episcleral vessels. The episclera becomes progressively thinner toward the back of the eye.

Scleral Stroma

The scleral stroma consists of collagen bundles associated with a few elastic Þbers. Between the bundles are a few Þbroblasts with occasional melanocytes. Collagen bundles, larger in diameter than those in the episclera, vary in thickness and form whorls and loops. Although they usually run parallel with the surface, many crisscross freely with each other, forming a feltlike structure. Because of the interlacing of the collagen bundles, the sclera presents a dull white color. Near the cornea and the optic nerve canal, the bundles tend to run in concentric circles. In other portions, they form patterns of loops running mainly in a meridional direction. This arrangement permits adjustment to the changes in intraocular pressure and to the stress produced by the extraocular muscles.

Lamina Fusca

The lamina fusca is the portion of the sclera that is adjacent to the uvea; collagen bundles become smaller and the number of elastic Þbers is increased. A large number of melanocytes are present, giving to this portion a faintly brown color. The lamina has grooves that provide a passage for the ciliary vessels and nerves. It is separated from the outer aspect of the choroid by a potential space, the perichoroidal space, Þlled by Þne collagen Þbers that provide a weak attachment between both layers.

1.3.1.3 Blood Supply and Emissary Canals

The elegant dissections performed by Leber in 1903 form the foundation of our current understanding on the ocular anterior segment circulation [20]. Anatomical techniques involving Indian ink injections [21] and vascular casting using neoprene and methyl methacrylate [22Ð24], combined with scanning electron microscopy [25Ð 27], conÞrmed LeberÕs Þndings and provided a unique static method for three-dimensional analysis of the ocular microvasculature. Anterior segment ßuorescein angiography [28Ð32], low-dose

ßuorescein anterior segment angiography [33], low-dose ßuorescein anterior segment videoangiography [34], and ßuorescein anterior segment videoangiography with a scanning angiographic microscope [35] helped in the study of circulatory dynamics of the anterior segment circulation, such as ßow direction and ßow velocity.

Vascular Distribution

Results of studies using static anatomical techniques show that the blood supply of the anterior segment of the eye has distinctive characteristics. Except for the perforating vessels, the sclera is a relatively avascular structure. It has a low metabolic requirement because of the slow turnover rate of its collagen. The episcleral blood supply is derived mainly from the anterior ciliary arteries anterior to the insertions of the rectus muscles and from the long and short posterior ciliary arteries posterior to these insertions. Scleral stroma contains capillary beds but is supplied by the episcleral and, to a lesser degree, choroidal vascular networks.

The arteries, veins, and nerves traverse the sclera through emissary canals [36]. These canals or passageways are separated from the sclera by a thin layer of loose connective tissue. There are more emissary canals superiorly at 12 oÕclock and inferiorly at 6 oÕclock than nasally and temporally. The fewest occur in the temporal quadrant. Emissary canals provide a passageway for extraocular extensions of intraocular tumors [37].

On the surface of the eye, the muscular arteries, which arise from the ophthalmic artery, run forward as the anterior ciliary arteries. The anterior ciliary arteries pass through the sclera just in front of the insertions of the rectus muscles in a slightly oblique direction from posterior to anterior. Each rectus muscle has two anterior ciliary arteries, except the lateral rectus muscle, which has only one (Fig. 1.7). The seven anterior ciliary arteries meet via their lateral branches 1Ð5 mm behind the limbus and form the anterior episcleral arterial circle, which feeds the limbal, anterior conjunctival, and anterior episcleral tissues (Fig. 1.11). The anterior episcleral arterial circle broadly resolves into limbal arcades, an anterior conjunctival plexus, a superÞcial episcleral

Fig. 1.11 Scanning electron micrograph of a vascular cast, showing the formation of the major vascular plexuses of the anterior segment. The anterior ciliary arteries (ACA) meet via their lateral branches 1Ð5 mm behind the

limbus and form the anterior episcleral arterial circle (EC), which feeds the limbal, anterior conjunctival, and anterior episcleral tissues (Courtesy of Van Buskirk EM)

plexus, and a deep episcleral plexus (Fig. 1.12). Limbal arcades and anterior conjunctival plexus usually share their origins and form the most superÞcial layer of vessels. The superÞcial episcleral plexus lies within the parietal layer of the episclera and anastomoses at the limbus with the conjunctival plexus, branches of the same plexus, and with the deep episcleral plexus (Fig. 1.13). The deep episcleral plexus lies within the visceral layer of the episclera and anastomoses with branches of the same plexus. In addition, extensions of the remaining anterior ciliary arterial branches perforate the limbal sclera through emissary canals and meet the long posterior ciliary arteries in the ciliary muscle to form the major arterial circle of the iris (Fig. 1.14). The anterior episcleral arterial circle and the major arterial circle of the iris communicate by scleral perforating anterior ciliary arterial branches, which do

not form a capillary bed in the sclera but rather provide nutrients to the uveal tract (Fig. 1.15).

The limbal venous circle collects blood from the anterior conjunctival veins and limbal arcades, and drains into radial episcleral-collecting veins. The episcleral-collecting veins also receive blood from anterior episcleral veins and perforating scleral veins. Perforating scleral veins emerge from SchlemmÕs canal, from which they receive aqueous humor. They penetrate the sclera through different emissary canals than do the arteries. These canals, over the ciliary body, often also carry the ciliary nerves. As the episcleral-collecting veins run posteriorly across the sclera, they form the anterior ciliary veins, which leave the anterior surface of the globe over the rectus muscles.

The two long posterior ciliary arteries (medial and lateral), which also arise from the ophthalmic artery, enter the sclera 3.6 mm nasal to the optic

Fig. 1.12 Scanning electron micrograph (×40) of a vascular cast of the anterior vascular plexuses of the eye. Note that the anterior ciliary arteries (ACA) bend toward the major iris arterial circle (arrows) anterior to the ciliary body (CB) and the iris (I). The superÞcial episcleral plexus (SE) and the deep episcleral plexus (DE) communicate via connecting vessels (Courtesy of Fryczkowski AW)

nerve and 3.9 mm temporal to the optic nerve (Figs. 1.14 and 1.16). The 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. 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 [26, 28, 29]. This Þnding seems to indicate that the anterior ciliary arteries contribute to the iris supply and that this contribution is critical in sectors of the globe that receive inadequate long posterior ciliary artery perfusion (vertical meridian). The superior and inferior anterior uvea are, therefore, at greater risk of ischemia after superior and inferior anterior ciliary occlusion following ligation of the respective muscles. The greater prevalence of emissary canals containing perforating anterior ciliary arteries in the vertical meridia [36] may compensate for this deÞcit.

Fig. 1.13 Scanning electron micrograph (×24) of a vascular cast. Iris vessels (white arrows) are seen posterior to the anterior ciliary artery (ACA) and its branches, as well

as the superÞcial episcleral (SE) and deep episcleral (DE) plexuses and their interconnectors (Courtesy of Fryczkowski AW)