Cavernous Sinus: two paired venous spaces located within the skull base in the sphenoid bone. These multi-compartmental spaces are surrounded by dura and located directly adjacent to the pituitary gland. Several critical structures pass through the cavernous sinuses including the internal carotid arteries on their way to the brain, as well as several cranial nerves including cranial nerves III (oculomotor), IV (trochlear) and VI (abducens) which are responsible for eye movements, as well as parts of cranial nerve V (trigeminal) which is responsible for facial sensation. Pituitary tumors and other skull base tumors such as meningiomas and chordomas can sometimes invade the cavernous sinus, surround the carotid arteries and cause impairment of the cranial nerve function with resultant double vision and/or facial pain or facial numbness.