Wren's...perspectograph, an ingenious device that allows a draftsman, squinting through a sight, to trace the lineaments of any object or landscape...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".

The accuracy of Wren's illustrations was largely due to the method for preserving anatomical specimens, described by Willis in the passage above. This method was actually devised by Wren; it  retained the original shape  of the specimens, which could then be dissected and studyied more closely. Willis and his colleagues also used the method to inject coloured dyes into the carotid artery, in order to visualize the distribution of blood vessels in the brain.

Figure 1a (above) includes a numbering system for the cranial nerves which remained in use for more than 100 years, and shows the Circle of Willis, the ring of arteries at the base of the brain which supplies the organ with blood. It also  precisely demarcates the hippocampus and surrounding structures for the first time. 

The figure comes from the Wellcome Trust's collection of images. For more about Wren and Willis, see Carl Zimmer's excellent Soul Made Flesh. See also the newly launched Wellcome Library blog.