artery, which pierces the tarsal plate at the subtarsal fold. The ascending branches pass up over the fornix to the globe, where they become the posterior conjunctival arteries. These anastomose with the anterior conjunctival arteries from the limbus and together they supply the bulbar conjunctiva (Fig. 1.2).

The marginal arcades send perforating branches through the tarsus to the conjunctiva at the subtarsal fold. These divide into marginal and tarsal secondary branches, which run perpendicularly either to feed the very vascular zone at the lid margin or to meet with corresponding branches of the peripheral arcades. The tarsal conjunctiva is well supplied with blood, hence its red color. The fornix is red and the bulbar conjunctiva is colorless unless congested. The conjunctival capillaries mainly are of the nonfenestrated continuous type that occurs in skeletal muscle and iris. These capillaries are excellent sites to externally view vaso-occlusive processes. In sickle cell disease, vaso-occlusions in conjunctival capillaries are called comma signs and are an excellent diagnostic characteristic for sickle cell disease [32].

1.1.3.2 The Conjunctival Veins

The palpebral veins drain the tarsal conjunctiva, fornix, and posterior bulbar conjunctiva. In the upper lid, a venous plexus between the tendons of the levator drains into the veins of the levator and superior rectus and then into the ophthalmic vein. Immediately behind the limbal arcades and anterior to the episcleral arterial circle lies a perilimbal venous circle, which collects blood from the limbus, marginal corneal arcades, and the anterior conjunctival veins. The circle drains into radial episcleral collecting veins and then into the veins of the rectus muscles.

1.2Uveal Tract

The ophthalmic artery branches to form the posterior ciliary arteries and the central retinal artery. There are two types of ciliary arteries supplying the uveal tract: 15–20 short posterior

ciliary arteries and two long posterior ciliary arteries (Fig. 1.3). Venous outflow of the uveal tract consists of four vortex veins that drain all of the choroid and most of the anterior uveal tract. Some uveal blood from the iris and ciliary body drains through the scleral plexus of anterior scleral veins. The four vortex veins are at the equator and drain to the superior and inferior orbital veins.

1.2.1The Iris

The iris is the most anterior portion of the uveal tract. It lies in the frontal plane of the eye between the anterior and posterior chamber and is bathed on both surfaces by the aqueous humor.

1.2.1.1 The Major Arterial Circle of the Iris

The iris receives its arterial blood supply from the major arterial circle (MAC), which lies in the stroma of the ciliary body near the iris root (Fig. 1.4). Branches from the major arterial circle enter the periphery of the iris through its root. The blood vessels of the iris are radially oriented with slightly sinuous courses, which allow them to accommodate to the movements of the pupil. The arteries arise mainly from the MAC of the iris. Some authors report that the major circle is not always continuous in man, and the same is true in the macaque monkey. Some arteries also arise directly from the perforating branches of the anterior ciliary arteries after they have pierced the sclera to reach the ciliary body (Figs. 1.1 and 1.2). Anastomosing occasionally, they converge radially from ciliary to pupillary margin. In pupillary miosis, their course is straight, but they become sinuous (or serpentine) as the pupil dilates. The collarette of the iris is a thickened area (0.6 mm), which overlies an incomplete vascular circle (circulus vasculosus iridis minor), and is where the ciliary zone is separated from the papillary zone of the iris (Fig. 1.5) [45]. At the collarette, a few anastomoses occur and, with corresponding venous anastomoses, make an incomplete vascular circle, the so-called circulus

PCAs

Divides more proximally on temporal side

Distal SPCAs

Paraoptic short posterior ciliary arteries

2º and 3º branches

Retinal of PCA SPCAs

vessels Pia

Dura Proximal circle

circle of Zinn-Haller

Fig. 1.3 The arterial supply of the uveal tract. The ophthalmic artery provides blood to posterior ciliary arteries (PCAs). The paraoptic and distal short posterior ciliary arteries (SPCAs) are branches of the PCAs. From the paraoptic SPCAs arise the proximal and distal (Haller and Zinn) arterial circles which supply blood to the capillaries of the optic nerve head and pia directly. The long PCAs (LPCAs) arise from the PCAs and supply most of the blood to major arterial circle of the iris (MAC) and recur-

rent arteries to the anterior choroid. The temporal long ciliary arteries (LPCAs) also supply the MAC and give off a recurrent branch to the posterior choroids. The major arterial circle in the ciliary body supplies the iris and ciliary processes. The anterior ciliary arteries (ACA) branch to form the intramuscular arterial circle (IMC), which supplies the outer and more posterior part of the ciliary muscle. Branches of the LPCAs and ACAs anastomose often (From Wolff [45], p. 374, with permission)

arteriousus iridis minor. Most vessels reach the pupillary margin where, after breaking up into capillaries, they bend around into the veins. The iris venous channels roughly follow the arterial ones, the larger veins lying in the anterior stroma, and the smaller ones near the dilator muscle. The collector trunks from these veins enter the ciliary body, where the blood then drains into the vortex system and the ciliary plexus.

The two long posterior ciliary arteries (medial and lateral) arise from the ophthalmic artery and enter the globe. These arteries, together with the nerves, traverse the sclera

through emissary canals in an oblique manner, from posterior to anterior, and enter the suprachoroidal space at the equator. They run forward to give arterial supply to the ciliary body and the iris. In addition, they meet the anterior ciliary arteries to form the major arterial circle of the iris (Figs. 1.2 and 1.4). The major arterial circle of the iris is located in the stroma of the ciliary body and gives arterial supply to the iris. Surgery on the vertical, but not the horizontal, rectus muscles may give rise to ischemic defects in the iris. The superior and inferior anterior uvea is at greater risk of ischemia after superior