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.[3] 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).[3] 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.[3] Swammerdam did not believe the results of his own experiment, suggesting that they were the result of artifact.[3] 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".[3] This idea was an important step toward the current understanding of how nerves actually cause muscle contraction.[3]