Alkaloid use of Ephedra and opium poppies. Also, coca leaves have been used by South American Indians since ancient times. Extracts from plants containing toxic alkaloids, such as aconitine and tubocurarine, were used since antiquity for poisoning arrows. Studies of alkaloids began in the 19th century. In 1804, the German chemist Friedrich Sertürner isolated from opium a "soporific principle" (), which he called "morphium" in honor of Morpheus, the Greek god of dreams; in German and some other Central-European languages, this is still the name of the drug. The term "morphine", used in English and French, was given by the French
The Interpretation of Dreams after its publication, Freud considered "On Dreams" as a shortened version of "The Interpretation of Dreams". The English translation of "On Dreams" was first published in 1914 and the second English publication in the James Strachey translation from 1952. Freud investigates the subject of "displacement" and our inability to recognize our dreams. In chapter VI, page 659, he states: "It is the process of displacement which is chiefly responsible for our being unable to discover or recognize them in the dream-content" and he considers the issue of displacement in chapter VIII, page 671 as: "the most striking of the dream-work."