A fountain of Parnassus sacred to the Muses. Its waters had the power of inspiring with the gift of poetry those who drank of them.
The drooping Muses [Sir Industry] Brought to another Castalie, Where Isis many a famous nursling breeds, Or where old Cam soft paces o'er the lea In pensive mood.
Thomson: Castle of Indolence, ii. 21.
“Isis” means the University of Oxford, and “Cam” the University of Cambridge, so called from the respective rivers on which they stand.
Source: Dictionary of Phrase and Fable, E. Cobham Brewer, 1894