Semicircular ducts

The semicircular ducts are the part of the membranous labyrinth that are contained in the semicircular canals of the osseous labyrinth, concerned with rotational equilibrium. Like the other parts of the membranous labyrinth, the ducts contain endolymph and are surrounded by perilymph.

The crura membranacea (membranous crura) is the name given to the two ends of each semicircular duct as they pass into the utricle.

The ducts fill only about one-quarter of the diameter of each canal except at the ampulla, the dilated lateral end of each duct, which occupies all of the diameter of the canal. The ampulla of each duct contains the ampullary crest which is the position of the hair cells and associated cupula, which in turn stimulate primary sensory neurons when the head turns and produces endolymph motion. These neurons synapse in the vestibular ganglia.

Arterial supply

• labyrinthine artery (from the basilar artery) vestibular branches join the vestibular branches of CN VIII

Venous drainage

• cochlear aqueduct vein and vestibular aqueduct vein; from these vessels the venous blood empties into the superior and inferior petrosal sinuses or the internal jugular vein 

Innervation

The vestibular branches of the vestibulocochlear nerve (CN VIII) supply the semicircular ducts

• superior (anterior) semicircular duct supplied by the superior division of the vestibular nerve
• posterior semicircular duct supplied by the inferior division of the vestibular nerve
• lateral (horizontal) semicircular duct supplied by the superior division of the vestibular nerve