Here is a brief tour of the human eye, from front to back:
Lids and Lashes
Our eyelids protect our eyes from the environment and keep them moist by spreading tears across the front surface of the eye when we blink. A thin, slippery, and transparent lining called the conjunctiva covers the inner surface of the eyelids. The conjunctiva folds back onto the eye at the back of the eyelid. The continuous nature of the conjunctiva from the inner surface of the eyelids to the outer surface of the eye makes it impossible for a contact lens or other material to become lost or trapped behind the eye.
Our eyelashes also have a protective function, and help to keep dust and debris away from the eye. Glands at the base of the eyelashes secrete oils into the tear film. These oils help the eye stay moist by slowing the evaporation of tears.
Cornea and Sclera
The cornea is the clear central portion of the front of the eye. It is located directly in front of the pupil and the iris (the colored part of the eye that surrounds the pupil). About 75 percent of the focusing ability of the eye is due to the curvature of the cornea.
The sclera is the tough outer coating of the eye that surrounds the cornea. The conjunctiva covers the sclera, but not the cornea. Tiny blood vessels lie between the transparent conjunctiva and the white sclera. When our eyes are dry or irritated, these blood vessels dilate, causing the eye to appear reddened. Eye drops that “get the red out” work by causing these small blood vessels to constrict so they become less visible.
Image courtesy of National Eye Institute, National Institutes of Health
Pupil and Iris
The pupil is the opening through which light passes into the eye. The iris is the pigmented tissue that surrounds the pupil and determines the color of our eyes. Brown eyes are highly pigmented; blue eyes have very little pigment. The distribution of pigment in the iris varies widely from person to person, causing an almost limitless variety of eye colors and patterns.
The pupil controls how much light enters the eye. Two muscles within the iris determine the size of the pupil. Contraction of one muscle makes the pupil larger; contraction of the other makes it smaller. Control of the size of the pupil is completely automatic. In dark conditions, the pupil dilates (gets larger) to allow more light into the eye. In bright sunlight, the pupil constricts (gets smaller) to protect the retina from too much light.
The space between the iris and the back of the cornea is called the anterior chamber. It’s filled with a clear, watery fluid called aqueous. A structure behind the iris called the ciliary body produces the aqueous fluid constantly. The aqueous passes through the pupil into the anterior chamber, where it drains from the eye through small openings in the chamber at the angle where the iris and cornea meet. If the aqueous cannot drain properly, pressure inside the eye increases. This increased pressure can lead to glaucoma.
Image courtesy of National Eye Institute, National Institutes of Health
The crystalline lens is located directly behind the pupil. It’s about the same size and shape as an M&M candy and is suspended from the ciliary body by thin fibers called zonules.
The crystalline lens works like a variable-focus camera lens. When we look at distant objects, the lens is held in its normal, somewhat flattened shape by tension in the zonules that connect it to the ciliary body. When we want to look at close objects, a circular muscle in the ciliary body contracts, relaxing the normal tension in the zonules. With less tension pulling it outward, the crystalline lens becomes thicker and more curved. This change in shape of the crystalline lens adds focusing power to the eye, allowing it to bring near objects into clear focus. The process is called accomodation.
Accommodative ability decreases with age. The ciliary muscle continues to operate normally, but the crystalline lens becomes harder and less elastic as we get older. Most of us notice that between ages 40 and 45, small print becomes increasingly more difficult to read. This normal, age-related change is called presbyopia (literally, “aging eye”). Presbyopia is easily treated with reading glasses, progressive or bifocal eyeglass lenses, and monovision or bifocal contact lenses.
Clouding of the normally transparent crystalline lens is called a cataract. Though the exact cause of the most common type of cataracts is unknown, they are associated with aging. Cataracts can also be caused by injuries to the eye, general health problems, or long-term use of certain medications. In cataract surgery, the cloudy crystalline lens is removed and replaced with a clear plastic lens implant.
In addition to eliminating blurred vision due to a cloudy crystalline lens, cataract surgery can eliminate nearsightedness, farsightedness and astigmatism, decreasing or eliminating your need for eyeglasses or contact lenses. New lens implants are available that can even restore some of your ability to focus on near objects, reducing the need for reading glasses.
The large space inside the eye between the crystalline lens and the retina is filled with a transparent material called the vitreous. Early in life, the gel-like vitreous resembles Jell-O (R) in texture. Suspended in the vitreous is a meshwork of fine fibers of a material called collagen. This collagen meshwork is thicker at the back of the eye, where it is referred to as the posterior vitreous membrane (PVM). The PVM attaches the vitreous to the retina and the optic nerve.
As we age, the vitreous becomes more watery. This causes the collagen fibers to move more freely and clump together. When this occurs, the fibers are sometimes seen as “floaters” – small, dark, irregularly-shaped images that drift in front of our eyes.
With time, the posterior vitreous membrane may pull away from its attachment to the retina. This pulling can cause flashes of light and a significant increase in floaters. In some cases, it can cause a detachment of the retina and loss of vision.
Note: If you experience a rapid onset of flashes, floaters or both, see you eye doctor immediately. A detached retina is a medical emergency that needs immediate attention.
The retina is the delicate light-sensitive inner lining of the back of the eye. The retina transforms light (that has been focused by the cornea and crystalline lens) into electrical impulses. These impulses are then transmitted to the brain by the optic nerve to create our sense of sight.
Cells in the retina that convert light into electrical energy are called photoreceptors. There are two kinds of photoreceptor cells – rods and cones. Rods are primarily responsible for our peripheral vision. Cones are concentrated at the most sensitive part of the retina called the macula and are responsible for our central vision and color perception.
A branching network of blood vessels rests on the surface of the retina. Your eye doctor evaluates the condition of these vessels during your eye exam. The appearance of blood vessels on the retina is a good indicator of the condition of blood vessels throughout the body. Therefore, a routine eye exam is an important part of your general health care. Your eye doctor may be the first health care provider to recognize early signs of arteriosclerosis, hypertension, diabetes and other disorders.