In Biblia Naturae Swammerdam's research on muscles was published.
Swammerdam played a key role in the debunking of the balloonist theory, the idea that 'moving spirits' are responsible for muscle contractions.
 The idea, supported by the Greek physician Galen, held that nerves were hollow and the movement of spirits through them propelled muscle motion.
René Descartes furthered the idea by basing it on a model of hydraulics, suggesting that the spirits were analogous to fluids or gasses and calling them 'animal spirits'.
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.
According to this hypothesis, muscles would grow larger when they contract because of the animal spirits flowing into them.
 To test this idea, Swammerdam placed severed frog thigh muscle in an airtight syringe with a small amount of water in the tip.
He could thus determine whether there was a change in the volume of the muscle when it contracted by observing a change in the level of the water (image at right).
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.
The idea that nerve stimulation led to movement had important implications for neuroscience by putting forward the idea that behaviour is based on stimuli.

Swammerdam's research had been referenced before its publication by Nicolas Steno, who had visited Swammerdam in Amsterdam.
Swammerdam's research concluded after Steno had published the second edition of Elements of Myology in 1669, which is referenced in Biblia Naturae.
A letter from Steno to Malpighi from 1675 suggests that Swammerdam's findings on muscle contraction had caused his crisis of consciousness.
Steno sent Malpighi the drawings Swammerdam had done of the experiments, saying "when he had written a treatise on this matter he destroyed it and he has only preserved these figures.
He is seeking God, but not yet in the Church of God."
In Biblia Naturae Swammerdam's research on muscles was published. Swammerdam played a key role in the debunking of the balloonist theory, the idea that 'moving spirits' are responsible for muscle contractions.  The idea, supported by the Greek physician Galen, held that nerves were hollow and the movement of spirits through them propelled muscle motion. René Descartes furthered the idea by basing it on a model of hydraulics, suggesting that the spirits were analogous to fluids or gasses and calling them 'animal spirits'. 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. According to this hypothesis, muscles would grow larger when they contract because of the animal spirits flowing into them.  To test this idea, Swammerdam placed severed frog thigh muscle in an airtight syringe with a small amount of water in the tip. He could thus determine whether there was a change in the volume of the muscle when it contracted by observing a change in the level of the water (image at right). 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. The idea that nerve stimulation led to movement had important implications for neuroscience by putting forward the idea that behaviour is based on stimuli.
Swammerdam's research had been referenced before its publication by Nicolas Steno, who had visited Swammerdam in Amsterdam. Swammerdam's research concluded after Steno had published the second edition of Elements of Myology in 1669, which is referenced in Biblia Naturae. A letter from Steno to Malpighi from 1675 suggests that Swammerdam's findings on muscle contraction had caused his crisis of consciousness. Steno sent Malpighi the drawings Swammerdam had done of the experiments, saying "when he had written a treatise on this matter he destroyed it and he has only preserved these figures. He is seeking God, but not yet in the Church of God."
Jan Swammerdam