Definition: The arachnoid layer is the middle layer of the meninges, membranes that cover the brain and spinal cord. The arachnoid layer acts as a cushion between the dura mater and the pia mater. The arachnoid layer is so named because it resembles a spider web. Its porous makeup allows cerebrospinal fluid to flow through it, which bathes the brain and spinal cord in nutrients.
Bleeding in the arachnoid layer is known as a subdural hematoma, a potentially life-threatening form of closed head injury.
Pronunciation: ah-RACK-noyd
