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. Giovanni Alfonso Borelli performed an experiment to test the idea that muscle is inflated by air. He slit the muscle of an animal under water and watched to see whether bubbles of air would rise to the surface. Since no bubbles were seen to rise, this experiment helped to refute the balloonist theory. 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 ions), and not air or fluid, is the impetus behind muscle movement. See also Passions of the Soul References History of neuroscience Muscular system