The subarachnoid space, is a space between two of the 3 meninges or the membranes that cover and protect the brain and spinal column or cerebrospinal column. It lays below the arachnoid Membrane (the middle membrane) and above the pia mater (the inner membrane). It is independently noted because it contains a free flowing fluid known as cerebrospinal fluid (CSF) along with small fibrous structures known as trabeculae, which stabilize the space so that the CSF can circulate freely cushioning and protecting the cerebrospinal column from concussion and other damage and because it is the only actual space within the meninges themselves.