The meninges are three layers of protective tissue called the dura mater, arachnoid mater, and pia mater. The dura mater is the most superior/outer of the meningeal layers. Its name literally translates to "hard mother" and it is tough and inflexible. This tissue forms several structures that separate the cranial cavity into compartments and protect the brain from displacement. The arachnoid is the middle layer of the meninges. In some areas, it projects into the sinuses formed by the dura mater. The subarachanoid space lies between the arachnoid and pia mater. It is filled with cerebrospinal fluid. All blood vessels entering the brain, as well as cranial nerves pass through this space. The pia mater is the innermost layer of the meninges. Unlike the other layers, this tissue adheres closely to the brain, running down into the sulci and fissures of the cortex.