_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.
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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.