Meninges
The pia mater (or "pia") is the layer of the meninges that is closest to the brain and spinal cord.  It is a thin, delicate layer ("pia mater" means "tender mother" in Latin).  The pia mater sticks very closely to both the brain and spinal cord. The pia mater is made of cells that fluid cannot get through.  This prevents cerebrospinal fluid from reaching the brain.