Mercurialis perennis is a herbaceous, rhizomatous, downy perennial plant found all over Europe and Caucasus (Wikipedia)
Dogs mercury is poisonous in the fresh state (internal use), though thorough drying or cooking is said to destroy the poisonous principle.
The fresh juice of the whole plant is emetic, ophthalmic and purgative. It is used externally to treat menstrual pain, ear and eye problems, warts and sores. It is used in the treatment of rheumatism, dropsy, diarrhoea and disorders of the gall bladder and liver (https://pfaf.org/).
More about the medicinal properties and mechanism of action here and here
The genus name comes from the Latin Mercurius, the Roman mythological deity, and -alis, belonging to. One interpretation has been that the plant was discovered by him; it also reveales its medicinal virtues. The common name was given in the Middle Ages as the plant was considered good for nothing as far as medical use was concerned, being fit only therefore for dogs (Herb Society).