Q._______ layer is in close contact of CNS in human being.

The brain (inside the cranium) and spinal cord (inside the vertebral canal) are both wrapped, inner to their bony coverings, in three protective connective-tissue membranes -- the meninges. The dura mater is the outermost, tough, thick, fibrous, non-vascular layer, attached to the cranium's inner surface. The arachnoid mater is the thin, web-like, non-vascular middle layer. The pia mater is the innermost, delicate, highly vascular membrane lying in direct contact with the nervous tissue itself. Cerebrospinal fluid (CSF), secreted by the choroid plexuses of the pia mater and by ependymal cells lining the brain's ventricles and the spinal cord's central canal, fills the subarachnoid space between arachnoid and pia and forms a cushion around and within the CNS. Slightly alkaline (specific gravity ~1.005, total volume ~100-120 cc), CSF is continuously produced and drained (via three openings in the medulla …