Osseous ampullae

The bony semicircular canals are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule. They are unequal in length, compressed from side to side, and each describes the greater part of a circle. Each measures about 0.8 mm. in diameter, and presents a dilatation at one end, called the osseous ampulla, which measures more than twice the diameter of the tube. They open into the vestibule by five orifices, one of the apertures being common to two of the canals.