Final answer:
The valvelike structures that protrude through the dura mater to absorb CSF into venous blood are called arachnoid granulations. They play a critical role in the venous return system by facilitating the reabsorption of CSF into the dural sinuses, which then drain into the jugular veins.
Step-by-step explanation:
The structures that are valvelike and protrude externally through the dura mater to absorb cerebrospinal fluid (CSF) into venous blood are the arachnoid granulations. These are part of the venous return system and are crucial for maintaining the balance of CSF in the body. The arachnoid granulations extend into the dural sinuses, specifically, they emerge into the sinuses where the reabsorption of CSF occurs, ultimately flowing into the venous blood that eventually drains through the jugular veins. The dura mater itself is a protective membrane that surrounds the central nervous system and supports the venous sinuses, facilitating the blood drainage from the brain back to the heart.