Balloonist theory

[1] 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.

This theory has now been superseded by the mainstream scientific community due to the establishment of neuroscience, which is supported by empirical evidence.

In 1667, Jan Swammerdam, a Dutch anatomist famous for working with insects, struck the first important blow against the balloonist theory.

[3] Balloonist theory took a second hit from Francis Glisson who performed an experiment in which a man flexed a muscle under water.

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.

Swammerdam's illustration of a nerve-muscle preparation. He placed a frog thigh muscle in a glass syringe with a nerve protruding from a hole in the side of the container. Irritating the nerve caused the muscle to contract, but the level of the water, and thus the volume of the muscle, did not increase.