A 38-year-old woman from Mumbai presents with fever, severe bilateral periorbital edema, and chemosis for 3 days. She has a history of untreated sinusitis. On examination, she has bilateral ptosis, restricted eye movements in all directions, and bilateral ophthalmoplegia. Her pupils are dilated and unreactive. Blood cultures grow Staphylococcus aureus. CT brain shows bilateral cavernous sinus enhancement with thrombosis. Which of the following anatomical features explains why infection can spread rapidly from the paranasal sinuses to the cavernous sinus?