Meninges and Cerebrospinal Fluid

1. Central nervous system (CNS) : Both the brain (inside the cranium) and the spinal cord (inside the vertebral canal) are protected by three meninges, layered from outside in. The dura mater is the outermost, tough, thick, fibrous and non-vascular membrane, attached to the inner surface of the cranium and separated from the layer beneath it by the serous-fluid-filled subdural space. The arachnoid mater is the thin, non-vascular, web-like middle layer; below it lies the narrow subarachnoid space, filled with cerebrospinal fluid (CSF), which in turn separates it from the pia mater. The pia mater is the innermost, delicate, highly vascular membrane lying in close contact with the actual nervous tissue of the brain and spinal cord.

CSF (cerebrospinal fluid) : Cerebrospinal fluid (CSF) is a lymph-like extracellular fluid secreted by the choroid plexuses of the pia mater and by the ependymal cells lining the brain's ventricles and the spinal cord's central canal; three openings in the roof of the medulla oblongata allow it to drain from the brain to the exterior of the CNS. Slightly alkaline, with a specific gravity of about 1.005, roughly 100-120 cc of CSF surrounds and fills the CNS at any time, continuously produced and continuously drained into the bloodstream (nervous tissue itself has no lymphatic vessels). Along with the meninges, CSF cushions the brain and spinal cord like a shock absorber against mechanical injury, keeps intracranial pressure constant, helps exchange nutrients and waste between blood and brain tissue, supplies oxygen to the brain, and protects it from desiccation.