Erasistratus (/ˌɛrəˈsɪstrətəs/; Greek: Ἐρασίστρατος; c. 304 – c. 250 BC) was a Greek anatomist and royal physician under Seleucus I Nicator of Syria. Along with fellow physician Herophilus, he founded a school of anatomy in Alexandria, where they carried out anatomical research. He is credited for his description of the valves of the heart, and he also concluded that the heart was not the center of sensations, but instead it functioned as a pump. Erasistratus was among the first to distinguish between veins and arteries. He believed that the arteries were full of air and that they carried the "animal spirit" (pneuma). He considered atoms to be the essential body element, and he believed they were vitalized by the pneuma that circulated through the nerves. He also thought that the nerves moved a nervous spirit from the brain. He then differentiated between the function of the sensory and motor nerves, and linked them to the brain. He is credited with one of the first in-depth descriptions of the cerebrum and cerebellum.