canals are located more superiorly at 12 o’clock and inferiorly at 6 o’clock than nasally and temporally. Emissary canals provide a passageway for extraocular extensions of intraocular tumors.

On the surface of the sclera are the arteries of the muscles that arise from the ophthalmic artery and 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 (Fig. 1.1). Each rectus muscle has two anterior ciliary arteries, except the lateral rectus muscle, which has only one. The seven anterior ciliary arteries meet via their lateral branches 1–5 mm behind the limbus and form the anterior episcleral arterial circle, which feeds the limbal, anterior conjunctival, and anterior episcleral tissues (Fig. 1.2). The anterior episcleral arterial circle broadly resolves into limbal arcades, an anterior conjunctival plexus, a superficial episcleral plexus, and a deep episcleral plexus.

Limbal arcades and anterior conjunctival plexus usually share their origins and form the most superficial layer of vessels (Fig. 1.2). The superficial 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. The deep episcleral plexus lies within the visceral layer of 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. 1.2). The anterior episcleral arterial circle and the major arterial circle of the iris communicate by scleral perforating anterior ciliary arterial branches, which provide nutrients to the uveal tract.

Intrascleral arteries of the limbus, an incomplete arterial circle, accompany the canal of Schlemm, derived from the superficial and deep terminal branches of the anterior ciliary arteries. The arterioles show a nonfenestrated endothelial lining and a 1–2 layer medial coat.

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 Schlemm’s 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 through the rectus muscles (Fig. 1.1).

1.1.3The Conjunctiva

The conjunctiva is a thin, translucent mucous membrane that covers the inner surfaces of the upper and lower lids and extends to the limbus on the surface of the globe. Three regions within the conjunctiva are recognized: the palpebral or tarsal region, which lines the inner surface of the lids; the fornical region, which lines the upper and lower surfaces of the recess or cul-de-sac known as the fornix; and the bulbar region, which lines the surface of the sclera between the fornix and the limbus. The conjunctiva has two structural components throughout all regions: the surface epithelium and the substantia propria.

1.1.3.1 The Conjunctival Arterial Supply

Most of the arterial channels are arterioles, which terminate in a complex subconjunctival capillary network. The arterial supply of the conjunctiva is from the peripheral tarsal arcades, the marginal tarsal arcades, the anterior ciliary arteries, and the deep ciliary system. The peripheral perforating branches of the peripheral tarsal arcade pass above the tarsal plate, pierce the palpebral muscle, and divide into ascending and descending conjunctival branches.

The descending branches supply the proximal two-thirds of the tarsal conjunctiva, anastomosing with the shorter branches of the marginal