The anterior segment is that part of the eye anterior to the ora serrata, the most anterior extension of the neural retina, and includes the anterior and posterior chambers and the structures that define them. These include the cornea and sclera, the iris, the lens, the ciliary body, and the connections between the basal lamina of the ciliary processes and the lens capsule (thick basal lamina of the lens epithelium) that form the suspensory ligament of the lens, the zonular fbers. The posterior chamber is bounded posteriorly by the anterior surface of the lens and anteriorly by the posterior surface of the iris. The ciliary body forms the lateral boundary. Aqueous humor flows through the pupil into the anterior chamber, which occupies the space between the cornea and the iris, and drains into the canal of Schlemm.
Anterior segment, eye, human, H&E 45; inset 75.
A portion of the anterior segment of the eye, shown in this figure, includes parts of the cornea (C ), sclera (S), iris (I ), ciliary body (CB), anterior chamber (AC ), posterior chamber (PC ), lens (L), and zonular fibers (ZF ).
The relationship of the cornea to the sclera is illustrated to advantage here. The junction between the two (arrows) is marked by a 924 change in staining, with the substance of the cornea appearing lighter than that of the sclera. The corneal epithelium (CEp) is continuous with the conjunctival epithelium (CjEp) that covers the sclera. Note that the epithelium thickens considerably at the corneoscleral junction and resembles that of the oral mucosa. The conjunctival epithelium is separated from the dense fibrous component of the sclera by a loose vascular connective tissue. Together, this connective tissue and the epithelium constitute the conjunctiva (Cj). The epithelial-connective tissue junction of the conjunctiva is irregular; in contrast, the undersurface of the corneal epithelium presents an even profile.
Just lateral to the junction of the cornea and sclera is the canal of Schlemm (CS; see also figure below). This canal takes a circular route about the perimeter of the cornea. It communicates with the anterior chamber through a loose trabecular meshwork of tissue, the spaces of Fontana. The canal of Schlemm also communicates with episcleral veins. By means of its communications, the canal of Schlemm provides a route for the fluid in the anterior and posterior chambers to reach the bloodstream.
The inset shows the tip of the iris. Note the heavy pigmentation on the posterior surface of the iris, which is covered by the same double-layered epithelium as the ciliary body and ciliary processes. In the ciliary epithelium, the outer layer is pigmented and the inner layer is nonpigmented. In the iris, both layers of the iridial epithelium (IEp) are heavily pigmented. A portion of the iridial constrictor muscle (M ) is seen beneath the epithelium.
PLATE 106 • EYE III: ANTERIOR SEGMENT
Anterior segment, eye, human, H&E 90; inset 350.