Arachnoid

The arachnoid mater is the second layer of the meninges found inner to the dura mater. It encases the brain and spinal cord, forming a delicate and transparent membrane. Unlike the dura mater, it doesn't extend into the brain's grooves, except at specific locations where partitions of the dura mater exist (like the falx cerebrum, cerebellum, and tentorium cerebellum).

Internally, the arachnoid membrane connects to the inner vascular layer called the pia mater. The area beneath the arachnoid membrane is known as the subarachnoid space. Fibrous septa within this space link the outer arachnoid membrane with the inner pia mater

It's important to recognize that the subarachnoid space, situated between the arachnoid and pia mater, contains cerebrospinal fluid (CSF) and the brain's blood vessels, including the circle of Willis. At certain locations the arachnoid membrane is widely separated from the underlying pia mater, creating dilated subarachnoid spaces called the cisterns.

At specific points along the superior sagittal sinus, the arachnoid membrane extends through the inner layer of the dura mater, creating openings into the sinus. These extensions of the arachnoid mater into the dural venous sinuses are called arachnoid villi or arachnoid granulations. Each arachnoid villus acts as a one-way valve, permitting CSF to flow from the subarachnoid space into the dural venous sinus but not the other way around.