“…Notwithstanding the early EM studies that left open the possibility that the iris vessels and ciliary epithelium were “leaky”, a host of studies, completed in several species, showed that iris vessels and the non-pigmented ciliary epithelium were impermeable to horseradish peroxidase. (Vegge, 1971a,b, Smith, 1971, Smith and Rudt, 1973; Uusitalo, Palkama and Stjernschantz, 1973; Raviola, 1974, Pederson, 1974; Freddo and Raviola, 1982a) (Figure 1 – top). The outcomes of these studies established that the anatomical equivalents of the BAB, as it was originally described, were the tight junctions of the iris vasculature and the non-pigmented ciliary epithelium (Raviola, 1977, Bill, 1986).…”