First description of a clear distinction between what is now known as gray and white matter in the central nervous system. The work also includes the first attempt to illustrate the brain in a sagittal view. The nine dramatic woodcuts in this work are strikingly original and not derived from Vesalius, as were most of the anatomical works in this period. The second issue of the sheets dated 1586 include an engraved portrait of Piccolomini on its title page that is very different from the portrait on the title page of the first issue of 1585, as well as a change in the dedicatee from Jacopo Boncompagni, Duke of Sora, to Pope Sixtus V, who ascended to the papacy in 1585. Digital facsimile of the 1586 issue from the Österreichische Nationalbibliothek at this link.