posterior wall tympanic cavity
(mastoid wall) contains a “doorway” or entrance (aditus) to the tympanic or mastoid antrum which is a chamber that is lined with mastoid air cells. There is also a bony pyramid called the “pyramidal eminence” which serves as an attachment site for the stapedius muscle which attaches to the stapes and acts to dampen vibration. This muscle is innervated by CN VII – the facial nerve (branchial motor fibers). Inside the posterior wall is the facial canal which transmits the facial nerve as it courses inferiorly to the stylomastoid foramen to form the 5 motor branches to muscles of fascial expression. As it passes through this vertical canal, it will give off the chorda tympani nerve which courses along the lateral wall of the tympanic cavity. The chorda tympani will exit the middle ear cavity by passing through the petrotympanic fissure which opens into the infratemporal fossa.