Psychiatry in its literal sense is the medical treatment of mental disorders, which necessarily implies the scientific investigation of the nature and causes of these disorders. Psychiatry is therefore indissolubly linked with the medical and biological sciences, and participates not only in their solidarity, but also in their unrest, for every advance in these sciences intimately affects its outlook. In addition it has undergone—and is still undergoing—vicissitudes peculiar to itself which are unknown in other departments of medicine.