Cerebro-spinal fluid filled in the the Sub-arachnoid space.
The brain is covered by three membrane or meninges. The innermost membrane is called piamater. It is thin and very delicate and supplies with blood vessels. The piamater is present very close to the brain. Next to the piamater is the arachnoid membrane, which is a spider webby-like structure. The outermost membrane is called the duramater and is presently close to the inside of the skull.
The space present between piamater and the arachnoid membrane is called the subarachnoid space. The subarachnoid space is filled with cerebrospinal fluid, which serves as a cushion for the central nervous system and protects it from shock. The fluid also provides a medium for the exchange of respiratory gases, food materials, waste, etc between the brain and the body.