Ibn al-Haytham (circa 1027, published in 1083). The oldest known drawing of the nervous system shows a large nose at the bottom, eyes on either side, and a hollow optic nerve that flows out of each one towards the back of the brain.
Santiago Ramón y Cajal (1899). Courtesy of Dr. Juan A. de Carlos. In a series of experiments that founded the modern field of neuroscience, Cajal mastered the intricacies of Golgi's staining technique and established the basic anatomy of the neuron and its part in the nervous system. This drawing shoes the basic components of a Purkinje neuron: a dense arborization of 'dendrites' that flows into the oval 'soma', which in turn sends out a thin 'axon'.
Ryan Draft, Jeff Lichtman and Joshua Sanes (2007). Image taken from a transgenic 'Brainbow' mouse that enables neuroscientists to distinguish between neighboring, densely packed neurons by illuminating them in different colors. This photomicrograph reveals the disposition of axons that regulate the contraction of certain muscles.
Andy Fischer (2008). This image of a chick's retina reveals the three basic stages of visual processing by the circuit in the eye that detects light and transforms it into signals the brain can understand. At the top of the image are the retina's photoreceptor cells (in gray)--the familiar rods and cones--that capture photons of light and translates them into electrical currents.
Alfonso Rodríguez-Baeza and Marisa Ortega-Sánchez (2009). Photomicrograph of the microscopic blood vessels that shuttle oxygen and nutrients to neurons in the brain, obtained with a scanning electron microscope. This sample, from Human cerebral cortex, shows a large blood vessel at the surface of the brain (top), which sends down thin, densely branched capillaries to deliver blood throughout the entire cortex.