The semicircular canals are part of the inner ear.
They are lined with cilia (microscopic hairs) and filled with a liquid substance, known as endolymph. Every time the head moves, the endolymph moves the cilia. This works as a type of motion sensor, as the movements of the cilia are communicated to the brain. As a result, the brain knows how to keep the body balanced, regardless of the posture.
The semicircular canals contain three main parts: the horizontal, posterior, and superior canals. Each of these canals provides a separate sense of directional balance. The superior canal detects head rotations on the anterior-posterior (side-to-side movement, like tilting the head toward the shoulders) axis. The posterior canal detects rotations on the sagittal plane (forward and backward movement, like sit-ups). The horizontal canal senses movement on a vertical basis, as the head rotates up-and-down on the neck.
Damage or injury to the semicircular canals may be twofold. If any of the three separate parts do not work, a person can lose their sense of balance. A loss of hearing may also result from any damage to these semicircular canals.