What exactly do the cavernous sinuses do?

Big blood vessel: The cavernous sinuses are actually not the stereotypical sinuses that most people talk about. If you have sinusitis, you will have inflammation of your paranasal sinuses. The cavernous sinus is a collection of veins just outside your brain. It is associated with important nerves and the carotid artery. In general it collects blood from the brain and helps send it back to the heart.

Cavernous sinus: The cavernous sinuses are at the base of the skull and are a space where the brain drains venous blood plus nerves that control your eye movements also travel through.