Section 401: The basal turn of the cochlea (c) is disappearing. The stapedius muscle (sm) is still present. The chorda tympani (ct), running in a separate canal, is close to the mastoid segment of the facial nerve (VIIf). The chorda tympani separates from the lateral surface of the facial nerve about 0.5 cm superior to the stylomastoid foramen. This area is generally not included in human temporal bone specimens.
Surgical perspective - A surgical route to the petrous apex (PA) is anterior to the facial nerve, inferior to the cochlea, posterior to the internal carotid artery (ICA) and anterior to the jugular foramen (JF). In this section the jugular foramen appears as condensing bone which is just superior to the foramen. Most of the inferior portion of the tympanic bone must be removed to expose the petrous apex via this route.