like many prominent medical figures in the 18th century, william cullen took a great interest in the nervous system. he defined the nervous system as an "animated machine" whose main function is to "perform a variety of motions," communicate and interact with "external bodies." cullen believed that the nervous system was composed of four elements: the medullary substance, consisting of the brain and the spinal cord, the membranous nerves, the sensory nerves, and the muscular fibers.