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Parcours historique > From Galvani's Frogs to Volta's pile

Galvani and "animal electricity"
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By Christine Blondel and Bertrand Wolff
Translation by Andrew Butrica

The cause of muscular contractions

Throughout the eighteenth century the question of the physical and chemical phenomena that lay at the heart of muscular movement was the subject of wide debate among naturalists as well as more generally in the scientific world. Mechanistic interpretations in vogue during the late seventeenth century, such as that of Descartes, explained this movement through the theory of "animal spirits". These subtle fluids, sent from the brain, ran through the nerves, which acted much like tubes, to cause muscles to swell and thereby resulted in them contracting.
These interpretations increasingly were under attack, and new avenues of research opened that involved chemistry and electricity. Thus, for example, Lavoisier showed that when a muscle was active, the organism increased its consumption of oxygen and simultaneously its production of carbon dioxide and water vapor.
The discovery of the Leyden jar in the middle of the eighteenth century raised the question of the possible action of the "electric fluid" on living bodies and immediately raised interest far beyond the scientific world. [See The Enigma of the Leyden jar and the vidéo The terrible jolt ]

Leyden jar discharges caused strong muscular contractions and unexpected sensations: could they cure paralysis and other diseases? Newspapers announced numerous electrical cures, and by the end of the century electric machines proliferated in hospitals. Medical electricity also was practiced by men who were neither doctors nor physicists [See Medical Electricity in the Journal des Savants]. Marat (the future revolutionary), priests, and popular science demonstrators of electrical phenomena became electrical healers. The reality of the healing observed was discussed by several scholars such as Benjamin Franklin and the abbé Nollet. Still, the question remained open.

While the "electrical healers" experimented on human beings, anatomists applied electricity to the members of animals on the dissecting table. A fledgling "electrical science" broke out in the field of animal physiology. Thus in 1756 in Padua, Caldani studied the effects of an electrical discharge on the heart and various muscles. The frog turned out to be the animal the most sensitive for revealing the action of electricity on nerves.

The idea of an analogy between the nervous fluid and electricity permeated contemporary thinking: its very invisibility, its ability to flow through a body without leaving a trace, its extraordinary speed of transmission. But the analogous actions of the two fluids were not enough to prove their identical nature. Thus, for instance, Albrecht von Haller, the foremost authority on physiology at the time, wrote in the articles Nerves (“Nerf”) and Nervous fluid ("Fluide nerveux") in the Encyclopédie (Suppléments, 1776-1777):

"A solid fiber could be traveled over in truth by an electric current, but the phenomena of animal bodies do not seem to allow the animal spirit to be an electric fluid." In effect, "Electrical matter obeys other laws [than those of nervous fluid]: it is not conserved by the ligatures, it does not remain in the nerves, it spreads into neighboring spaces devoid of similar matter, and it regains its equilibrium."

The arguments of Von Haller, especially the interruption of the nervous fluid by a simple ligature, defended vigorously the peculiarity of the nervous fluid and reflected the intensity of the debate.

Galvani's laboratory

Luigi Galvani (1737-1798), professor of anatomy at Bologna, in his turn became interested in the influence of electricity on nerves. No wonder, then, that a look at his laboratory reveals an electrical machine, Leyden jars, and frogs " prepared in the usual manner," that is to say, maintaining the femoral nerves of the legs connected to the spinal cord (Plate I, lower left).

Galvani, Commentary..., 1791, Plate I

Figure Ω: Frog prepared for experiment  
Figure 1: Electrical Machine
Figure 2: Iron wire E, penetrating the spinal cord, is in contact with iron rod G and is extended by the long conducting wire KK.
Figure 3: A prepared frog is enclosed in glass jar A. A very long iron wire E E E can be connected at C to the iron wire B implanted in its spinal cord.
Figure 5: Leyden jar  
Figure 6: Experiment in the vacuum

His laboratory notes show that he began these experiments in November 1780, but it was only in 1791, when he was confident that he had accumulated enough evidence in favor of the existence of animal electricity, that Galvani revealed the results of his persistent and scrupulous experiments, often carried out with the assistance of his wife and his two nephews. His work was published in Latin, as was still common in the field of medicine: De viribus electricitatis in motu musculari: Commentarius (Commentary on the Forces of Electricity in their Relation to Muscular Motion). Some unpublished memoirs read before the Bologna Academy of Sciences as well as his manuscripts give us some insight into the chronology of his discoveries.

The action at a distance of an electric spark on a frog

His starting point is an astonishing observation mentioned in his notes of 1781 and related in his Commentary as follows:

"I dissected a frog and prepared it [...] [and] I placed the frog on the same table as an electrical machine [...] so that the animal was completely separated from and removed at a considerable distance from the machine's conductor [conductor C in Plate I supported by an insulating candle stick]. When one of my assistants by chance lightly applied the point of a scalpel to the inner curural [femoral] nerves, DD, of the frog, suddenly all the muscles of the limbs were seen so to contract that they appeared to have fallen into violent tonic convulsions. Another assistant who was present when we were performing electrical experiments thought he observed that this phenomenon occurred when a spark was discharged from the conductor of the electrical machine [B]. [...] I became extremely enthusiastic and eager to repeat the experiment so as to clarify the obscure phenomenon and make it known. I myself, therefore, applied the point of the scalpel first to one then to the other crural nerve, while at the same time one of the assistants produced a spark; the phenomenon repeated itself in precisely the same manner as before. [...] Excited by the novelty of the phenomenon, we began to make tests and experiments of various kinds, but always using the same scalpel [...] Nor was this additional diligence without its reward, for we discovered that the answer to the problem lay in the part of the scalpel we held in our fingers. Since the scalpel had a bone handle, we found that when this handle was held in the hand, no movements were produced at the discharge of a spark. They did occur, however, when the fingers touched the metal blade or the iron nails that secured the blade of the instrument."

Galvani varied the conditions of the experiment in various ways and concluded:

"... it became clear to us that not only was a conducting substance that touched the nerves required to produce the phenomenon, but one also of determined size and length."

The conducting circuit formed by the experimenter and the scalpel can be replaced by a long iron wire (Plate I, fig. 2, wire KK). The experiment is reproduced in this fashion in the video "From the experiments of Galvani to the pile of Volta." .

But, how can a spark trigger the contraction of a thigh from a distance? Was there no "access to the electric fluid of the machine, in any way that this might occur, by the animal and its conductors?" So that "all pathways [were] removed from the fluid of the machine," Galvani imagined all sorts of arrangements, including that shown in Figure 3, in which the frog is placed in a jar on the opposite side of a wall traversed by an iron wire (EE). What if, despite everything, "the machine, the animal, and the conductor communicated with each other through the air?" He then arranged two jars as shown in Figure 6 placed under the cover of a vacuum pump:

"... I generated a spark, sometimes when the air had been removed, sometimes when it had not been removed. Contractions occurred in either case ..."

The phenomenon resisted any explanation within the limits of knowledge of the period. It was not until the end of the nineteenth century that science established that an electric spark brought about a short discharge of electromagnetic waves. This discharge is the origin of the parasitic frequencies that happen to AM (amplitude modulation) radio transmissions, and they are caused by either lightning discharges or the poorly shielded electric motors of automobiles. In this way, the conducting wire connected to the nerve of Galvani's frog acted as a receiving antenna and electrically stimulated the nerve.

The action of atmospheric electricity

Galvani possessed many works on electricity in his library. He knew well that lightning is a discharge of electricity of the same kind as the "artificial" electricity produced by electrical machines.

"Having already set forth our discoveries on the effects of artificial electricity on muscular contractions, we wanted nothing better than to investigate whether so-called atmospheric electricity produces the same phenomena or not, or more precisely whether lightning flashes like discharged sparks excite muscular contractions when the same techniques are used."

Galvani, Commentary..., 1791, Plate II

Right: Frog is placed in a glass container. The end of the iron wire AA touches the frog's limbs, while the other is extended to the water of the well.
Center: Frog is placed on a table covered with an oily coating. An iron wire connects it to the wall. The phenomena are identical.

On a stormy day in 1786, he set up his prepared frogs, equipped with their conducting wires, on his terrace (Plate II):

"Whenever lightning flashed, all the muscles simultaneously fell into numerous violent contractions. These contractions preceded and as it were gave warning of the thunder to follow, just as the flash and illumination of lightning is wont to do."

Once again, Galvani increased the number of tests while varying their parameters. He noted in particular that a threatening sky, even without the presence of a lightning storm, sometimes led to the same effect. In this instance, sensitive electrometers that he had placed on his terrace "gave clear signs of electricity." He concluded:

"The thing having been carefully examined, it appears that artificial electricity and atmospheric electricity act in the same fashion."

Muscular contractions with no source of electricity!

During the experiments carried out on his terrace, it dawned on Galvani to observe the occasional contractions during calm weather.

"Since I had upon occasion remarked that prepared frogs, which were fastened by brass hooks in the spinal cord to an iron railing which surrounded a certain hanging garden of my home, fell into the usual contractions not only when lightning flashed, but even at times when the sky was quiet and serene, I surmised that these contractions had their origin in changes that occur during the day in the electricity of the atmosphere. "

Was it possible for atmospheric electricity to accumulate slowly in the frog on a clear stormless day? To get to the bottom of the question, Galvani continued his observations "for numerous days." "Tired of waiting in vain," he pressed the copper hooks set in the spinal cord against the iron bars and next observed “frequent contractions," which seemed "unrelated to the electrical state of the atmosphere.".

"Now since I had observed these contractions only in the open air [...] I was on the point of postulating that such contractions result from atmospheric electricity slowly insinuating itself in the animal, accumulating there, and then being rapidly discharged when the hook comes in contact with the iron railing. For in experimenting, it is easy to be deceived and to think we have seen and detected things that we wish to see and detect."

To verify or refute the role of atmospheric electricity, Galvani returned to his laboratory:

"... when I brought the animal into a closed room, placed it on an iron plate, and began to press the hook which was fastened in the spinal cord against the plate, the same contractions and movements occurred as before."

So, atmospheric electricity was out of the question! Galvani increased his experiments. Finally he used two curbed rods of different metals, one in contact with the nerve, the other in contact with the muscle, the two rods forming what he called a "conducting arc" and which later came to be known as a galvanic arc. He varied the choice of metals and contractions occurred whenever the free ends of the two metallic rods were brought into contact.

"... the use of a diversity and variety of metals is much more satisfactory than the use of one and the same metal in obtaining or increasing muscular contractions [...] If [.. .] one of them is iron and the other copper, or better still silver [...] the contractions will be greater and of longer duration."

[In the video the experiment is conducted with copper and zinc.]

Galvani, Commentary..., 1791, Plate III

Figure 9 : Animal is placed on a glass plate [F]. A small piece of tin foil is placed at A and another of brass at C. Contractions occur when these pieces are connected by the conducting arc D (copper covered with silver foil).
Figure 10 : Hook C is made of copper. The arc AA consists of two parts, one of which is an insulator [glass or resin rod]. No contractions observed.
Figure 11 : "If a frog is so held in the fingers by one leg that the hook fastened in the spinal cord touches a silver plate, and if the other leg falls down freely on the same plate, the muscles are immediately contracted at the instant that this leg makes contact. Thereupon the leg is raised, but soon, however, it becomes relaxed of its own accord and again falls down on the plate. As soon as contact is made, the leg is lifted again for the same reason and thus it continues alternately to be raised and lowered so that to the great astonishment and pleasure of the observer, the leg seems to function like an electric pendulum."
Figure 12 : "Contractions can be produced more clearly and quickly” when one uses two metallic arcs, one of which is made of copper and the other of silver."
Figure 13 : This arrangement eliminates the possibility of mechanical stimulation when metal is brought in contact with a nerve or muscle. The frog is placed on a pane of glass both sides of which are coated with metal foil. At H the nerves and the spine are bent so that they are in contact with the foil underneath, while the muscles lie on the top foil. It is when the two foil sheets are connected by the conductor consisting of the two metal rods and the body of the operator that the contractions occur, thus without the animal having been touched.
Figure 14 : The glass tube K is filled with various liquids. There were no contractions with oil-filled tubes.

Galvani, Commentary..., 1791, Plate IV

Figure 17: Contractions are observed even when the conducting arc's length is sizeable (here it includes both individuals).
Figure 18: The frog is placed on a glass plate covered with two sheets of different metals, F being silver and G copper.
Figure 19: At BB two glass containers filled with water form part of the conducting arc.
Figures 20 et 21: Muscle contractions can be obtained with warm-blooded animals.
Fig. 21: A chicken leg. At B is the femoral nerve and at D the thigh muscles.

It is important to note – and this will play an important role in the controversy that followed – that in a first account of the observation made on the terrace in the absence of a storm, the hook planted in the marrow was made of iron, like the bars of the balcony. In his laboratory, Galvani also observed contractions with an arc consisting of a single metal, but they were far weaker than with two different metals. On the other hand, if he replaced one of the metals with an insulator, nothing happened which established in his mind the electrical nature of the phenomenon. But, as this was not the “artificial” electricity generated by a machine, nor atmospheric electricity, what was the source of this electricity?

"These results provoked in me a great astonishment and began to make me suspect [the existence of] an electricity inherent in the animal itself. [And, after a long series of experiments ...] it seemed to me that one could conclude, without any hesitation, that this electricity [...] was in the prepared animal."

Galvani thus formulated the hypothesis of an "animal electricity" that originated in the brain and that was discharged when nerve and muscle were connected by metals. Previous experiments on the effects of the spark from an electrical machine or from lightning showed that this animal electricity "was excited by common electricity."

Some accounts portray Galvani as seeking to confirm an a priori hypothesis of animal electricity from the beginning of his experiments. So it is not insignificant to mention a note written in 1781, in which, in agreement with (in his words) "the immortal Haller," Galvani wrote: "the fluid occupying the nerves is not electrical [...and] animal spirits are not electric fluid." However, his point of view was not inflexible seeing as he saw fit, in certain public lectures, to put forward as probable the sameness of nervous fluid and electric fluid. Following his experiments on the action at a distance of electrical machine sparks and lightning, he considered the frog to be a simple electrical detector. The frog represented "the most sensitive electrometer ever discovered," but was not a source of electricity. It was the series of experiments with the metallic arc that in 1791 led him to adopt the hypothesis of the existence of a form of electricity unique to animals.

"Conjectures and some Conclusions"

This is the title of the last part of the Commentary in which Galvani describes his conclusions at great length. A half a century later, these were considered to be foundational by the celebrated German physiologist Emil Dubois-Reymond (1818-1896) who summarized them as follows:
1. Animals have an electricity which is specific to them, called animal electricity.
2. It emanates from the brain, and it is distributed by the nerves.
3. It is the internal substance of the nerve that conducts electricity, while the fatty outer layer is an insulator that prevents its dispersion.
4. Electricity is received by the muscles, which can be compared to a large number of Leyden jars: the outer surface of each muscle fiber is like the negative (outer) side of the bottle, and the inner surface like the positive (inner) component of the bottle. The conducting arc is the "the most effective [means] for triggering the discharge" of the Leyden bottle.
5. Muscular movement results from the discharge of the "muscular Leyden jar" via the nerve.

But Galvani, beyond what appeared to him to be "sufficiently established," emphasized the vague areas of his interpretation, expressed certain hypotheses, and even offered some possible therapeutic applications:
- What are the similarities and differences between animal electricity and ordinary electricity? The electricity of the torpedo fish (the electric ray): is it different from that of other animals or is it just more powerful? [The great excitement caused by this astonishing fish, which had been known since Antiquity, was its supposed ability to relieve certain pains. Its electrical nature had been established in the 1770s by the Englishman Walsh.]
- What happens inside a muscle fiber when it contracts? This "is very difficult to know and very vague." How does animal electricity act during a mechanical action on the nerve, or during the action at a distance of a spark? How does it work in voluntary movements ordered by the brain? [All these questions remained open].
- Were some illnesses, such as apoplexy or epilepsy, caused by electrical disturbances? Did changes in atmospheric electricity have an influence on our health? What are the virtues, respectively, of positive electricity and negative electricity? An understanding of animal electricity should give rise to "the invention of new methods of administering electricity, more effective than those discovered so far."

But "now let us cease and call an end to conjecture, " Galvani concluded, "these were the facts in particular that I discovered and wished to communicate to learned men about the effects of electricity [...] on muscular motion."

Further readings

GALVANI, Luigi. Commentary on the effect of electricity on muscular motion, trad. Robert Montraville Green, Cambridge, Mass. : E. Licht, 1953. [Read on Internet Archive]

Electricité animale ou électricité métallique ? La controverse Galvani-Volta et l'invention de la pile

COHEN, I. B. Introduction to GALVANI, Luigi. Commentary on the effects of electricity on muscular motion, Norwalk: Burndy Library, 1953.

PERA, Marcello. The Ambiguous Frog: The Galvani-Volta Controversy on Animal Electricity, Princeton: Princeton University Press, 1996.

A bibliography of "secondary sources" on the history of electricity.

French version: March 2007 (English translation: November 2011)


© 2005 CRHST/CNRS, conditions d'utilisation. Directeur de publication : Christine Blondel. Responsable des développements informatiques : Stéphane Pouyllau ; hébergement Huma-Num-CNRS