The membrana aracnoides, or arachnoid membrane, is one of the three protective layers surrounding the brain and spinal cord, known collectively as the meninges. It is located between the outer layer, the dura mater, and the inner layer, the pia mater. The arachnoid membrane is a thin, web-like structure that helps protect the central nervous system. This membrane contains a space called the subarachnoid space, which is filled with cerebrospinal fluid (CSF). This fluid acts as a cushion, providing support and protection to the brain and spinal cord while also facilitating the exchange of nutrients and waste.