Explanation

Piamater is thin innermost vascular and pigmented sheath that lies in contract with brain. At two places, it is fused with roof of brain to form choroid plexuses for secreting cerebrospinal fluid (CSF). Arachnoid is thin webby and porous non-vascular sheath. A narrow sub-arachnoid space occurs between arachnoid and piamater. It contains cerebrospinal fluid (CSF) and connective tissue strands.