The clear fluid that surrounds, cushions, and protects the brain, spinal cord, and meninges.  Cerebrospinal fluid has additional functions that include: gives buoyancy to the brain; thus reducing net weight/relieving pressure at the base of the brain stem, excretion of waste products, and acts as an endocrine medium by transporting hormones to other areas of the brain that can be carried to remote sites of the brain where needed.