Internal acoustic meatus

The internal acoustic meatus (also internal auditory meatus, IAM) is a canal within the temporal bone that transmits nerves and vessels from within the posterior cranial fossa to the auditory apparatus. Contents of the IAM include:

• facial nerve
• vestibulocochlear nerve
• vestibular ganglion
• labyrinthine artery (usually a branch of the AICA or basilar artery)

The opening of the IAM is located within the cranial cavity, near the posterior surface of the temporal bone. The margins of the opening are smooth and rounded and the canal is short (1 cm), running laterally to the bone. At the distal end of the canal, it splits into three distinct openings, one of which is the facial canal.

Nerves

There are five nerves that run through the AIM:

• nervus intermedius (sensory component of CN VII)
• facial motor root (motor component of CN VII)
• cochlear nerve (component of CN VIII)
• inferior vestibular nerve (component of CN VIII)
• superior vestibular nerve (component of CN VIII)

Their position is most constant in the lateral portion of the meatus which is anatomically divided by the falciform crescent. This horizontal ridge divides the canal into superior and inferior portions.

• superior: facial nerve and superior vestibular nerve (SVN); the facial nerve is anterior to the SVN and is separated from it laterally by Bill's bar, a vertical ridge of bone
• inferior: cochlear nerve and inferior vestibular nerve (IVN); the cochlear nerve is situated anteriorly

Ganglion

In addition to the three nerves which enter it, it also contains the vestibular ganglion (ganglion of Scarpa). From here three bundles emerge (superior and inferior division of the vestibular nerve and the nerve from the posterior semicircular canal - see Vestibulochoclear nerve (CN VIII) for further details)