(4 syl.). A white wand carried by Roman officers when they went to treat for peace. The Egyptians adorned the rod with a male and female serpent twisted about it, and kissing each other. From this use of the rod, it became the symbol of eloquence and also of office. In mythology, a caduceus with wings is placed in the hands of Mercury, the herald of the gods; and the poets feign that he could therewith give sleep to whomsoever he chose; wherefore Milton styles it “his opiate rod” in Paradise Lost, xi. 133.
So with his dread caduceus Hermës led From the dark regions of the imprisoned dead; Or drove in silent shoals the lingering train To Night's dull shore and Pluto's dreary reign.
Darwin: Loves of the Plants, ii. 291.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
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