of melanocytes, and the size and density of melanin granules within the melanocytes.6 Studies in which melanocyte counts have been done between irises of various colors and from different races have shown that the number of melanocytes is fairly constant.7,8 Color seems to be determined by the number of melanin granules within the melanocytes and the area they occupy.6 The type of melanin present and the arrangement of the connective tissue components can also affect the transmission and reflection of light contributing to iris color.9 An iris appears blue for the same reason that the sky is blue; the wavelength seen results from light scatter caused by the arrangement and density of the connective tissue components. Other iris colors are caused by the amount of light absorption, which depends on the pigment density within the melanocytes. If the iris is heavily pigmented, the anterior surface appears brown and smooth, even velvety, whereas in a lighter iris, the collagen trabeculae are evident and the color ranges from grays to blues to greens depending on the density of pigment and collagen. A freckle or a nevus is an area of hyperpigmentation, an accumulation of melanocytes, and frequently is seen in the anterior border layer. In all colored irises, the two epithelial layers are heavily pigmented. Only in the albino iris do the epithelial layers lack pigment.3

Clinical Comment: Pigmentary

Dispersion­ Syndrome

IN PIGMENTARY DISPERSION SYNDROME  pigment granules are shed from the posterior iris surface and are dispersed into the anterior chamber. They can be deposited on the iris, lens, or corneal endothelium or in

the trabecular meshwork, where they might compromise aqueous outflow.4 Significant pigment loss will be evident on transillumination of the iris when the red fundus reflex shows through in the depigmented areas (Figure 3-10).

Clinical Comment: Heterochromia

HETEROCHROMIA  of the iris is a condition in which one iris differs in color from the other or portions of one iris differ in color from the rest of the iris. This can be congenital or

a sign of uveal inflammation. If congenital, a disruption of the sympathetic innervation may be suspected.4,10 A history regarding iris coloration should be elicited.

C I L I A R Y B O D Y

When viewed from the front of the eye, the ciliary body is a ring-shaped structure. Its width is approximately 5.9 mm on the nasal side and 6.7 mm on the temporal side.2 The posterior area of the ciliary body, which terminates at the ora serrata, appears fairly flat, but the anterior

FIGURE 3-10

Retro-illumination of the iris showing defects in the pigmented epithelial layers of the iris. (From Spalton DJ, Hitchings RA, Hunter PA, eds. Atlas of clinical ophthalmology, London, 1984, JB Lippincott)

ciliary body contains numerous folds or processes that extend into the posterior chamber. In sagittal section, the ciliary body has a triangular shape, the base of which is located anteriorly; one corner of the base lies at the scleral spur, the iris root extends from the approximate center of the base, and portions of the base border both the anterior and posterior chambers. The outer side of the triangle lies against the sclera, and the inner side lines the posterior chamber and a small portion of the vitreous cavity (Figure 3-11). The apex is located at the ora serrata.

The ciliary body can be divided into two parts: the pars plicata (corona ciliaris) and the pars plana (orbicularis ciliaris). The pars plicata is the wider, anterior portion containing the ciliary processes (see Figure 3-10). Approximately 70 to 80 ciliary processes extend into the posterior chamber, and the regions between them are called valleys of Kuhnt. A ciliary process measures approximately 2 mm in length, 0.5 mm in width, and 1 mm in height, but there are significant variations in all measurements.1

The pars plana is the flatter region of the ciliary body. It extends from the posterior of the pars plicata to the ora serrata, which is the transition between ciliary body and choroid. The ora serrata has a serrated pattern, the forward-pointing apices of which are called teeth or dentate processes. The rounded portions that lie between the teeth are called oral bays (Figure 3-12, A). The dentate processes are elongations of retinal tissue into the region of the pars plana.

The zonule fibers course from the ciliary body to the lens. Some of these fibers insert into the internal limiting membrane of the pars plana region and travel forward through the valleys between the ciliary processes. Some attach to the internal limiting membrane of the valleys of the pars plicata (Figure 3-12, B). The ciliary body is

Partitions and layers of ciliary body.

attached to the vitreous base, which extends forward approximately 2 mm over the posterior pars plana.1

SUPRACILIARIS (SUPRACILIARY LAMINA)

The supraciliaris is the outermost layer of the ciliary body adjacent to the sclera. Its loose connective tissue is arranged in ribbonlike layers containing pigmented melanocytes, fibroblasts, and collagen bands1 (Figure 3-13). The arrangement of these bands allows the ciliary body to slide against the sclera without detaching from or stretching the tissue. The arrangement of the supraciliaris allows for the accumulation of fluid within its spaces, which may cause a displacement of the ciliary body from the sclera. Damage to the layer caused by trauma may result in a ciliary body detachment.

CILIARY MUSCLE

The ciliary muscle is composed of smooth muscle fibers oriented in longitudinal, radial, and circular directions. Interweaving occurs between fiber bundles and from layer to layer, such that various amounts of connective tissue are found among the muscle bundles.1 The longitudinal muscle fibers (of Brücke) lie adjacent to the supraciliaris and parallel to the sclera. Each muscle bundle resembles

a long narrow V, the base of which is at the scleral spur, whereas the apex is in the choroid. The tendon of origin attaches the muscle fibers to the scleral spur and to adjacent trabecular meshwork sheets. The insertion of the longitudinal ciliary muscle is in the anterior one third of the choroid in the form of stellate-shaped terminations or “muscle stars.”1,2 Inner to the longitudinal muscle fibers, the radial fibers form wider, shorter interdigitating Vs that originate at the scleral spur and insert into the connective tissue near the base of the ciliary processes.1 This layer is a transition from the longitudinally oriented fibers to the circular fibers.

The innermost region of ciliary muscle, (Müller’s) annular muscle, is formed of circular muscle bundles with a sphincter type of action. These fibers are located near the major circle of the iris. Figure 3-14 shows the relationship between these regions of the ciliary muscle and surrounding structures.

The ciliary muscle is dually innervated by the autonomic nervous system. Parasympathetic stimulation activates the muscle for contraction, whereas sympathetic innervation likely has an inhibitory effect that is a function of the level of parasympathetic activity.11-13

CILIARY STROMA

The highly vascularized, loose connective tissue stroma of the ciliary body lies between the muscle and the epithelial layers and forms the core of each of the ciliary

A

a

b

FIGURE 3-12

A, Inner aspect of the ciliary body shows pars plicata (a) and pars plana (b). Ora serrata is at c, and posterior to it, retina exhibits cystoid degeneration (d). Bays (e) and dentate processes (f) of ora are shown; linear ridges or striae (g) project forward from dentate processes across pars plana to enter valleys between ciliary processes. Zonular fibers arise from pars plana beginning 1.5 mm from ora serrata. These fibers curve forward from sides of dentate ridges into ciliary valleys, then from valleys to lens capsule. Zonules coming from the valleys on either side of a ciliary process have a common point of attachment on lens. Zonules attach up to 1 mm from the equator posteriorly and up to 1.5 mm from the equator anteriorly. At equatorial border, attaching zonules give a crenated appearance to lens. Ciliary processes vary in size and shape and often are separated from one another by lesser processes. Radial furrows (h) and circular furrows (i) of peripheral iris are shown. B, Anterior view of the ciliary processes showing zonules attaching to lens. Zonules form columns (a) on either side of ciliary processes (b), which meet on a single site (c) as they attach to lens. These two columns form a triangle, the base of which is on ciliary body and apex of which is on lens. Zonules form tentlike structure (d) as they become attached to lens capsule. Equatorial surface of lens is crenated (e) by attachment of zonule. Iris is pulled upward, revealing its posterior surface containing radial folds (f) and circular furrows (g). (From Hogan MJ, Alvarado JA, Weddell JE: Histology of the human eye, Philadelphia, 1971, Saunders.)

processes; it is continuous with the connective tissue that separates the bundles of ciliary muscle. Anteriorly, the stroma is continuous with iris stroma. It thins in the pars plana, where it continues posteriorly as choroidal stroma. The major arterial circle of the iris is located in the ciliary stroma anterior to the circular

muscle and near the iris root. This circular artery is formed by the anastomosis of the long posterior ciliary arteries and the anterior ciliary arteries. The stromal capillaries are large and fenestrated, particularly in the ciliary processes, and most are located near the pigmented­ epithelium.1

FIGURE 3-14

1 3

2