Science to Live By: Our Senses (Part Two): Sight


© J. Dirk Nies, Ph.D.

Part of the eye
Part of the eye

British musket balls were screaming toward his ill-supplied troops on the morning of June 17, 1775 when Colonel Prescott commanded his American patriots, in legendary words remembered to this day: “Don’t fire until I tell you! Don’t fire till you see the whites of their eyes!”

The human eye is a most remarkable organ. Our eyes are very particular about what “light” they see, responding only to a very narrow band of the electromagnetic spectrum. Radiation with wavelengths shorter than blue light—ultraviolet and x-rays—or longer than red light —infrared, microwave and radio waves – is invisible to our eyes. It is worthy of note that the limited range of radiation we can see is centered smack dab in that very region of the spectrum the sun shines energy most intensely upon the earth!

Possessing the simple shape of a sphere and built like a balloon filled with clear liquid, the human eye is astonishingly complex with dozens of different parts working together in harmony to afford us the gift of vision.

Five parts of the eye are visible when looking in the mirror: the cornea, iris, pupil, sclera and conjunctiva. The cornea is the convex, transparent covering over the front of the eye. Located behind the cornea is the iris, a flat, colored, adjustable, ring-shaped membrane. The pupil is the black circular opening at the center of the iris. The sclera is the white of the eye; the tough, fibrous tissue enveloping the eyeball (except the cornea). And the conjunctiva is the thin, clear, mucous membrane covering the sclera.

The most colorful of these five structures is named after the Greek goddess Iris, the wing-footed messenger who personified the rainbow.

The variety of human eye color is completely unexpected given that the pigment present in the iris is melanin, the same brown material that colors skin and hair. Eyes ranging from light brown to black derive their color from the level of melanin present in the iris; the more melanin, the darker the color.

Blue Eyes
Blue Eyes

But unlike our skin or hair, the iris takes on hues ranging from blue, to green, to hazel when little or no melanin is present. These colors arise not from pigmentation, but via a light scattering phenomenon similar to the process that imparts blueness to the sky. Do you recognize whose azure eyes these are? Yes, it’s Ol’ Blue Eyes, Frank Sinatra.

The pupil, which widens and constricts to control the amount of light entering the eye, is named from the Latin pupilla, which means “little doll.” When the Romans looked into another’s eyes, they saw a puppetlike reflection of themselves. The old Hebrew expression for pupil is similar: eshon ayin, which means “little man of the eye.”

Another vital part of the eye is the lens. Found just behind the iris and pupil, the lens focuses light on the back of the eye. Cataracts that cloud the lens are the most common cause of blindness.

Two clear liquids are found within the interior of the eye. The aqueous humor, a transparent gelatinous fluid, fills the small space between the cornea and the lens. Behind the lens, the eye is filled with vitreous humor, a clear gelatinous substance. Chronically elevated fluid pressure within the eye can lead to glaucoma, the second-leading cause of blindness after cataracts.

The retina, the inside lining of the eye, is covered with two varieties of vision-empowering cells; rods and cones. Rod cells provide us with black-and-white vision in low light. Cone cells are responsible for color vision. Rod and cone cells contain photo-reactive pigments such as rhodopsin that change structure when exposed to light. These structural changes generate electrical impulses that travel down retinal nerve fibers. These fibers gather together at the back of the eye to form the optic nerve, which conducts these electrical impulses to the brain.

The place where the optic nerve exits the retina is called the optic disk. There are no rods or cones at this location, creating a blind spot. We are unaware of this visual deficiency because each eye compensates for the blind spot of the other eye.

Color blindness, the decreased ability or inability to see color or perceive color differences, most commonly arises from improper development of retinal cones. Diabetic retinopathy, the most common diabetic eye disease and a leading cause of blindness in American adults, is caused by adverse changes in the blood vessels of the retina.

The macula is a special area of the retina. Located at the back of the eye, the macula provides us with central vision. Within the macula is a tiny depression densely covered with cone cells called the fovea. It is the fovea that gives us the ability to see objects in sharp detail. Age-related macular degeneration (AMD) is a progressive eye condition that leads to significant impairment of one’s central vision, eventually leaving only peripheral vision intact and making it difficult or impossible to read, drive or recognizes faces. As many as 15 million Americans are living with AMD.

Here a few suggestions that can help keep your eyes healthy.

Protect your eyes. Prevent eye injury when playing sports or working with power tools by wearing safety glasses or appropriate face shields. On bright sunny days, wear UV protective sunglasses, not just darker lenses. Lenses that only darken will just make your pupils dilate allowing damaging UV rays to enter the eye. Also, don’t smoke. Smoking can double the risk of AMD.

Nourish your eyes. Eat dark colored and leafy vegetables. Carrots, sweet potatoes and spinach are loaded with beta-carotene, a nutrient correlated with reducing the risk of macular degeneration. Consume flaxseed oil or fish, such as salmon and sardines, to supply omega-3 fatty acids that promote the health of blood vessels in the eyes.

Rest your eyes. Glare from computer screens and other electronic devices can cause muscle fatigue in the eyes. We blink less when we are attentively focused on a screen, causing our eyes to become dry. To combat dry eyes, make a conscious effort to blink a few times every minute.

Our eyes are our most treasured sense organ. Nearly three-fourths of our body’s sense receptors are dedicated to the eyes, facilitating vision and allowing us to process the enormous amount of information contained in the light waves emanating from the world around us. With our eyes, we perceive depth of field, we distinguish intensity of light, we discriminate between millions of colors, and we pick out the face of a dear friend in a crowd. Like the goddess Iris, our eyes convey messages from the heavens and the earth to our souls, allowing us, in the words of William Blake,

To see a World in a Grain of Sand

And a Heaven in a Wild Flower.