Medical Definition of spleen
: a highly vascular ductless abdominal organ of vertebrates that resembles a gland in organization but is closely associated with the circulatory system, that plays a role in the final destruction of red blood cells, filtration and storage of blood, and production of lymphocytes, and that in humans is a dark purplish flattened oblong object of a soft fragile consistency lying near the cardiac end of the stomach and consisting largely of blood and lymphoid tissue enclosed in a fibroelastic capsule from which trabeculae ramify through the tissue of the organ which is divisible into a loose friable red pulp in intimate connection with the blood supply and with red blood cells free in its interstices and a denser white pulp chiefly of lymphoid tissue condensed in masses about the small arteries
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