Great developments in study of the central and peripheral nervous system took place throughout the 19th century, especially in Britain. These studies, together with other factors, gave rise to the development of a somaticism of mental illness. The followers of this school of thought, most notably Henry Maudsley, supported the idea of materialism, based on the theory of evolution. They were opposed by those who believed in the duality of body and mind whith its different variants. These two visions constituted the centre of the theoretical debate of psychiatry at that time. The author pays special attention to the first stance and reaches the conclusion that this process of knowledge made mental illness to be considered as an organic disease and contribuited to the entry of the lunatic wholly into the sphere or medical action.