Anatomy of Dry Eyes

Tears: Humans produce three types of tears – basal or continuous tears which moisten the eye; reflex tears which spring into action when the eye is irritated by some foreign object and emotional tears. Tears are formed in the glands that surround the eyes.

Continuous tears do much more than water your eyes. Let’s chart the course of this marvelous fluid as it is produced, spread, and expelled through the lacrimal system.

The main tear gland is found in the depression just above the outer corner of your eye. This spongy gland, along with 60 others, creates a precision film made up of three layers—mucus, aqueous, and oil.

The inner layer, the mucous, makes a smooth surface so the lid glides across the exposed eyeball. The aqueous layer is the thickest of the three, containing many important ingredients including oxygen, vital to the cornea. Also add a dose of lysozyme and 11 other enzymes found in tears. Lysozyme is a bacteria fighter par excellence. It keeps the eye white and clear.

The finishing touches on this tear will be supplied by 30 Meibomian glands, those little yellow dots lining both lids in single file behind the lashes. The glands secrete the oil layer, so thin that it doesn’t distort your vision, yet keeps the tear film from evaporating and causing uncomfortable dry spots on the eye between blinks. In fact, some people have an inadequate supply of oil, and their tears evaporate much faster than normal.

When the lid sweeps down over the eye it draws out just the right blend of ingredients, and spreads them evenly across the eye in three layers. The lids meet perfectly so that the entire surface of the exposed eye is bathed in this soothing wash.

What happens to the used tears? A close look at your eye will show a tiny hole in the inner corner, the punctum, that drains the excess tears into a channel leading to the tear sac. From there the tears pass down the back of the nose and throat, where the tears are absorbed by the mucous membranes. Blinking causes the tear sac to act like a pump, which propels the tears into the canal and downward.

Eye Anatomy: The eyeball, or “globe,” is round, except at the front, where it has a bulge. This bulge contains the light-gathering apparatus of the eye. The ‘skin’ of the entire eyeball is opaque [nontransparent] to light except at this bulge, where it is normally a beautifully clear and round window called the cornea.

Behind the cornea is the colored iris, with a hole, or pupil, at its center. The iris automatically increases or decreases the size of the pupil to control the amount of light entering the eye.

Just behind the iris is the crystalline lens. This works together with the cornea to focus light at the rear of the eyeball, where it is converted into electrical impulses that are transmitted to the visual center of the brain. It is the brain, not the eyes, that actually does the “seeing.”

Back of the lens the eyeball is filled with vitreous humor. This is a transparent jellylike substance made up mostly of water, with a tiny percentage of solids.

The “skin” of the eyeball consists of three layers. The outermost layer is the sclera. It is tough, fibrous and opaque over most of the eye, preventing light from entering. At the front, however, the sclera becomes the transparent cornea.

The middle layer of this skin is highly complicated. At the front of the eyeball it separates into other structures, including the iris. However, over four fifths of the eyeball it forms an essentially continuous layer called the choroid.

The third or innermost layer of the eyeball’s three-ply skin is the retina. The retina is a paper-thin membrane that gives the light-images entering the eye the shape, color and texture that the brain perceives. Though “paper thin,” the retina consists of many distinct layers. It, consists of three main layers of cells: (1) nerve cells toward the central cavity, (2) light-sensitive cells in the middle, and (3) pigment-containing cells toward the outside near the choroid.”

The light-sensitive cells in the retina number many millions. Each eye contains some 130 million rods that respond to dim light and transmit only shades of gray; the 7 million cones, concentrated largely at the center of the retina in the fovea, react to bright light and are responsible for color vision.

Tiny nerve fibers extend from the rods and cones in all parts of the eye. These come together at the rear of the eyeball to make up the optic nerve, which connects with the brain.