the next addition to the human understanding of pulmonary circulation came with the ancient greeks. the physician alcmaeon (520 - 450 bce) proposed that the brain, not the heart, was the connection point for all of the vessels in the body. he believed that the function of these vessels was to bring the spirit (pneuma) and air to the brain. empedocles (492 - 432 bce), a philosopher, proposed a series of pipes impermeable to blood but continuous with blood vessels which carried the pneuma throughout the body. he proposed that this spirit was internalized with pulmonary respiration. the physician hippocrates (460 - 370 bce) developed the view that the liver and spleen produced blood, which traveled to the heart to be cooled by the lungs that surrounded it. hippocrates described the heart as having two ventricles connected by an interventricular septum. he depicted the heart as the connecting point for all the vessels of the body and proposed that some vessels carried only blood, while others also carried air. these air-carrying vessels were the pulmonary veins, which brought air to the left ventricle, and the pulmonary artery, which carried air to the right ventricle and blood to the lungs. he also proposed two atria of the heart that functioned to capture air. hippocrates was one of the first to begin to accurately describe the anatomy of the heart and to describe the involvement of the lungs in circulation, but his descriptions of the process of pulmonary circulation and of the functions of the parts of the heart were still largely incorrect.