The brain for Galen is the ηγεμOνiÓνand ãOΧηof the nervous system. Anatomical demonstration enabled him to advance his claims for its nervous function. His conception of the brain and the nerves hinges on his employment of some psychic pneuma. Galen's system of pneumatic physiology is, however, largely dependent on the demonstration of a functioning vascular system. This paper focuses on two vascular structures-essential for the proper functioning of the brain and nerves-which carry the psychic pneuma. They arc the retiform plexus, (...) or rete mirabile, and the choroid plexuses. The former is a well-established arterial structure at the base of the brain in some higher mammals ; the latter is a vascular plexus in the ventricles of the brain. Taken together, the account Galen gives of these structures is both internally consistent and convincing and sheds useful light on Galen's understanding of the brain and the nerves. (shrink)