Apparently Flower had never entertained the idea of taking up the profession of an army surgeon as a permanency, and after his return to London he definitely resigned military service, with the intention of settling down to private medical practice in the Metropolis. In the spring of 1857 he passed the examination qualifying for the Fellowship of the Royal College of Surgeons; and about this time, or perhaps immediately on his return to London, he joined the staff of the Middlesex Hospital as Demonstrator in Anatomy. During the next year (1858) he was elected to the post of Assistant-Surgeon to the same Institution, where he resumed the Curatorship of the museum and was also appointed Lecturer on Comparative Anatomy. Although a large portion of his time while at the hospital was devoted to surgical and other duties connected with the medical profession, his Lectureship and Curatorship required that he should devote a considerable amount of attention to the more congenial study of Comparative Anatomy. It was during his connection with the Middlesex Hospital that his first scientific work was published, this being the well-known and useful little volume entitled _Diagrams of the Nerves of the Human Body_, which appeared in 1861, and has passed through three editions. During this period of his career he also contributed to Holmes’ _System of Surgery_ an article on “Injuries to the Upper Extremities,” which contained certain original observations with regard to dislocations of the shoulder-joint; and he likewise wrote an essay on the same subject to the Pathological Society, as well as several articles on various surgical subjects to the medical journals of the day. But even at this comparatively early period of his career Flower’s published scientific work was by no means strictly confined to his ostensible profession, for his two first papers on Comparative Anatomy—the one “On the Dissection of a Galago”(Lemur); and the other “On the Posterior Lobes of the Cerebrum of the Quadrumana”—appeared during the period in question. During this period, as the writer of his obituary notice in the “Record” of the Royal Society well remarks, there is little doubt that Flower had breathing time, after his Crimean experiences, to collect his energies and gather up a store of valuable information which stood him in good stead in later years, when he had frequently less leisure to devote to pure study.