Oxford scholars Martin Kemp and Nathan Flis have a short essay about the anatomical drawing of Christopher Wren in Nature this week.
Wren's perspectograph was probably used to trace the topography of the brain, its nerves and blood vessels, just as he used a telescope coupled to a micrometer to map the heavens and create the first lunar globe. However, it would be wrong to think that Wren used his instruments and great skill as a draftsman to create the equivalent of a photograph of a dissection — his representations are a succinct synthesis of what Willis and his collaborators had deduced about the structure of the brain over of period of time. To arrive at this point, they exploited the latest techniques of injection to preserve and dye the brain's tissues and vessels. As Willis explained, "let a dyed liquor, and contained in a large squirt or pipe, be injected upwards in the trunk [of an artery] of one side: after once or twice injecting, you shall see the tincture or dyed liquor to descend from the other side by the trunk of the opposite artery".
Carl Zimmer covers this material well in his book, Soul Made Flesh: The Discovery of the Brain--and How it Changed the World which covers the roots of neuroscience in the milieu of the founders of the Royal Society.