The semicircular ducts are three membranous canals that resemble two-thirds of a circular arch, each occupying its own bony semicircular canal oriented perpendicular to the others.
Each semicircular duct opens in the saccule through a dilated end called the ampulla. The semicircular ducts, the same as the semicircular canals, are placed on three different planes, where they make 90-degree angles with each other. Since their specific arrangement, each semicircular duct is responsible for the movements directed within the plane they are aligned.
Within their ampullar ends, the semicircular ducts have receptors for balance. These receptors together with the balance receptors in the utricle and saccule perceive statokinetic triggers. Then the impulses are taken to the central nervous system through the vestibulocochlear nerve (CN VIII).
The anterior semicircular duct (Latin: ductus semicircularis anterior) is a membranous tube located in the bony labyrinth of the inner ear forming about two thirds of a circle. It is oriented in the vertical plane and somewhat perpendicular to the petrous part of the temporal bone, situated medially and laterally over the roof of the utriculus. The anterior semicircular duct detects rotations of the head in around the lateral axis (nodding the head a.k.a. the "Yes" movement).
The lateral semicircular duct (Latin: ductus semicircularis lateralis) is a membranous tube forming about two thirds of a circle and is oriented in the horizontal plane. It has two openings in the lateral wall of the utriculus. The lateral semicircular duct detects the rotation of the head around a vertical axis (the "No" movement).
The posterior semicircular duct (Latin: ductus semicircularis posterior) is a membranous tube forming about two thirds and oriented in the vertical plane and in a plane parallel to the longitudinal axis of the petrous part of the temporal bone, directed downwards and laterally behind the utriculus. The posterior semicircular duct detects rotation of the head around the left-right (frontal) axis (tilting the head over from left to right shoulders).