The nutrition of the intraocular tissues is accomplished by the retinal vessels, the uveal vessels, and by the aqueous humor. Both morphologically and physiologically the retinal vessels are similar to those in the brain. The endothelial cells of the capillaries are attached to each other by tight junctions, the resistance vessels respond poorly to a large number of drugs, and the blood flow through the retina is autoregulated and very little affected by the sympathetic nervous system. The blood vessels of the iris also have morphological and permeability characteristics similar to those in the brain but they are under a strong influence from the sympathetic nerves and react to many drugs. The blood flow is autoregulated. The blood vessels of the choroid and the ciliary processes are similar to those in the small intestine and in the kidney. The endothelial cells of the capillaries are fenestrated; the vessels respond to sympathetic nervous stimulation and to a large number of vasoactive drugs. Autoregulation of the blood flow is intermediate in the ciliary body and very poor or absent in the choroid...