The meningeal layer lays over the spinal arachnoid mater. The other two meningeal membranes are the dura mater and the arachnoid mater. In subdural hematoma, bleeding occurs between the dura and the arachnoid mater. CSF circulates in the subarachnoid space (between arachnoid and pia mater). CSF returns to the vascular system by entering the dural venous sinuses via arachnoid granulations. Cerebrospinal fluid (CSF) flows under the arachnoid in the subarachnoid space. The space between the arachnoid and the underlying pia mater is called the subarachnoid space. Intracranial arachnoid cysts usually occur adjacent to the arachnoidal cistern. The dura mater is a membrane that envelops the arachnoid mater. The cerebrospinal fluid that surrounds the spinal cord is contained by the arachnoid mater. Subarachnoid hemorrhage involves bleeding into the space between the arachnoid membrane and the pia mater. Meningitis is the inflammation of the pia and arachnoid mater. Craniopharyngiomas are distinct from Rathke's cleft tumours and intrasellar arachnoid cysts. Most arachnoid cysts are asymptomatic and do not require treatment. There are some cases where hereditary disorders have been connected with arachnoid cysts.