Sequential microdissection and scanning electron microscopy of whole primate ocular lumenal methyl methacrylate corrosion castings were performed to determine the anatomic basis of collateral arterial blood flow to the anterior uvea. Collateral anastomoses were observed at three sites: (1) the episclera where the anterior ciliary arteries interconnect at the limbus to form the episcleral circle; (2) the ciliary muscle where anastomoses between the perforating anterior ciliary and long posterior ciliary arteries comprise the intramuscular circle, the most extensive of the three collateral arterial rings, and (3) the root of the iris where circumferentially oriented vessels derived from the intramuscular circle form the "major arterial circle". Of these, the "major arterial circle" is the most discontinuous. This multilevel collateral system probably accounts for the rarity of anterior segment ischemia following all but the most extensive extraocular muscle surgeries.