Sir William Osler (1849-1919), Canadian physician and discoverer of the minute platelet cells that make blood clot. Osler studied medicine in Toronto, Canada, obtaining his degree in 1872. In 1873 he discovered blood platelets, and in 1875 was appointed professor at McGill University. He subsequently held professorial posts at the University of Pennsylvania, Johns Hopkins University and Oxford University, UK. As a teacher he pioneered new training methods, including tuition in the wards and laboratory techniques, and wrote the perennially popular textbook, The Principles and Practice of Medicine (1892).