Erasistratus Of Ceos (flourished c. 250 bc) was a Greek anatomist and physician in Alexandria, regarded by some as the founder of physiology.

Known especially for his studies of the circulatory and nervous systems, Erasistratus noted the difference between sensory and motor nerves, but thought that the nerves were hollow tubes containing fluid. He believed that air entered the lungs and heart and was carried through the body in the arteries, and that the veins carried blood from the heart to the various parts of the body. He correctly described the function of the epiglottis and the valves of the heart, including the tricuspid, which he named.

Erasistratus was the first major exponent of pneumatism, which was based on the premise that life is associated with a subtle vapour called the pneuma.