1860-1945

American physiologist and biochemist who isolated and characterized heparin, a substance that inhibits blood clotting, and conducted pioneering studies of the biochemistry of the blood. The activity of heparin preparations is now measured in terms of Howell units. He was the first to describe the "Howell-Jolly bodies" that are sometimes seen in erythrocytes, especially after a splenectomy. Howell demonstrated the presence of amino acids in the blood and lymph and conducted pioneering studies into the preparation and properties of thrombin, antithrombin, and prothrombin.