The Wonder of Sight — Understanding How Eyes Work

Eyesight is our richest and most significant means of learning about the surrounding world. Eyes capture the shapes and colours of life and are responsible for about 75% of all that people perceive.

Eyes are designed to respond to light, which originates with the sun. When the eyes are open upon an illuminated world, they are able to sense about ten million different shades of colour. As extensions of the brain itself, the eyes gather up to a billion pieces of information each second they are open. All parts of an eye are created to fulfill their specific roles in the amazing process of capturing, focusing and 
processing light.

Cornea: The cornea is the clear front window of the eye, which allows light to enter the eye. In a normal cornea's tissue, the cells are arranged so neatly that they allow light to pass through without distortion. The cornea does not contain any blood vessels since they would prevent the cornea from being completely clear. Instead of relying on blood for nourishment as do other bodily tissues, the cornea receives oxygen directly from the air in front of it and is nourished by the clear fluid, known as the aqueous humor, which flows behind it. The cornea takes light rays and bends them into the eye, providing about 60% of the eye's total focusing power.

Sclera: The sclera is the sturdy white tissue that forms the outer wall of most of the eye. Behind the white surface is a dark layer, known as the choroid, replete with blood vessels that nourish many of the eye's tissues. This dark choroidal layer and the pigmented layer of the retina also serve to trap light that enters the eye so that it can be processed into images.

Iris: The iris is located behind the cornea — it is the pigmented tissue that gives colour to the eye (e.g., blue, brown, green). The iris is a ring of circular and radial muscles with an opening in the centre called the pupil. The iris changes the size of the pupil in response to various light conditions to control the amount of light entering the eye. In very bright conditions, the pupil narrows to the size of a pinhole, and in dark conditions, it can dilate to almost a centimetre to allow any available light to enter the eye for image production.

Lens: The lens is suspended directly behind the pupil by tiny elastic-like strands called zonules. The lens fine focuses the light that passes through it and provides ~40% of the eye's focusing power. The lens consists of a clear gel-like protein surrounded by a transparent outer membrane known as the lens capsule. Through the actions of the ciliary muscle, the zonules loosen and tighten to change the thickness of the lens so that it can focus on objects at varying distances. The lens also serves to block any harmful ultraviolet light rays from entering the inner eye.

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Ciliary Body: This little known part of the eye is tissue located just behind the outer rim of the iris and attached firmly to the inside of the sclera (white wall of the eye). The ciliary body is made up of two parts — a circular muscle that works with the zonules to change the shape of the lens and a gland-like element that produces aqueous humor liquid.

Aqueous Humor: Aqueous humor is a clear solution produced mostly by the ciliary body, behind the iris. The aqueous humor carries nutrients to the tissues in front of the lens and helps to maintain internal eye pressure. This nourishing liquid flows between the lens and the iris and through the pupil before it enters into a drainage canal in the peripheral areas of the eye, just in front of the iris. From there, the aqueous humor is absorbed into the blood stream.

Vitreous: The vitreous is a clear gel-like substance that fills the large back chamber of the eye, between the lens and the retina. In adult life, vitreous has no known function since it does not seem necessary to maintain space or help the eye maintain its structure.

Retina: The retina, a direct extension of the brain, is a delicate light-sensitive membrane that lines the back inside wall of the eye. The retina's function is to catch the light rays entering the eye and change the images into electrical impulses for transmission to the brain. The retina contains two types of light receptors — cones and rods. The cones, which are responsible for responding to detail, colour and daylight conditions are located in the centre of the retina, known as the macula, where most of the light is focused. The rods, which are active in dim light conditions, are concentrated in the peripheral areas of the retina where they respond to peripheral light rays and provide peripheral (side) vision. A normal retina contains ~125 million rods and ~6 million cones.

The macula is the specialized central area of the retina that is responsible for acute central vision. An indentation at the centre of the macula, called the fovea, allows light to come in almost direct contact with the photoreceptors without having to pass through much tissue in front of them. Because of a high concentration of cone cells, this area produces vision 100 times more detailed than any other part of the retina. The macula is responsible for central vision and the precise vision required to read, drive or focus clearly on an object.

Optic Nerve: The optic nerve connects the back of the eye with the visual cortex in the brain where visual signals are interpreted. The optic nerve's function is to carry the electrical impulses from the retina to the brain. Because there are no light receptors in the small portion of the retina where the optic nerve is located, a blind spot exists in everybody's peripheral vision. However, since both eyes work together, there is no break in the scope of vision since each eye compensates for the other's blind spot. The brain also contributes to fill in the blind spot with the same background as is seen around it.

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The Eyelids: Besides being used to protect the eye and block out light when the eyes are closed, the eyelids serve to spread lubrication over the eye through a continuous and frequent blinking action during waking hours.

The Lubrication Network: A three-layer tear film, produced by three different glands, serves to lubricate the eye. The meibomian glands on the edge of the eyelids produce the outer lipid (oily) layer, which reduces evaporation and keeps the tear surface smooth. The lacrimal glands in the conjunctiva (the protective membrane lining the inside of the eyelid and the front part of the sclera) as well as above the eye but below the eyebrow, produce the middle aqueous (watery) layer of tear film. These middle watery tears have lubricating, anti-bacterial and cleansing qualities — these are the tears shed in response to irritation or emotion. The goblet cells (small glands in the conjunctiva) produce the inner mucin (sticky) layer, which enables the watery tears to adhere to and spread evenly over the surface of the eye - moistening and protecting the conjunctiva and cornea.

The Eye Muscles: The movement of the eye is controlled by a complex set of six muscles, which allow the eye to move freely in all directions. Muscles are attached to the eyeball on the top, bottom and on each side to pull the eye in each of these directions. Besides these four basic muscles, there are two muscles attached to the eye at angles — one on the top and one on the bottom — to help raise and lower the eye as well as allow the eye to rotate clockwise and counterclockwise. The muscles of the two eyes work together harmoniously as a well-directed team.

When the eye is open, light passes through the cornea. The cornea takes a wide spectrum of light and bends it uniformly through the pupil to the lens. The lens, signaled by the brain, adjusts its thickness to fine focus the light onto the sensitive macular area of the retina. The photoreceptors catch the light images and encode them into electrical messages, which are sent to the brain via the optic nerve. Upon receiving the information, the brain decodes the impulses and vision becomes a vivid reality.

Since vision is a priceless asset, it is wise to follow some simple precautions, which can help the eyes to remain healthy for as long as is possible.

Eat a healthy, balanced diet: Like the rest of the body, the eyes require many nutrients to function properly. Eyes benefit from the same type of diet that benefits the heart (i.e., a diet low in fat and cholesterol, but high in whole grains, fresh fruits and vegetables). Reducing or eliminating animal fats from the diet as well as increasing the consumption of whole grains (particularly oats and barley), legumes (such as lentils, kidney beans and peanuts), nuts, fruits as well as orange and dark green vegetables will contribute to the health of the entire body.

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Exercise regularly: Physical activity benefits the whole body in many ways. One benefit of exercise is that it stimulates the circulatory system, which contributes to good eyesight; since, it provides the tiny blood vessels in the eye with a steady flow of blood to nourish and oxygenate the eye's delicate components. Exercise also seems to help regulate internal eye pressure. Exercise programs do not need to be exhausting, expensive or elaborate. Simple, moderate exercise (such as regular walking or swimming programs) can provide significant health benefits. NOTE: If health problems are a concern, a doctor should be consulted before beginning an exercise program.

Avoid harmful substances: Cigarette smoking is the single greatest cause of preventable disease. Not only does it cause numerous forms of cancer and various respiratory problems, it also contributes to the development of cataracts and retinal diseases, which can lead to blindness. Furthermore, smokers are less likely to benefit from treatment for retinal disease. Alcohol, high blood pressure, high blood sugar and high blood cholesterol are also risk factors for developing some eye conditions (e.g., cataracts, macular degeneration, etc.).

Wear sunglasses outdoors: Bright sunlight is so intense that it can adversely affect the lens and retina of eyes that are continually exposed to it. Wearing sunglasses outdoors can maintain eye health. If worn to protect the eyes, sunglasses should block out ultraviolet A and B radiation. Sunglasses that provide protection from blue light have also been shown to reduce the risk of developing some retinal diseases. In order to ensure a pair of sunglasses provides the desired level of protection, when selecting and purchasing sunglasses, it may be advisable to consult with an eye care professional trained to dispense eyewear.

Avoid eye injuries: All reasonable measures should be taken to prevent injuries to the eye in potentially dangerous employment and recreational situations. Of particular danger are activities involving metal-to-metal or metal-to-stone hammering, which can propel metal fragments through the wall of the eye. Masks and goggles save many eyes. If an accident does occur, medical attention should be sought immediately. It is a good idea to have information regarding emergency eye care readily available, since some action may be required before a doctor is available.

Arrange for eye exams: Everyone should have regular eye exams so that subtle diseases and degenerative conditions can be diagnosed early, when treatments are more likely to be effective. Regular eye exams are vital for diabetics because they are at increased risk for many eye diseases. Any change in the condition of the eyes indicates a need for an eye exam to determine the cause. The number and types of evaluative procedures during an eye examination vary with factors such as age, family history, past and present problems as well as the purpose of the examination. 

Wear proper corrective lenses: Any individual, whose eyes do not focus clearly at all distances, should wear appropriate corrective lenses to improve vision. Spectacles should be properly fitted and kept clean. When contact lenses are used, it is essential that all recommended instructions for contact lens care, wear and cleaning be followed carefully. If contact lenses are used improperly, infections can occur and serious damage can be done to the eyes.

Give eyes a break: There is good reason to believe that eyes need to rest when they have been working hard. People who spend extended periods of time engaged in close work (e.g., reading, sewing or using a computer) should vary their focus by looking at a distant object periodically. This eye resting practice seems to be particularly important for young children.