The lens is a transparent biconvex disc suspended behind the iris by the ciliary zonules that extend from the ciliary body. Contraction and relaxation of the ciliary muscles changes the shape of the lens, allowing for visual accommodation (focus on near and far objects). The lens surface is surrounded by a proteoglycan and type IV collagen-rich lens capsule to which the ciliary zonules attach. Below the lens capsule on the anterior surface is the lens epithelium, a simple cuboidal epithelium. At the posterior edge near the lens equator, the lens epithelium proliferates and terminally differentiates, giving rise to lens fibers that lack organelles and are filled with crystallins. The tightly packed lens fibers provide for transparency of the lens.

• Examine the lens in the images below and this slide. Identify the capsule (LC) and simple cuboidal lens epithelium (LE), differentiating nucleated lens fibers (DLF), and mature non-nucleated lens fibers (MLF).

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.

• Next, examine the images at the right and the iris on this slide, noting the continuity with the ciliary body. Identify the heavily pigmented epithelium (PE), the stroma (S), the sphincter pupillae muscle (SPM, a circular bundle of smooth muscle near the pupil), and the dilator pupillae muscle (DPM, a very thin layer consisting of contractile processes of myoepithelial cells). Identify the anterior (AC) and posterior (PC) chambers, meeting at the pupil.
   

Now for the cornea.