The brain is surrounded by mesodermal coverings, the meninges. The outer layer is the tough pachymeninx, or dura mater (A1). The inner layer is the soft leptomeninx which consists of two sheets, the arachnoidmater (A2) and the pia mater (A3).
The pia mater (A3) is the meningeal cover-ing that contains the blood vessels. It borders directly on the brain and forms the me-sodermal side of the pia–glia barrier. From here vessels enter the brain, and they are surrounded by the pia mater for some dis-tance (pial funnel).