The arachnoid mater is one of the three protective membranes, known as meninges, that surround the brain and spinal cord. It is located between the outer layer, the dura mater, and the inner layer, the pia mater. The arachnoid mater is a delicate, web-like structure that helps cushion the central nervous system. This membrane contains a space called the subarachnoid space, which is filled with cerebrospinal fluid (CSF). The CSF acts as a shock absorber, providing protection and maintaining a stable environment for the brain and spinal cord. The arachnoid mater also plays a role in the circulation of CSF.