_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.
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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.
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