Balloonist theory
Encyclopedia
Balloonist theory was a theory in early neuroscience
that attempted to explain muscle
movement by asserting that muscles contract by inflating with air or fluid. The Greek physician Galen
believed that muscles contracted due to a fluid flowing into them, and for 1500 years afterward, it was believed that nerves were hollow and that they carried fluid. René Descartes
, who was interested in hydraulics
and used fluid pressure to explain various aspects of physiology
such as the reflex arc
, proposed that "animal spirits" flowed into muscle and were responsible for their contraction. In the model, which Descartes used to explain reflex
es, the spirits would flow from the ventricles of the brain, through the nerves, and to the muscles to animate the latter.
In 1667, Thomas Willis
proposed that muscles may expand by the reaction of animal spirits with vital spirits. He hypothesized that this reaction would produce air in a manner similar to the reaction that causes an explosion, causing muscles to swell and produce movement.
Also in 1667, Jan Swammerdam
, a Dutch anatomist famous for working with insects, struck the first important blow against the balloonist theory. Swammerdam, who was the first to experiment on nerve-muscle preparations, showed that muscles do not increase in size when they contract (if a substance such as animal spirits flowed into muscles, their volume should increase when they contract). Swammerdam placed severed frog thigh muscle in an airtight syringe with a small amount of water in the tip. He could thus determine whether there was a change the volume of the muscle when it contracted by observing a change in the level of the water (image at right). When Swammerdam caused the muscle to contract by irritating the nerve, the water level did not rise but rather was lowered by a minute amount; this showed that no air or fluid could be flowing into the muscle. Swammerdam did not believe the results of his own experiment, suggesting that they were the result of artifact. However, he concluded in his book The Book of Nature II that "motion or irritation of the nerve alone is necessary to produce muscular motion". This idea was an important step toward the current understanding of how nerves actually cause muscle contraction.
Balloonist theory took a second hit from Francis Glisson
who performed an experiment in which a man flexed a muscle under water. The water level did not go up (in fact it went down slightly), further supporting the conclusion that no air or fluid could be entering the muscle.
The idea that muscle is inflated by air was further debunked when Giovanni Alfonso Borelli
slit the muscle of an animal under water and watched to see if bubbles of air would rise to the surface; of course, none did.
The invention of the microscope
allowed preparations of nerves to be viewed at high magnification, showing that they are not hollow.
In 1791 Luigi Galvani
learned that frogs' muscles could be made to move by the application of electricity
. This finding provided a basis for the current understanding that electrical energy (carried by ion
s), and not air or fluid, is the impetus behind muscle movement.
Neuroscience
Neuroscience is the scientific study of the nervous system. Traditionally, neuroscience has been seen as a branch of biology. However, it is currently an interdisciplinary science that collaborates with other fields such as chemistry, computer science, engineering, linguistics, mathematics,...
that attempted to explain muscle
Muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...
movement by asserting that muscles contract by inflating with air or fluid. The Greek physician Galen
Galen
Aelius Galenus or Claudius Galenus , better known as Galen of Pergamon , was a prominent Roman physician, surgeon and philosopher...
believed that muscles contracted due to a fluid flowing into them, and for 1500 years afterward, it was believed that nerves were hollow and that they carried fluid. René Descartes
René Descartes
René Descartes ; was a French philosopher and writer who spent most of his adult life in the Dutch Republic. He has been dubbed the 'Father of Modern Philosophy', and much subsequent Western philosophy is a response to his writings, which are studied closely to this day...
, who was interested in hydraulics
Hydraulics
Hydraulics is a topic in applied science and engineering dealing with the mechanical properties of liquids. Fluid mechanics provides the theoretical foundation for hydraulics, which focuses on the engineering uses of fluid properties. In fluid power, hydraulics is used for the generation, control,...
and used fluid pressure to explain various aspects of physiology
Physiology
Physiology is the science of the function of living systems. This includes how organisms, organ systems, organs, cells, and bio-molecules carry out the chemical or physical functions that exist in a living system. The highest honor awarded in physiology is the Nobel Prize in Physiology or...
such as the reflex arc
Reflex arc
A reflex arc is a neural pathway that controls an action reflex. In higher animals, most sensory neurons do not pass directly into the brain, but synapse in the spinal cord...
, proposed that "animal spirits" flowed into muscle and were responsible for their contraction. In the model, which Descartes used to explain reflex
Reflex
A reflex action, also known as a reflex, is an involuntary and nearly instantaneous movement in response to a stimulus. A true reflex is a behavior which is mediated via the reflex arc; this does not apply to casual uses of the term 'reflex'.-See also:...
es, the spirits would flow from the ventricles of the brain, through the nerves, and to the muscles to animate the latter.
In 1667, Thomas Willis
Thomas Willis
Thomas Willis was an English doctor who played an important part in the history of anatomy, neurology and psychiatry. He was a founding member of the Royal Society.-Life:...
proposed that muscles may expand by the reaction of animal spirits with vital spirits. He hypothesized that this reaction would produce air in a manner similar to the reaction that causes an explosion, causing muscles to swell and produce movement.
Debunking
Jan Swammerdam
Jan Swammerdam was a Dutch biologist and microscopist. His work on insects demonstrated that the various phases during the life of an insect—egg, larva, pupa, and adult—are different forms of the same animal. As part of his anatomical research, he carried out experiments on muscle contraction...
, a Dutch anatomist famous for working with insects, struck the first important blow against the balloonist theory. Swammerdam, who was the first to experiment on nerve-muscle preparations, showed that muscles do not increase in size when they contract (if a substance such as animal spirits flowed into muscles, their volume should increase when they contract). Swammerdam placed severed frog thigh muscle in an airtight syringe with a small amount of water in the tip. He could thus determine whether there was a change the volume of the muscle when it contracted by observing a change in the level of the water (image at right). When Swammerdam caused the muscle to contract by irritating the nerve, the water level did not rise but rather was lowered by a minute amount; this showed that no air or fluid could be flowing into the muscle. Swammerdam did not believe the results of his own experiment, suggesting that they were the result of artifact. However, he concluded in his book The Book of Nature II that "motion or irritation of the nerve alone is necessary to produce muscular motion". This idea was an important step toward the current understanding of how nerves actually cause muscle contraction.
Balloonist theory took a second hit from Francis Glisson
Francis Glisson
Francis Glisson was a British physician, anatomist, and writer on medical subjects. He did important work on the anatomy of the liver, and he wrote an early pediatric text on rickets...
who performed an experiment in which a man flexed a muscle under water. The water level did not go up (in fact it went down slightly), further supporting the conclusion that no air or fluid could be entering the muscle.
The idea that muscle is inflated by air was further debunked when Giovanni Alfonso Borelli
Giovanni Alfonso Borelli
Giovanni Alfonso Borelli was a Renaissance Italian physiologist, physicist, and mathematician. He contributed to the modern principle of scientific investigation by continuing Galileo's custom of testing hypotheses against observation...
slit the muscle of an animal under water and watched to see if bubbles of air would rise to the surface; of course, none did.
The invention of the microscope
Microscope
A microscope is an instrument used to see objects that are too small for the naked eye. The science of investigating small objects using such an instrument is called microscopy...
allowed preparations of nerves to be viewed at high magnification, showing that they are not hollow.
In 1791 Luigi Galvani
Luigi Galvani
Luigi Aloisio Galvani was an Italian physician and physicist who lived and died in Bologna. In 1791, he discovered that the muscles of dead frogs legs twitched when struck by a spark...
learned that frogs' muscles could be made to move by the application of electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
. This finding provided a basis for the current understanding that electrical energy (carried by ion
Ion
An ion is an atom or molecule in which the total number of electrons is not equal to the total number of protons, giving it a net positive or negative electrical charge. The name was given by physicist Michael Faraday for the substances that allow a current to pass between electrodes in a...
s), and not air or fluid, is the impetus behind muscle movement.