Cavernous sinus: A large channel of venous blood creating a "sinus" cavity bordered by the sphenoid bone and the temporal bone of the skull. The cavernous sinus is an important structure because of its location and its contents which include the third cranial (oculomotor) nerve, the fourth cranial (trochlear) nerve, parts 1 (the ophthalmic nerve) and 2 (the maxillary nerve) of the fifth cranial (trigeminal) nerve, and the sixth cranial (abducens) nerve.

A cavernous sinus thrombosis is a blood clot within the cavernous sinus. This clot causes the cavernous sinus syndrome.

The cavernous sinus syndrome is characterized by edema (swelling) of the eyelids and the conjunctivae of the eyes and paralysis of the cranial nerves which course through the cavernous sinus.

The orbital cavity is drained by paired veins called the superior and inferior ophthalmic veins. The superior ophthalmic vein arises at the inner angle of orbit and follows the course of ophthalmic artery into the cavernous sinus, a greatly dilated (widened) channel of venous blood. The inferior ophthalmic vein arises from a venous network at the forepart of orbit and divides into two branches one of which also ends in the cavernous sinus.