nextSir Christopher Wren was an English astronomer, mathematician and architect. He is most famous for designing Saint Paul's Cathedral after the Great Fire of London (1666). In 1665, Wren administered an intravenous injection of tincture of opium to a dog by means of a bladder attached to a sharpened quill. Wren also experimented with canine blood transfusions. However, safe blood transfusion would be feasible only after Austro-American immunologist Karl Landsteiner (1868–1943) discovered the major blood groups and developed of the ABO system of blood-typing.