Fig. 18 The uveal blood vessels. The blood supply of the eye is derived from the ophthalmic artery. Except for the central retinal artery that supplies the inner retina, almost the entire blood supply of the eye comes from the uveal vessels. There are two long posterior ciliary arteries, one entering the uvea nasally and one temporally along the horizontal meridian of the eye near the optic nerve (a). These two arteries give off three to five branches (b) at the ora serrata that pass directly back to form the anterior choriocapillaries. These capillaries nourish the retina from the equator forward. The short posterior ciliary arteries enter the choroid around the optic nerve (c). They divide rather rapidly to form the posterior choriocapillaris that nourishes the retina as far anteriorly as the equator (the choriocapillaris is not shown in this drawing). This system of capillaries is continuous with those derived from the long posterior ciliary arteries. The anterior ciliary arteries (d) pass forward with the rectus muscles, and then pierce the sclera to enter the ciliary body. Before joining the major circle of the iris, they give off 8 to 12 branches (e) that pass back through the ciliary muscle to join the anterior choriocapillaris. The major circle of the iris (f) lies in the corona ciliaris and sends branches posteriorly into the ciliary body as well as forward into the iris (g) and limbus (h). Some branches join the episcleral system of vessels (i). The circle of Zinn (j) is formed by pial branches (k) as well as branches from the short posterior ciliary arteries. The circle lies in the sclera and furnishes part of the blood supply to the optic nerve and disc. The vortex veins exit from the eye through the posterior sclera (l) after forming an ampulla (m) near the internal sclera. Venous branches that join the anterior and posterior part of the vortex system are meridionally oriented and are fairly straight (n), whereas those joining the vortices on their medial and lateral sides are oriented circularly about the eye (o). The venous return from the iris and ciliary body (p) is mainly posterior into the vortex system, but some veins cross the anterior sclera and limbus (q) to enter the episcleral system of veins. (Hogan M, Alvarado J, Weddell J: Histology of the Human Eye—An Atlas and Textbook. Philadelphia: WB Saunders, 1971:326)