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This article starts with the description of a medical case: the removal of a brain tumor carried out in 1886 in London at the National Hospital for the Paralysed and Epileptic. This medical case is recorded in the Casebooks that today can be found in the archive of the hospital. Firstly, there is the description of the patient's state of health and of the intracranical surgery performed by Victor Horsley who referred himself to David Ferrier's cortical maps. Secondly, there is the reconstruction of the theoretical path that led Ferrier, in the 1870s, to prove on an experimental basis the existence of different localised cerebral functions in specific cortical areas. These cerebral localizations are then compared with the model that, at the beginning of the century, contained their first theoretical seed: the Organology of Franz Joseph Gall.