By the start of the 18th century, brain scientists were beginning to develop a better understanding of the complex anatomy of the nervous system. The physiology of the brain, however---or the way in which the brain functions---was still an area dominated by speculation and lacking in experimental evidence. One extremely important but unanswered question at the time involved the physiology of the nerves. Scientists in the beginning of the 18th century still relied on observations made by the ancient Greeks when attempting to explain nerve function, but those insights did not seem to be matching up with recent laboratory discoveries.
The dominant hypothesis regarding nerve function at the beginning of the 1700s centered around the ambiguous concept of animal spirits. The notion of animal spirits is thought to have originated with the ancient Greeks, and was advocated by Galen---whose influence may have helped the doctrine remain dominant for over 1500 years.
The animal spirits hypothesis suggested that the hollow nerves of the body were filled with spirits---invisible, intangible substances that acted in a mysterious manner to cause movement or allow sensation to occur. According to Galen's view, a form of spirits called natural spirits were produced in the liver after the consumption of food. Natural spirits then were sent to the heart, where they were converted to vital spirits. Vital spirits were carried in the carotid arteries to the brain to either the ventricles or to a complex of arteries at the base of the brain that Galen called the rete mirabile, or "wonderful net." In one of these locations, the vital spirits were converted to animal spirits---the highest form of spirits. The animal spirits were then stored in the ventricles until they were needed.
Although the animal spirits hypothesis was still the prevailing hypothesis at the start of the 18th century, investigators were not having success in experimentally verifying the existence of the spirits. This led to the exploration of other hypotheses, like Thomas Willis' idea that the nerves carried fluid that dripped onto muscles to stimulate them. These new hypotheses, however, also did not seem to stand up to experimental scrutiny. But this changed early in the 18th century when some scientists began to suggest that electricity was the enigmatic substance that filled the nerves.
At the time, appreciation for the wonders of electricity was rapidly growing. The first devices that could produce and store electricity, known respectively as friction machines and Leyden jars, appeared in the first half of the 18th century. These contraptions could be used to create dazzling displays, and became a popular novelty at social engagements. It was also soon recognized, however, that electricity had some potential medical applications. It seemed to be particularly effective at stimulating the muscles of paralyzed limbs to contract.
This led some to hypothesize that electricity was the substance that flowed through the nerves. This hypothesis was bolstered when it was verified that the shocks produced by electric fish (e.g. the electric ray) were caused by actual electricity, as it proved that electricity could exist within the confines of an animal's nervous system. It was at this time, when excitement about electricity as a mechanistic component of the nervous system was beginning to grow, that Luigi Galvani would make his seminal contributions to the field.
Galvani was a doctor and professor of anatomy at the University of Bologna in Italy. In the 1770s, he began to explore electricity and its association with the nerves, conducting his experiments in his own home and mostly with frogs as the subjects.
In 1791, after 10 years of research into the subject, Galvani published the work that would make him famous, his Commentary on the Effects of Electricity on Muscular Motion. In the treatise, Galvani described a series of experiments that made a strong case for the natural involvement of electricity in the nervous system. First, Galvani discussed an observation that occurred serendipitously. He had placed a dissected frog on a table next to an electrical machine, and when one of his assistants touched the frog's nerves with a metal scalpel at the same time as the electrical machine emitted a spark, the frog's leg muscle contracted, causing a convulsive movement of the limb.
Galvani further explored the ability of electricity to cause muscle contractions. He found that when a wire was stretched from the electrical machine to the frog's leg, a convulsion was also elicited. He extended the finding to mammals, observing that similar types of contractions could be generated in chickens and sheep.
Galvani then began to investigate the effects of natural sources of electricity, showing that lightning (as conducted by a lightning rod and down a wire) was also capable of producing muscle contractions when it was given a path to a frog's limbs. These experiments were all interesting, but they were not groundbreaking on their own. Other investigators had observed the ability of electricity to elicit muscle movement. But the next experiments Galvani conducted, and the resultant deductions he made, are what really caused his research to stand apart from the rest.
As a way to attach conductors or hang the frogs outside his home for experiments with lightning, Galvani had fastened brass hooks to the frogs' spinal cords. He was inside with a frog that had a brass hook attached to it when he pressed the frog, along with the hook, up against a metal plate. To Galvani's surprise, the frog exhibited the same type of convulsive movements the application of electricity had caused. This suggested the movements were not dependent on some external source of electricity, and led Galvani to make the deduction that "the electricity was inherent in the animal itself."
Galvani went on to hypothesize that this "animal electricity" was produced by the brain and distributed by the nerves to the muscles (the brain as the "source" of electricity would not stand up to scrutiny once researchers began to better understand the electrical properties of neurons). He also postulated that the nerves must be covered with a fatty insulatory material---a hypothesis that preceded the discovery of that insulatory material (i.e. myelin) by over 60 years.
Galvani's findings and deductions were very influential. Other hypotheses about nerve function, like the animal spirits doctrine, began to fall out of favor. Although many questions about the electrical properties of the nervous system remained, at least now investigators had a mechanism for nerve function that could be observed and measured (unlike the elusive animal spirits). Galvani's discoveries would form the foundation of the modern study of nerve function.
Galvani was not able to fully appreciate the significance of his observations or the popularity they engendered. His wife died in the same year he published his findings (1791). He was devastated and never seemed to be the same emotionally. He spent the next seven years defending his conclusions from critics, especially the well-known well-known Alessandro Volta, who incessantly attacked Galvani's work as insufficient to support the deductions he made. Galvani died in 1798, uncertain of how important his discoveries would become and unaware that they would be an essential piece in the foundation modern neuroscience has been built upon.
Finger S. Minds Behind the Brain. New York, NY: Oxford University Press; 2000.
Galvani L. Commentary of the effects of electricity on muscular motion. Foley MG, translator. Norwalk, CT: Burndy Library; 1953.