Examination of a histologic preparation does not clearly reveal the arrangement of the muscle fibers. Rather, the organizational grouping is based on microdissection techniques.
Ciliary processes are ridgelike extensions of the ciliary body from which zonular fibers emerge and extend to the lens.
Ciliary processes are thickenings of the inner vascular region of the ciliary body. They are continuous with the vascular layers of the choroid. Scattered macrophages containing melanin pigment granules and elastic fibers are present in these processes (Plate 106, page 924). The processes and the ciliary body are covered by a double layer of columnar epithelial cells, the ciliary epithelium, which was originally
FIGURE 24.8 • Structure of the iris. a. This schematic diagram shows the layers of the iris. Note that the pigmented epithelial cells are reflected as occurs at the pupillary margin of the iris. The two layers of pigmented epithelial cells are in contact with the dilator pupillae muscle. The incomplete layer of fibroblasts and stromal melanocytes is indicated on the anterior surface of the iris. b. Photomicrograph of the iris showing the histologic features of this structure. The lens, which lies posterior to the iris, is included for orientation. The iris is composed of a connective tissue stroma covered on its posterior surface by the posterior pigment epithelium. The basal lamina (not visible) faces the posterior chamber of the eye. Because of intense pigmentation, the histologic features of these cells are not discernible. Just anterior to these cells is the anterior pigment myoepithelium layer (the dashed line separates the two layers). Note that the posterior portion of the myoepithelial cells contains melanin, whereas the anterior portion contains contractile elements forming the dilator pupillae muscle of the iris. The sphincter pupillae muscle is evident in the stroma. The color of the iris depends on the number of stromal melanocytes scattered throughout the connective tissue stroma. At the bottom, note the presence of the lens. 570.
derived from the two layers of the optic cup. The ciliary epithelium has three principal functions:  Secretion of aqueous humor Participation in the blood–aqueous barrier (part of the blood–ocular barrier  Secretion and anchoring of the zonular fbers that form the suspensory ligament of the lens
The inner cell layer of the ciliary epithelium has a basal lamina facing the posterior and vitreous chambers. The cells in this layer are nonpigmented. The cell layer that has its basal lamina facing the connective tissue stroma of the ciliary body is heavily pigmented and is directly continuous with the pigmented epithelial layer of the retina. The double-layered ciliary epithelium continues over the iris, where it becomes the posterior pigmented epithelium and anterior pigmented myoepithelium. The zonular fibers extend from the basal lamina of the nonpigmented epithelial cells of the ciliary processes and insert into the lens capsule (the thickened basal lamina of the lens).
The cells of the nonpigmented layer have all the characteristics of a fluid-transporting epithelium, including complex cell-to-cell junctions with a well-developed zonular occludens, extensive lateral and basal plications, and localization of Na/K-ATPase in the lateral plasma membrane. In addition, they have an elaborate rER and Golgi complex consistent with their role in secreting the zonular fibers. The cells of the pigmented layer have a less developed junctional zone and often exhibit large, irregular lateral intercellular spaces. Both desmosomes and gap junctions hold together the apical surfaces of the two cell layers, creating discontinuous “luminal” spaces called ciliary channels.