151207-N-GL720-002 BETHESDA, Maryland (Dec. 7, 2015) Chief of Naval Operations (CNO) Adm. John Richardson addresses the audience during the retirement ceremony for Vice Adm. Matthew Nathan, Navy surgeon general and chief, U.S. Navy Bureau of Medicine and Surgery. Nathan retired after 31 years of service. (U.S. Navy photo by James Rosenfelder/Released)