Clinical note: Oxidative changes in the lens fibers are common as one ages and can lead to opacity of lens tissue called a cataract, which eventually produces blurred vision. In surgery for cataracts, such lenses are broken up and aspirated out through a slit in the upper cornea, leaving the zonule and thick posterior capsule of the lens in place to hold an implanted plastic lens. The two images to the right show what a cataract lens looks like and what a person sees with such a lens.

Examine the iris, noting the continuity with the ciliary body.

• Identify the heavily pigmented surface epithelial layers, the stroma, the constrictor muscle at the pupil, and the dilator muscle of the pupil (Fig. 23-11). Identify the anterior and posterior chambers meeting at the pupil.

What cells of the iris determine the color of one's eyes?

What substance moves through the pupil?

Why are the two muscular structures in the iris difficult to study histologically?

Now for the cornea.