(Chapter 12) Footnote 28:

The so-called rete mirabile (Gk. diktuoeide\j ple/gma, retiform plexus), on which see Singer & Rabin 1946, pp. xliii-xlv. Ascribed by Galen to “those around Herophilus” ( De usu pulsuum 5.155.6, see von Staden 1989, p. 223), the reticular plexus became an important fixture of Galenic physiology, supposedly converting vital spirit to animal spirit; this network of blood vessels at the base of the skull exists primarily in ungulates — never in humans — serving to cool the blood. Vesalius had depicted this imagined feature of human anatomy in Tabula III of Tabulae anatomicae sex (1538); but on p. 642 of the 1543 Fabrica, he admits “I cannot sufficiently marvel at my own stupidity; I who have so labored in my love for Galen that I have never demonstrated the human head without that of a lamb or ox, to show in the latter what I could not in the former, lest forsooth I should fail to display that universally familiar plexus” (tr. Singer & Rabin, ibid.). Mundinus (1316) and Massa (1536), meekly following Galen, had affirmed its existence in humans; only Berengario da Carpi expressed his scepticism ( Commentary on Mundinus, 1521, and Isagogae breves, 1522, see Lind 1959, p. 147). Jacob Sylvius In Hippocratis et Galeni physiologiae anatomicam isagoge, written about the same time as Vesalius’ Fabrica, acknowledged its absence in humans but implied that human anatomy had changed since Galen: it “still appears today in brutes” (fol. 57r). The reluctance of Renaissance authorities to challenge Galen openly concerning this feature testifies to his enduring authority. The 1555 edition omits this reference to the reticular plexus, substituting this phrase describing the larger branch of the carotid artery: “running out of the inner space of the skull.”