Something You Might Want to Know About the Iris

The human iris ranges in size from 11-13 mm.

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At the limbus, the transitional tissue where the transparent cornea meets the white opaque sclera, the degree of clarity of the cornea determines how much of this diameter is visible during viewing. The visible iris is somewhat ovoid, with the top and bottom partially covered by the limbus, even though the anatomical iris is round. This ovoid appearance is more pronounced at the bottom of the cornea and in older eyes (Warwick, 1976). This “arcus senilis” is an opaque, grayish ring at the periphery of the cornea. It is important for artists to remember that the limbus and eyelid cast shadows on the iris. Anatomically speaking, the iris is generally conical in shape. It is pushed slightly forward by the lens that surrounds its central portion. Illustrations that show this feature floating independently illustrate how misunderstood it is. An artificial iris is typically given a conical shape by using the optic properties of the prosthetic cornea. 

 

As a result, the way light strikes the surface of the iris is affected. Illustrations depict light from the upper left; thus, a painting or drawing of an eye will focus more on the upper right iris in light. During biomicroscopy, the iris best displays its three-dimensionality at a magnification of 40X (Daughman, 1999). 

 

At the collarette, the skin is thickest, while the pupillary margin and iris root are thinnest. The iris' radial striations result from the contraction of the sphincter muscle in the pupillary portion, whereas the circumferential folds in its peripheral portion are a result of the action of the dilator muscle. There are no continuous or perfect circles in these peripheral folds.


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