the greek physician galen (129 - c. 210 ce) provided the next insights into pulmonary circulation. though many of his theories, like those of his predecessors, were incorrect, his theory of pulmonary circulation dominated the medical community for hundreds of years after his death. galen contradicted erasistratus before him by proposing that arteries carried both air and blood, rather than air alone. he proposed that the liver was the point of origin for all blood vessels and that the heart was not a pumping muscle, but rather an organ which blood passed through. galen's theory included a new description of pulmonary circulation. in it, air was inhaled into the lungs where it became the pneuma. pulmonary veins transmitted this pneuma to the left ventricle of the heart to cool the blood simultaneously arriving there. this mixture of pneuma, blood, and cooling produced the vital spirits which could then be transported throughout the body via arteries. galen also proposed that the heat of the blood arriving in the heart produced noxious vapors which were expelled through the same pulmonary veins that first brought the pneuma. he wrote that the right ventricle played a different role than the left; it transported blood to the lungs where the impurities were vented out so that clean blood could be distributed throughout the body. though galen's description of the anatomy of the heart was more complete than those of his predecessors, it included several mistakes. most notably, galen believed that blood flowed between the two ventricles of the heart through small, invisible pores in the interventricular septum.