"At the medial canthus of the eye, there is a communication with the ophthalmic veins, which drain into the cavernous sinus 1. Blood from the frontal scalp normally flows via the facial vein. However, if flow via this passageway is occluded (e.g., due to thrombosis or pressure), blood above the obstruction will flow through the orbit and into the cavernous sinus 2."
"Infection of the upper lip, vestibule of the nose and eyelids spread by way of the angular, supraorbital and supratrochlear veins to the ophthalmic veins which drain directly into the cavernous sinus."