A 42-year-old man from Delhi presents with a 5-day history of left-sided headache, fever (39.2°C), and progressive left eye swelling. On examination, he has complete ophthalmoplegia affecting the left eye (all extraocular movements impaired), bilateral proptosis, and chemosis. Both pupils are dilated and non-reactive. He has a history of chronic sinusitis. MRI brain shows a filling defect in the left cavernous sinus on post-gadolinium T1 images. Which of the following anatomical relationships explains why both eyes are affected despite unilateral sinus pathology?