The subarachnoid space between the arachnoid mater and the pia mater is filled with CSF and contains the largest blood vessels serving the brain.The pia mater is next to the nervous tissue and contains a network of blood vessels.Meningitis is an infection of the meninges that occurs in the CSF of the subarachnoid space and that can result in increased intracranial pressure, seizures, and loss of consciousness. It can be diagnosed by examining the CSF obtained via a lumbar puncture for microbes or viruses.