Fig. 42-17. Superior aspect of the base of the skull. The right temporal bone (see fig. 42-19) has been removed, thereby displaying the head of the mandible. The floor of the cranial cavity presents three "steps"-the anterior, middle, and posterior cranial fossae-separated by two ledges: (1) the lesser wings of the sphenoid and (2) the upper border of the petrous part of the temporal bone on each side. Click for high resolution image.