Temporal Bone and Adjacent Cranial Base. Superior view of the middle and posterior cranial base formed by the sphenoid, temporal, and occipital bones. The temporal bone has five parts: the squamosal, petrous, mastoid, tympanic, and styloid parts. Only the squamosal, petrous, and mastoid parts are seen on the upper surface. The styloid and tympanic parts are not seen because they are on the lower surface. The upper surface of the squamosal part forms some of the floor and lateral wall of the middle cranial fossa. The lower surface is the site of the roof of the mandibular fossa in which the mandibular condyle sits. The petrous part of the temporal bone houses the internal acoustic meatus, acousticovestibular labyrinth, and the carotid and facial canals. The mastoid part contains the mastoid air cells and mastoid antrum. The squamosal part of the temporal bone joins anteriorly with the greater wing of the sphenoid bone to form the floor of the middle cranial fossa. The petrous part articulates medially with the body of the sphenoid bone and the clival portion of the occipital bone at the petroclival fissure to form the anterior wall of the posterior fossa. The sigmoid sulcus descends along the posterior surface of the mastoid portion of the temporal bone and turns forward on the upper surface of the occipital bone to enter the jugular foramen. The foramen lacerum, which is located at the junction of the temporal, sphenoid, and occipital bones, is usually covered below the terminal part of the carotid canal by cartilage. The foramina spinosum and ovale of the sphenoid bone are positioned anterior to the petrous apex. The greater petrosal nerve courses along the medial part of the petrosphenoid junction. (Image courtesy of AL Rhoton, Jr.)