The arachnoid mater is one of the three protective membranes, or meninges, that surround the brain and spinal cord. It is located between the outer layer, called the dura mater, and the inner layer, known as the pia mater. The arachnoid mater has a web-like structure, which gives it its name, and it helps to cushion the central nervous system. This membrane contains a space called the subarachnoid space, which is filled with cerebrospinal fluid. This fluid acts as a shock absorber, providing protection and nourishment to the brain and spinal cord. The arachnoid mater also plays a role in the circulation of cerebrospinal fluid, helping to maintain the overall health of the central nervous system.