gums situated between them, accompanied by bright flashes in
the eyes. And these will be distinctly perceived before contact,
and a second time on separating the ends of the rods, or when they
have again attained what may be called the striking distance. If
the rods be alternated, no effect whatever is produced.
Care must be taken, not to press the pieces hard to the gums; this
either hinders us from perceiving the convulsion, or prevents it. I
find too, that one rod, whether zinc or silver, is sufficient for
the communication, and even bringing the two pieces together, will
do as well, or perhaps better. But the rods are easier in the
management.
Asking pardon for the liberty I have taken, without having the
honour of your acquaintance, I am,
With great regard,
SIR,
Your most obedient
Humble servant,
JOHN ROBISON.
------------------------------------
F I N I S.
------------------------------------------------------------------------
Transcriber’s notes:
Obvious typographical errors were repaired. This included one word
duplicated across page boundaries.
Non-obvious “misspellings” were left alone.
Possible old spellings inside quotations were left alone.
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