STRUCTURE OF IRIS
The iris is a thin, circular structure located anterior to the lens, often compared with a diaphragm of an optical system. The center aperture, the pupil, actually is located slightly nasal and inferior to the iris center.
1 Pupil size regulates retinal illumination. The diameter can vary from 1 mm to 9 mm depending on lighting conditions. The pupil is very small (miotic) in brightly lit conditions and fairly large (mydriatic) in dim illumination.
The average diameter of the iris is 12 mm, and its thickness varies. It is thickest in the region of the collarette, a circular ridge approximately 1.5 mm from the pupillary margin.
This slightly raised jagged ridge was the attachment site for the fetal pupillary membrane during embryologic develop-ment.1,2 The collarette divides the iris into the pupillary zone, which encircles the pupil, and the ciliary zone,which extends from the collarette to the iris root. The color of these two zones often differs.
The pupillary margin of the iris rests on the anterior surface of the lens and, in profile, the iris has a truncated cone shape such that the pupillary margin lies anterior to its peripheral termination, the iris root.
The root, approximately 0.5 mm thick, is the thinnest part of the iris and joins the iris to the anterior aspect of the ciliary body .
1 The iris divides the anterior seg-ment of the globe into anterior and posterior chambers, and the pupil allows the aqueous humor to flow from the posterior into the anterior chamber with no resistance
OPTOMETRY-SHARP VISION
Optometrist