A, The tympanic bone forms the anterior, lower, and part of the posterior wall of the external canal. The facial nerve exits the skull through the stylomastoid foramen, which is located medial to the tympanomastoid suture. The spine of Henley approximates the deep site of the tympanic facial segment and the lateral canal. The mastoid antrum is located between the posterosuperior wall of the external canal and middle fossa floor deep to the depression behind the spine of Henle. B, A mastoidectomy has been completed to expose the capsule of the posterior and lateral canals and the tympanic and mastoid facial segments. C, The posterior and superior wall of the external canal and the tympanic membrane have been removed while preserving the malleus and chorda tympani. The mastoid segment of the facial nerve descends through the facial canal and gives rise to the chorda tympani, which passes upward and forward across the tympanic membrane and malleus neck. D, Enlarged view. The head of the incus articulates with the head of the malleus, the short process of the incus points backward toward the facial nerve, and the long process attaches to the stapes, which sits in the oval window. The stapedial muscle passes forward below the tympanic segment of the facial nerve and attaches to the neck of the stapes. E, The incus has been removed to expose the stapes sitting in the oval window. The chorda tympani crosses the neck of the malleus. The promontory is located superficial to the basal turn of the cochlea. The labyrinth and fundus of the internal meatus are located medial to the tympanic cavity. A line directed medially through the skull along the long axis of the external meatus will also approximate the site of the long axis of the internal meatus on the medial side of the promontory and acousticovestibular labyrinth. F, The stapes has been removed from the oval window. The handle of the malleus attaches to the tympanic membrane, the neck is crossed by the chorda tympani, and the head articulates with the incus, which has been removed. The tendon of the tensor tympani attaches to the upper part of the handle of the malleus. The stapedial muscle is housed within the pyramidal eminence and its tendon inserts on the stapedial neck. (Images courtesy of AL Rhoton, Jr.)