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One Miraculous Organ
By Frances Maguire Paist, Staff Writer

(Page 2 of 3)

The iris is the colored membrane of the eye, located between the cornea and the lens. It controls light levels inside the eye and because of microscopic pigment cells called melanin, varies in color from pale blue to dark brown. Flat in perspective, it divides the front (or anterior) chamber of the eye from the back (or posterior) chamber, and its color, textures and patterns are as unique to each individual as a fingerprint.

Directly in the middle of the iris is the pupil, the round black hole that changes size with the help of tiny muscles that control the amount of light entering the eye. The sphincter muscle, around the very edge of the pupil, contracts in bright light to constrict the pupil. The dilator muscle runs radially throughout the pupil to widen the eye in dim lighting.

The ciliary body lies just behind the iris, and tiny fiber guy wires called zonules are attached to it. One of its functions is the production of aqueous humor, the clear liquid that fills the space between the cornea and the iris. Aqueous humor nourishes the cornea and the lens and gives the front of the eye its shape. The ciliary body also helps change the shape of the lens. When it contracts, the zonules relax, the lens thickens and close-up vision is made possible. When it relaxes, the zonules contract, the lens becomes thinner and distance vision is made possible. 

Vitreous is a thick, transparent substance that fills the center of the eye. It is composed of water and is two-thirds of the eye’s volume, providing its form and shape. Firmly attached to certain areas of the retina, vitreous has the consistency of egg-white in children but thins as we age. As it thins, it can separate from the retina, and this causes the floaters with which older adults are familiar.

The choroids lie between the retina and the sclera and are composed of layers of blood vessels that nourish the back of the eye. They connect with the ciliary body towards the front of the eye and the edges of the optic nerve at the back of the eye. 

The lens is transparent and convex on both sides. Located behind the iris, it focuses light rays entering through the pupil to form an image on the retina. The nucleus (or innermost part of the lens) is surrounded by soft material called the cortex. The lens is encased in a capsular-like bag and suspended within the eye by zonules (those tiny guy wires mentioned earlier). In young people, the lens changes shape to accommodate for near or far vision, but by older adulthood, it is often no longer capable of the same flexibility and vision correction becomes necessary.

The retina is a thin, multilayered membrane lining the back two-thirds of the eye. Composed of millions of visual cells, it is connected by the optic nerve to the brain. The retina captures light rays that enter the eye and sends electrical impulses to the brain that result in sight. Rods and cones located in the retina function as photoreceptors. The approximately six million cones permit us to appreciate color, and the approximately 125 million rods function best in dim light and are responsible for peripheral and night vision.

The macula is an area of the eye near the center of the retina where visual perception is most acute. It is responsible for the critical focusing vision so necessary for seeing fine detail. One hundred times more sensitive to detail than the peripheral retina, it’s often called the “bull’s eye center” of the retina. The fovea is the most center portion of the macula.

The optic nerve, composed of thousands of nerve fibers that connect the macula and retina to the brain, carries electrical impulses to the processing center of the brain and converts them to sight. The visual portion of the optic nerve is called the optic disc and is connected to the back of the eye near the macula. 

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