(Chapter 5) Footnote 11:

In the 1555 edition, instead of “instrument of sense,” Vesalius wrote “vehicle of animal spirit and faculty,” animalis spiritus facultatisque vehiculum. This is Galen’s psychic pneuma, a vaporous substance refined first from inhaled air in the lungs, then to vital pneuma in the left ventricle of the heart, and finally in the brain to psychic pneuma (Vesalius’ animalis spiritus), which is sent out through the invisible lumen of the nerves. Psychic pneuma is what enables the nerves to impart sense and motion to the parts. First posited by the Hippocratics, pneuma was worked into a consistent theory of pneumatology by Herophilus’ contemporary Erasistratus of Ceos and modified by Galen. See Preface n. 48 and May 1968, 46 ff. The existence of separate animal spirits was challenged in 1556 by the neoteric physician Giovanni Argenterio, De somno et vigilia Book 2 chaps. 7-10. See Siraisi, Avicenna p. 339.