actions and clarified the components of the light reflex within the eye. Central nervous system Whytt's theories on the nervous system and its role in movement opposed many of the teachings that were in place in the 18th century. During that time, many physiologists still supported Descartes' theory of movement which hypothesized that muscle contraction was due to the activation of fluid in the nervous system called animal spirits. Physiologists, such as Whytt's colleague, Albrecht von Haller, also believed that muscles were capable of action independent of the nerves. Whytt strongly advocated against Descartes' theory, and explicitly denied the concept of animal spirits. Furthermore, he rejected Haller's theory by claiming that movement must depend on interconnecting nerves that lead to the brain or spinal cord. Whytt decided to prove his theory through experimentation. He replicated Stephen Hales' experiment that consisted of probing and examining the response of limbs in decapitated frogs. In Whytt's version of the experiment, he inserted a hot wire through the spine of the decapitated frog and observed that when the spine of the frog was destroyed, no form of pricking or cutting of the frog's limb elicited a response. If the frog's spine stayed intact as it did in Hales' experiment, the limbs continued to respond to the pricking and cutting. Additionally, Whytt tested to see if a response could still be created if certain sections of the spinal cord remained intact. The results show that as long as the spinal cord is partially undamaged, small responses in the limbs can still be produced. The experiment led Whytt to conclude that the spinal cord was a crucial component in facilitating response action to stimuli. The proof that movement still occurs after decapitation disproves Descartes' animal spirits in muscles. Likewise, the relationship between the spinal cord and response action in the limbs disproves Haller's theory of movement. The response movement will later be described as "reflex action" by Marshall Hall. Voluntary and involuntary motion Another theory that opposed Whytt's ideas during his time was the position of Stahl's animism. Animism downplayed the importance of the brain and nerves in movement and attributed it predominantly to the soul. Whytt acknowledges the presence and importance of the soul, but unlike Stahl's animism, he does not agree that motion is controlled exclusively in the soul. In 1745, Whytt published An Equiry Into the Cause Which Promote the Circulation of Fluids