Pia mater is the innermost of the three meninges; the protective membranes that cover the brain and spinal cord. The other two are dura mater (outermost) and arachnoid mater (middle).
Pia mater is a thin, translucent, mesh-like envelope that closely invests the brain, covering the gyri and descending into the deepest sulci. The only regions where the pia is absent are the natural openings between the ventricles, the foramen of Magendie and the foramina of Luschka.
In conjunction with other meningeal membranes, pia mater functions to cover and protect the Central Nervous System.