semicircular duct

n.
  1. Any of three small membranous tubes that lie within the bony labyrinth of the inner ear, form loops in planes at right angles to one another, and open into the vestibule.
The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.