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.[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.[2] In the model, which Descartes used to explain reflexes, the spirits would flow from the ventricles of the brain, through the nerves, and to the muscles to animate the latter.[3]