Eye Care Blog

In order to fully understand how a contact lens works, you should first have a basic knowledge and appreciation of the structure and workings of your eyes.

Anatomy of the Eye

The eye is often compared to a camera. There are many similarities, but the eye is actually much more complex and miraculous: it focuses and adjusts to light automatically, and the film never runs out. Moreover it picks up a constant stream of images, in contrast to the single image that registers on each frame of film; it is self-cleaning and “develops” film instantly —all in the space of a one-inch-diameter globe!

When you “see” something, your eyes actually pick up the light rays that are bouncing off that object. Whether you’re reading a comic book, watching the ballet, or admiring a distant landscape, here’s what happens: As the light rays enter your eye, they pass through and are refracted (bent) by the:

Cornea, the clear, dome-shaped tissue that covers the inner parts of the eye as a watch crystal covers the face of a watch, and which acts as the “window” of the eye. The cornea is in part responsible for focusing the light rays; unlike the crystalline lens, the shape and focusing power remain constant. The cornea, upon which the contact lens floats over a layer of tears, is the most sensitive part of the eye, since it has the highest density of nerve endings per square millimeter. Light rays then pass through the:

Pupil, the small hole that appears black at the center of the:

Iris, the blue-, green-, or brown-colored part of the eye. The pupil opens up (dilates) or closes down (constricts) automatically to adjust to the amount of light, like the shutter of a camera. The rays of light continue their journey through the:

Crystalline lens, which is clear and flexible and changes shape via muscle contraction and relaxation, depending upon the distance of the object from the eye. (This is called accommodation.) As the light is being focused by the lens, it passes through the:

Vitreous humor, the clear jellylike fluid that fills the inside of the eyeball and gives it its shape. It finally ends up focusing (or being out of focus, if your eye has a refractive error) on the:

Retina, or “film.” This thin, delicate sheet of light-sensitive tissue lining the back of the eye is connected to the:

Optic nerve, which transmits the image in the form of a chemical-electrical impulse to the brain for decoding and interpretation.

Some parts of the eye do not relate directly to the light rays that provide the visual image, but are important for the contact lens wearer.

The conjunctiva is a thin, clear tissue that covers the white part of the eye (sclera) and undersides of the lids. This continuous sheet forms the anatomical barrier that prevents a contact lens from becoming “lost” behind the eye.

Tears play a most important part in making contact lens wear a reality. There are actually three layers to the tears. The outermost layer is lipid, the product of the oil-producing Meibomian glands of the eyelids. It functions to retard evaporation of the middle layer of tears, the aqueous layer. This portion accounts for 40 percent of the tears. It is produced by the lachrymal glands, the main tear-producing glands of the eye. The aqueous layer contains dissolved minerals (sodium, chloride, potassium), protein, glucose, and the all-important immunoglobulins and lysozyme (the eye’s natural antibodies and enzymes), which protect the eye from harmful bacteria and fungi. The innermost layer which is closest to the eye is the mucin layer. It is produced by special glands (goblet cells) inthe conjunctiva. Mucin helps spread the tears evenly over the cornea.

When a contact lens is worn, it floats over a thin layer of tears, which cushions and contributes to the focusing power of the lens. Perhaps most important to the contact lens wearer is the tears‘ role in carrying a large portion of the oxygen to thecornea and carrying away carbon dioxide, the prime metabolic waste product.

The cornea itself has no blood vessels and so must rely on other sources—mainly the air and the tears—to obtain what the blood normally supplies to parts of the body. Since contact lenses diminish the amount of air reaching the cornea, the tears assume more of this responsibility in the contact lens wearer. Also, the aqueous layer cools the eye as it evaporates.

Some eyes may be deficient in tear volume or composition, causing severe problems in contact lens wear. For example, if there is too little of the aqueous layer, the gas exchange will be too far below normal; or because of a faulty supply of mucin,the tears, though sufficient, may not spread evenly over the cornea.

A perfect illustration of this problem is the young woman referred to me because she suffered from red, itchy eyes every time she attempted to wear contact lenses. At first an allergic reaction to the lens solutions was a possibility; so was an infection. However, after probing into her medical history it was determined that the birth control pills she was taking caused a deficiency in tear production. Further examination proved indeed that her eyes were very dry. She decided that contact lens wear was important enough for her to switch to another method of birth control. Soon after the changeover, the eyesbecame less dry and contact lens wear became comfortable.

The eyelids, to continue the camera metaphor, could be compared to a lens cap that protects from dust and scratches. Eyelids do that and more: through the blinking mechanism, they spread tears evenly over the cornea, enabling them to do their job of cleaning, lubricating, and pumping new tears to the cornea, thus enriching the oxygen supply and removingthe carbon dioxide.

Eyelashes (cilia) add further protection by acting as a barrier that prevents larger airborne particles from reaching thesurface of the eye.

What is 20 / 20 Vision?

You’ve probably heard that people who see “normally” have “20 / 20 vision.” But what exactly does that familiar label refer to? It’s simply a shorthand way of indicating how sharply your eyes can focus on a certain size letter at a distance of 20 feet. The first 20 refers to the distance at which the test is being given; the second 20 refers to the distance at which you should be able to clearly see letters of that size. So if you have 20 / 20 vision, you can see the letters in a specified line on the eye chart at 20 feet; if you have 20 / 40 vision, you can see clearly at 20 feet what someone with normal vision would see at 40 feet; and if you’re 20 / 150, you see at 20 feet what you should be able to see 150 feet away. (There are some rare cases of 20 / 10 vision, which is better than average; they can read movie marquees and road signs before anyone else.)

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