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. Galen lived from about A.D.130 to 200.
It was the 17th century philosopher, scientist and mathematician, René Descartes, who was explain various aspects of physiology such as the reflex arc, proposed that "animal spirits" flowed into muscle and were responsible for their contraction. In extending his speculations beyond the notions of his predecessors, Descartes relied extensively upon mathematical and mechanical models. He postulated that the heart supplied "animal spirits" to the nerves, which conveyed the spirits to the muscles, very much as water is conducted through pipes.
In 1667, Thomas Willis, a London physician, proposed that muscles may expand by the reaction of animal spirits with vital spirits. Although his speculations about the anatomical sites of reflection, memory, and phantasy had little enduring scientific impact, his investigations did contribute important information about the blood supply to the brain. The ring of blood ves-sels which he described at the base of the brain continues to bear his name (the circleof Willis) and still is an important landmarkof brain anatomy.
The end of the "balloonist" theories of nerve-muscle activities came, after Jan Swammerdam, a Dutch anatomist famous for working with insects, struck the first important blow against the balloonist theory. Jan Swammerdam, in the 1667, performed a most elegant series of experiments on this point, and proved that contracting muscles were not swollen by any influx of fluids from nerves.
Francis Glisson (1597-1677) disproved balloonist theories of nerve function by submerging a man's arm in water and measuring the displacement of water when the muscles were con-tracted. Because no change in water level could be observed, Glisson concluded that muscle contraction was not the result of fluid flowing into the muscle as was commonly thought.
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. In 1848, Emil du Bois-Reymond was the first to demonstrate that the nervous effect was an electrical phenomenon and that a wave of electrical negativity, an action potential, passes down the nerve.
Balloonist
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