Much of our knowledge of the cerebral circulation has been derived from studies of species other than human. There is increas ing recognition of species differences and concern that studies in animals may be misleading if unquestioningly applied to the human. A dramatic example of this occurred in the early his tory of the study of the circulation of the brain. Galen of Pergamo (131-201 AD) described a rete mirabile or "marvelous network" of blood vessels at the base of the human skull that he must have derived from observations of certain animals. This vascular structure was supplied by the...