Posterior to the equator, the sclera is perforated obliquely by the emissary canals for the vortex veins (Fig. 1.17). Each eye usually contains from four to seven veins. Vortex veins drain the venous system of the choroid, ciliary body, and iris. One or more veins are in each quadrant. The superior vortex veins exit 8 mm posterior to the equator, close to the most posterior edge of the insertion of the superior oblique muscle. The inferior vortex veins exit 6 mm posterior to the equator.

The short posterior ciliary arteries arise from the ophthalmic artery as it crosses the optic nerve. After dividing into 10Ð20 branches, they perforate the sclera around the entrance of the optic nerve (Fig. 1.18) and supply the choroid as far as the equator of the eye. The vessels pass directly to the choroid without forming a capillary bed in the sclera. Some of the branches in the sclera run toward the equator and anastomose with the branches of the long posterior ciliary arteries to supply the posterior episclera. However, this posterior episcleral plexus is so thin that it gives a poor supply to the underlying sclera in the equator. Most of the nutrient requirements of the sclera in this area are supplied by the choroidal

circulation. Some of the branches of the short posterior ciliary arteries around the optic disk form the incomplete vascular circle of ZinnHaller near the inner scleral rim. The emissary canals for the short ciliary vessels, arteries, and veins may be perpendicular, oblique, or spiral.

Circulatory Dynamics

The classic, static anatomical techniques have provided the foundation for our knowledge of the blood supply of the anterior segment of the eye (described in the preceding section). However, the direction of ßow in these vessels studied by circulatory dynamic techniques, such as anterior segment ßuorescein angiography, has excited controversy. Some studies support the traditional view [20] that the anterior ciliary artery ßow is from the region of the rectus muscles toward the inside of the eye, providing major perforating contributions to the intraocular circulation [35, 38Ð40]. This centripetal distribution is supported by corrosion plastic castings of the ocular vasculature, in which perforating anterior ciliary arteries divide extensively within the anterior uvea [24, 26, 41], and by the fact that anterior segment ischemia may arise from surgery on the vertical

Fig. 1.18 Diagrammatic representation of a transverse section of the globe, illustrating another view of the relationships between the short posterior ciliary, long

posterior ciliary, and anterior ciliary arteries and the short posterior ciliary, anterior ciliary, and vortex veins