The arachnoid is a membrane-like tissue structure located in the central nervous system. It is a translucent membrane made of connective tissue and is located deep in the dura mater. The arachnoid is a necessary structure in the brain that serves to protect brain tissue and can span the sulcus of the spinal cord and brain, divided into two parts, the spinal arachnoid and the cerebral arachnoid, which continue together with each other. Between the arachnoid and the soft membranes of the brain surface is the subarachnoid space, which is filled with cerebrospinal fluid, and the brain and spinal cord are suspended in the cerebrospinal fluid, which supports and cushions the brain and spinal cord and helps to metabolize and support them nutritionally. When a blood vessel on the surface of the brain ruptures and bleeds, it can cause blood to enter the subarachnoid space, resulting in a subarachnoid hemorrhage. A lumbar puncture also requires a needle to be inserted into the subarachnoid space to extract cerebrospinal fluid to help diagnose central nervous system disorders.