The connective tissue membrane called meninges, which are also called as the membranes of the brain, surrounds the brain and spinal cord. They are as follows:
Dura mater: It is the outermost meninx, which is tough, thick and fibrous. It is attached to the inner side of the cranium.
Arachnoid mater: It is middle, thin and vascular, formed of reticular connective tissue. It has spider web like appearance.
Pia mater: It is the innermost thin membrane lying in contact with CNS. It is highly vascular.
The arachnoid mater and dura mater are separated by sub-Dural space. In between pia mater and arachnoid mater sub arachnoid space is present. It contains lymph-like watery fluid called cerebrospinal fluid (CSF). Choroid plexus secretes it. Cerebrospinal fluid is alkaline and approximately 120ml in adults.