The localization of intravenously administered fluorescein was studied in rabbit and monkey by fluorescence microscopy of freeze-dried eyes. The blood vessels, large and small, of retina and optic nerve are impermeable to fluorescein. Although fluorescein diffuses rapidly throughout the choroid, none passes the pigment epithelium to enter the retina. In contrast, at the posterior scleral foramen some fluorescein enters the optic disc by diffusion from neighboring tissues.
Grayson MC, Laties AM. Ocular Localization of Sodium FluoresceinEffects of Administration in Rabbit and Monkey. Arch Ophthalmol. 1971;85(5):600-609. doi:10.1001/archopht.1971.00990050602014