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The Anatomy of the Inner Ear

The inner ear houses the vestibulocochlear organ that maintains balance and receives sound from the middle ear. It has two parts: the bony labyrinth and the membranous labyrinth. The bony labyrinth contains the otic capsule, which is denser than the surrounding temporal bone. It contains fluid called perilymph and three cavities, called the cochlea, vestibule, and semicircular canals:

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  • Cochlea: The cochlea is a shell-shaped opening with a cochlear duct, which is important for hearing. It has a spiral canal that begins at the vestibule and turns around a bony core called the modiolus, which has canals for blood vessels and branches of the cochlear nerve. It opens into the subarachnoid space via the cochlear aqueduct. It has a round window and a secondary tympanic membrane.

  • Vestibule: This small, oval chamber contains the utricle and the saccule along with parts of the vestibular labyrinth. An oval window is on its lateral wall. The vestibular aqueduct runs to the posterior surface of the temporal bone.

  • Semicircular canals: These three canals open up into the vestibule. Each is semicircular in shape and has a swelling called the bony ampulla.

The membranous labyrinth is within the bony labyrinth. It contains fluid called endolymph and is surrounded by the perilymph that fills the bony labyrinth. The membranous labyrinth includes the vestibular labyrinth, which contains is the utricle and the saccule (two vestibular sacs), three semicircular ducts in the semicircular canals, and the cochlear labyrinth (duct of the cochlea). These structures and fluids are involved in both balance and hearing.

The internal acoustic meatus is a canal that runs laterally within the temporal bone. It houses the vestibulocochlear nerve (CN VIII), which divides into its vestibular and cochlear divisions.

The nerves that supply the ear include the following:

  • Great auricular nerve and auriculotemporal nerve (CN V3): These nerves supply the skin of the auricle with a little help from the facial nerve. The auriculotemporal nerve also supplies a portion of the external acoustic meatus and the external surface of the tympanic membrane.

  • Auricular branch: This branch of the vagus nerve (CN X) innervates part of the external surface of the tympanic membrane.

  • Glossopharyngeal nerve (CN IX): This nerve innervates the linings of the middle ear and the pharyngotympanic tube.

The posterior auricular artery and the superficial temporal artery supply blood to the auricle. The ascending pharyngeal artery, the middle meningeal artery, and the artery of the pterygoid canal provide blood to the pharyngotympanic tube. Blood is drained from the ear by veins that accompany the arteries.

Lymphatic fluid from the auricle is drained into the superficial parotid lymph nodes, mastoid lymph nodes located near the mastoid process, and deep cervical lymph nodes.

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