E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
Walls have Ears.
The Louvre was so constructed in the time of Catherine de Medicis, that what was said in one room could be distinctly heard in another. It was by this contrivance that the suspicious queen became acquainted with state secrets and plots. The tubes of communication were called the auriculaires, and were constructed on the same principle as those of the confessionals. The Ear of Dionysius communicated to him every word uttered in the state prison. (See SPEAKING HEADS, 9.)