Which structure is a tough, fibrous membrane composed of two layers, the periosteal and the meningeal layers?

• A. Periosteal layer of dura mater
• B. Meningeal layer of dura mater
• C. Dura mater
• D. Arachnoid mater

Answer: (C)

Two-layered, tough fibrous membrane surrounding the brain and spinal cord is the dura mater. Its outer portion, the periosteal layer, lines the inner skull, while the inner portion, the meningeal layer, covers the brain and continues to form the dural folds such as the falx cerebri and tentorium. These layers are usually fused, but they separate in places to create dural venous sinuses and other folds. The arachnoid and pia maters are thinner, separate layers that lie beneath the dura, not a two-layered structure themselves. So the described structure is the dura mater.