After doing exceptional cerebral surgery abroad under Kocher at Bern and Sherrington at Liverpool, he began private practice in Baltimore. During his time with Kocher, he first encountered the Cushing reflex which describes the relationship between blood pressure and intracranial pressure. At the age of 32, he was made associate professor of surgery at Johns Hopkins Hospital, and was placed in full charge of cases of surgery of the central nervous system. Yet he found time to write numerous monographs on surgery of the brain and spinal column and to make important contributions to bacteriology. He made (with Kocher) a study of intracerebral pressure and (with Sherrington) contributed much to the localization of the cerebral centers. In Baltimore, he developed the method of operating with local anaesthesia, and his paper on its use in hernia gave him a European reputation. In 1911, he was appointed surgeon-in-chief at the Peter Bent Brigham Hospital in Boston. He became a professor of surgery at the Harvard Medical School starting in 1912. In 1913, he was made an honorary F.R.C.S. (London). He was elected a Fellow of the American Academy of Arts and Sciences in 1914. In 1915, before the Clinical Congress of Surgeons in Boston, he showed the possibility of influencing stature by operating on the pituitary gland.