Lateral View of a Right Temporal Bone. The squamosal part forms part of the lateral wall and floor of the middle fossa, the posterior part of the zygomatic arch, and the upper surface of the mandibular fossa. The tympanic part forms the posterior wall of the mandibular fossa; the anterior wall, lower wall, and part of the posterior wall of the external canal; and the floor of the tympanic cavity and adjacent osseous portion of the eustachian tube. The styloid process is ensheathed at its base by the tympanic part and projects downward, serving as the attachment of three styloid muscles. The mastoid part is located posterior to the external acoustic meatus and contains the mastoid air cells, which coalesce into a large cavity at the mastoid antrum. The retrolabyrinthine, translabyrinthine, and transcochlear approaches are directed through the mastoid part. The digastric muscle attaches medial to the mastoid tip in the digastric groove. The oval window is exposed in the medial wall of the tympanic cavity. (Image courtesy of AL Rhoton, Jr.)