Fig. 20 Schematic diagram of the blood–brain barrier at the optic nerve head. Plasma proteins may leak readily from the choroid via the highly fenestrated choriocapillaris and into the nerve head across the sclera, through the glial mantle, and directly, at the level of the choroid. Entry of proteins into the retina is blocked by the presence of a series of tight junctions between the lining glial cells and the adjacent pigment epithelium. (From Tso MOM, Shih C-Y, McLean IW. Arch Ophthalmol 93:815, 1975, with permission)