The bony semicircular canals are three in number, superior, posterior, and lateral, and are situated above and behind the vestibule. These canals are responsible for detecting angular acceleration of the head, and their orientation determines to which plane of rotation they are sensitive; for example, the horizontal canal picks up head rotation in the horizontal plane about the body's Y-axis. 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. Within an osseous ampulla there is a crista ampullaris, consisting of a thick gelatinous cap called a cupula and many hair cells. When the head rotates, the endolymph in the canals lags behind due to its inertia and acts on the cupula, which bends the cilia of the hair cells. This stimulation of the hair cells sends the message to the brain that angular acceleration is taking place. They open into the vestibule by five orifices, one of the apertures being common to two of the canals.