The ash-tree, whose roots run in three directions: one to the Asa-gods in heaven, one to the Frostgiants, and the third to the under-world. Under each root is a fountain of wonderful virtues. In the tree, which drops honey, sit an eagle, a squirrel, and four stags. At the root lies the serpent Nithhöggr gnawing it, while the squirrel Ratatöggr runs up and down to sow strife between the eagle at the top and the serpent. (Scandinavian mythology.)
The Nornas besprinkle The ash Yggdrassil.
Lord Lytton: Harold, bk. viii.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894
More on Yggdrasil' from Fact Monster: