Section 190: The mastoid segment (VIIf) of the facial nerve is visible on its vertical descent through the temporal bone. The chorda tympani (ct) has already separated from the facial nerve and is ascending toward the middle ear in a separate canal (i.e., iter chordae posterius). The non-sensory portion of the posterior (psc) and lateral (lsc) semicircular canals are partly surrounded by air cells (ac). Note that the posterior canal is vertical and approximately parallel to the posterior fossa surface of the temporal bone.