Herophilus and Erasistratus made new discoveries about the nervous system, and Erasistratus ex- plained the relationship between the system of arteries and the nerves by distinguishing between two kinds of pneuma. Air breathed moves through the ‘vein-like artery’ (the pulmonary vein) into the left ventricle of the heart, whence it is distributed as vital pneuma (pneuma zo ¯ tikon) through the arteries. Some of this is transformed into psychic pneuma (pneuma psychikon) in the brain and serves the living being’s cognitive and motor activities in the brain and nerves. 100 Chrysippus, like Herophilus and Erasistratus, identified the soul with pneuma. The psychic pneuma is a special t ype of corporeal spirit, which Chrysippus describes as a spirit with sufficiently high tension (tonos).