Anatomist Santiago Ramon y Cajal was the first to see–and illustrate–what neurons really do. His exquisitely detailed drawings changed our understanding of the brain and nervous system. Cajal relentlessly pursued his microcopic study of animal tissues, leading to an essential discovery: Brain signals jump from cell to cell rather than flow through a continuous web of fibers, as was believed at the time.
Discover Magazine has a fascinating (and beautiful) gallery of illustrations of neurons and neural tissue by the Nobel Prize-winning Cajal. Amazing stuff, especially since he made his discoveries armed only with curiosity and a microscope.