He needed to know which nerves emanated from the brain and which from the spinal cord, which muscles they activated, and which facial movements were connected to others. He would even try, when dissecting the brain, to figure out the precise location where the connections were made between sensory perceptions, emotions, and motions. By the end of his career, his pursuit of how the brain and nerves turned emotions into motions became almost obsessive. It was enough to make the Mona Lisa smile.