The opacity of the sclera, like that of other dense connective tissues, is primarily attributable to the irregularity of its structure. The sclera is pierced by blood vessels, nerves, and the optic nerve (see Fig. 24.2). It is 1 mm thick posteriorly, 0.3 to 0.4 mm thick at its equator, and 0.7 mm thick at the corneoscleral margin or “limbus.” The sclera is divided into three rather ill-defined layers:  The episcleral layer (episclera), the external layer, is the loose connective tissue adjacent to the periorbital fat.
 The substantia propria (sclera proper, also called
Tenon’s capsule), is the investing fascia of the eye and is composed of a dense network of thick collagen fibers.
 The suprachoroid lamina (lamina fusca), the inner aspect of the sclera, is located adjacent to the choroid and contains thinner collagen fibers and elastic fibers as well as fibroblasts, melanocytes, macrophages, and other connective tissue cells.
In addition, the episcleral space (Tenon’s space) is located between the episcleral layer and substantia propria of the sclera. This space and the surrounding periorbital fat allow the eye to rotate freely within the orbit. The tendons of the extraocular muscles attach to the substantia propria of the sclera.
The corneoscleral limbus is the transitional zone between the cornea and sclera.
At the junction of the cornea and sclera (Fig. 24.6 and Plate 107, page 926), Bowman’s membrane ends abruptly. The overlying epithelium at this site thickens from the 5 cell layers of the cornea to the 10 to 12 cell layers of the conjunctiva. The surface of the limbus is composed of two distinct types of epithelial cells: one type constitutes the conjunctival cells and the other corneal epithelial cells.
At this junction, the corneal lamellae become less regular as they merge with the oblique bundles of collagen fibers of the sclera. An abrupt transition from the avascular cornea to the well-vascularized sclera also occurs here.
The limbus region, specifically, the iridocorneal angle, contains the apparatus for the outflow of aqueous humor (Fig. 24.7). In the stromal layer, endothelium-lined channels called the trabecular meshwork (or spaces of Fontana) merge to form the scleral venous sinus (canal of Schlemm). This sinus encircles the eye (see Figs. 24.6 and 24.7). The aqueous humor is produced by the ciliary processes that border the lens in the posterior chamber of the eye. The fluid passes from the posterior chamber into the anterior chamber through the valvelike potential opening between the iris and lens. The fluid then passes through the openings in the trabecular meshwork in the limbus region as it continues its course to enter the scleral venous sinus. Collecting vessels in the sclera, called aqueous veins because they convey aqueous humor instead of blood, transport the aqueous humor to (blood) veins located in the sclera. Changes in the iridocorneal angle may lead to blockage in the drainage of aqueous humor causing glaucoma (see Folder 24.1). The iridocorneal angle can be visualized during eye examination using a gonioscope, a specialized optical device that utilizes mirrors or prisms to refect the light from the iridocorneal angle into the direction of the observer. In conjunction with a slit lamp or operating microscope, the ophthalmologist can examine this region to monitor various eye conditions associated with glaucoma.
The iris, the most anterior part of the vascular coat, forms a contractile diaphragm in front of the lens.