Background Research: The Human Eye

The eye is a slightly asymmetrical sphere, around an inch in diameter. Out of all the organs in the human body, the eye has been called the most complex organ. There must be light in order for your eyes to see. The human eye is similar to a camera. Light rays reflected off an object enter the eye through the cornea, a clear cover that can be compared to the glass of a camera’s aperture. The amount of light coming in through the cornea is controlled by the pupil, which is an opening that opens and closes similar to a camera shutter. At the back of your eye, the light is focused by the retina, which is a series of light-sensitive cells. The retina acts similar to camera film; it reacts to incoming light and converts it into electric signals which are sent to the brain through the optic nerve. When the brain receives the electrical signals, vision occurs and we can see.

There are many parts in the eyes. First we will start with the parts in the front part of the eye (the part of the eye you can see in the mirror). These are the iris, the cornea, the pupil, the sclera, and the conjunctiva. The iris is the colored part of the eye which surrounds the pupil. It is a membrane that lies between the cornea and the lens. The iris controls the amount of light that enters the eyes by widening or narrowing the pupil. When there is a strong amount of light, the iris sphincter muscles contract the pupil, and when it’s dark, the iris opens the pupil by using the dilator muscles. The diameter of the pupil also changes depending on how near or distant an object when focusing on it. The pupil decreases in diameter when focusing on near objects and increases in diameter when focusing on distant objects; this reflex is known as the Accommodation Reflex. The pupil is the dark circle shaped center of the eye. It is basically a hole in the iris that is widened and narrowed to control the amount of light that the retina receives.

The cornea is the clear, transparent part of the eye that covers the iris, pupil and anterior chamber (the part of the eye behind the cornea and in front of the iris and lens). It’s the part of the eye that lets light enter the eye allowing sight. The sclera is a membrane of tendon in the eyes (the outer coat of the eyeball), which is also known as the white of the eye. It surrounds the optic nerves at the posterior part of the eyeball and works to protect the inner and more sensitive parts of the eye such as the choroid and the retina. Then the last part of the eye that can be seen is the conjunctiva, which is a mucous membrane that covers the sclera, lines the visible parts of the eye, and the inside of the eyelids. Located in the conjunctiva are many glands that have an important part in the production of tear-film.

Now we will learn about the parts of the eye which we cannot see. The parts that we cannot see are the choroid, retina, vitreous body, macula, fovea, optic disc, lens, optic nerve, aqueous fluid, and ciliary body. The choroid is the layer located within the sclera. It consists of layers of blood vessels located between the sclera and the retina. The choroid provides nourishment to the retina and the back area of the eye with oxygen and nutrients. The macula and the front part of the optic nerve are dependent on the blood supplied by the choroid. The retina is the sensory membrane that lines the eye. The retina often also refers to the light sensitive tissue located in the back of the eyeball. It receives images formed by the lens and then converts them into signals that are sent to brain through the optic nerve. In the retina there are two kinds of light receptors, the cones and the rods. The cones are color sensitive and are located in the center of the retina. The cones mainly absorb stronger light. The rods are located peripheral to the fovea and absorb softer light in black and white.

The vitreous body is the part of the eye located between the lens and the retina. It’s a clear gel-like substance that fills the eyeball behind the lens. There is aqueous fluid that fills the space in front of the lens. Aqueous fluid (also known as aqueous humor) is produced at the back of the ciliary body. Its main function is to supply the cornea and the lens with nutrients and oxygen. This then brings us onto the next part of the eye, the ciliary body. The ciliary body is the part of the eye between the iris and the choroid. It contains the ciliary muscle, which is the muscle that controls the vision accommodation reflex. The ciliary body’s functions are accommodation, production of aqueous fluid, and holding the lens in place. The lens is located behind the cornea and is the nearly globular body in the eye. It is the part of the eye that focuses light rays onto the retina. The lens allows us to change the focus according to different distances such as when we change focus from the television screen to the coffee mug in our hand. The lens is attached to zonula threads, which are a mass of threads, and the zonula threads are then attached to the ciliary body. When we want to focus on a nearby object, a muscle in the ciliary body contracts causing the zonula threads to loosen, which in turn allows the lens to contract in diameter and thicken. This increases the acuteness of the lens.

The macula is the area around the fovea and is located near the middle of the retina. Due to the densely packed visual cells in the macula, the macula allows us to see objects in great detail or in high image resolution. Around the macular area, the visual cells are not as densely packed, however they are good enough to allow our peripheral vision to properly work. The next part of the eye is the fovea. The fovea is the most central part of the macula and is a depression in the retina. In the fovea, the visual cells located there are only cones that are packed very tightly. Due to that, the fovea provides acute eyesight, which is perfect for reading or watching television. Just outside the fovea there are rods, which are packed, and are active in conditions with low light.

The optic nerve consists of around 1,000,000 nerve threads and is a nerve that sends electrical impulses (or signals) from photoreceptor cells (rods and cones) in the retina to different parts of the brain. These parts of the brain are where the signals are interpreted into images. The optic nerves from both eyes are reconnected behind the eyes. Now the last part of the eye that we cannot see is the optic disc. The circular area where the optic nerve attaches to the eye is known as the optic disc or the optic nerve head. The eye’s blind spot is located here too, due to the absence of visual cells in the optic disc. All of the visual messages from the cones and rods leave the eyeball from this point and are then transferred through the nerve threads to the brain.

Some of the other minor parts of the eye are the anterior chamber, posterior chamber, suspensory ligament, medial rectus muscle, and lateral rectus muscle. The anterior chamber is the part of the eye behind the cornea and in front of the iris and lens, while the posterior chamber is the part behind the iris and in front of the lens. Next is the suspensory ligament, also called the zonule of the Zinn, which is a membrane of fibers (zonules) that hold the lens in place. The medial rectus muscle is the muscle that moves the eye towards the nose, and the lateral rectus muscle moves the eye away from the nose.

Now that the parts of the eye and their function(s) have been described, we will now go into the eye conditions and diseases. The most common eye conditions that everyone knows are nearsightedness (myopia) and farsightedness (hyperopia). Nearsightedness is something that affects a large amount of people. People with nearsightedness have difficulty seeing distant objects, but can clearly see objects that are near. Myopia is caused when the eyeball is “too long” for the lens or when the cornea has too much curvature. When one of these circumstances occur, the light entering the eye is not focused properly on the retina. The images focus in the front of the retina instead of directly on the retina, which causes blurred vision. The opposite of nearsightedness is farsightedness (or hyperopia). Farsightedness is the inability to see near objects clearly. People with farsightedness can see things at a distance more easily than things up close. Hyperopia occurs when the eye is “too short” for the lens, when the cornea is not curved enough, or when the lens sits farther back in the eye than usual. It can also occur when eye muscles have weakened with age. When any of these situations are met, it causes light entering the eye to be focused behind the retina instead of directly on it.

Another common eye condition is amblyopia, or lazy eye. It is a condition in which one eye sees better than the other. The weaker eye is called the “lazy eye” because the weaker eye may or may not wander. Amblyopia occurs during childhood and can be caused by several ways. When one of the eyes has better focus than the other, the brain is challenged with a blurry image and a clear image, so the brain will begin to ignore the blurry image. If this continues on for several months or years, it will cause deterioration in vision in the weaker eye. Another cause of lazy eye is strabismus. Strabismus (also known as crossed eyes) is an ocular misalignment, which means that one of the eyes turn inward or outward. Some people are born with strabismus and in some cases strabismus is caused as a result of a problem with the nervous system (esp. the part that controls the muscles of the eyes). It prevents both eyes from focusing together on an image and can cause double vision. To combat this, the brain usually ignores the image from the strayed eye, which then leads to deterioration of vision in that eye.

Astigmatism is a common and well known eye condition that is a defect that causes an inability to correctly focus light onto the retina. It is caused by an eye that is not completely round. A person’s eye is naturally shaped like a sphere and when light enters the eye it refracts evenly, which creates a clear view of the object. However, people with astigmatism have an eye shaped more like a football or the back of a spoon. This causes light that enters the eye to be refracted more in one direction that the other, which causes only part of the object to be in focus at one time.

A common eye disease among the elderly is age-related macular degeneration (AMD). It is the loss of central vision in both eyes. Macular degeneration is caused by deterioration of the retina and can critically damage vision. There is no cure for macular degeneration; however it can be treated with medications, vision aids, vitamins, and laser therapy.

A cataract is a clouding of the lens, which can cause changes in vision due to the fact that it hinders the passage of light through the lens. The symptoms of cataracts include fuzzy or cloudy vision and sensitivity to glare. Some other common or well-known eye conditions and diseases are conjunctivitis, diplopia, dry eye, and glaucoma. Conjunctivitis (also known as pinkeye) is an infection or inflammation of the conjunctiva. There are several different causes for pinkeye including: bacteria (such as gonorrhea or chlamydia), viruses, irritants (shampoo, dirt, smoke, pool chlorine, etc.), and allergies (dust, pollen, or a type of allergy that affects some contact lens wearers). Pinkeye that is caused by some bacteria or viruses can be spread very easily from person to person, however it is not a serious health risk as long as it is detected promptly.

Diplopia is also known as double vision. Double vision can be caused whenever there are problems with any part of the vision system (cornea, lens, muscles of the eye, nerves, and brain). Cornea problems such as infections of the cornea, corneal scars, and dryness of the cornea, often lead to double vision in only one eye. The abnormal surface of the eye causes distortion of incoming lights, which causes double vision. Problems with the lens also cause diplopia. The most common problem of the lens is cataracts. If cataracts are present in both eyes, it will cause images from both eyes to become distorted. Muscle problems such as when a muscle in one eye is weak, which causes the eye to be unable to move as smoothly with the healthy eye can cause double vision. These problems can result from myasthenia gravis (an illness that blocks the stimulation of muscles by nerves inside the head) and Graves’ disease (a thyroid condition that affects the muscles of the eyes). Problems of the nerves that control the eye muscles lead to double vision. Nerve problems can be caused by multiple sclerosis, Guillain-Barre syndrome (nerve condition), and uncontrolled diabetes. Then the last cause of diplopia is brain problems. The nerves that control the eye are connected directly to the brain, and visual processing takes place inside the brain. For these reasons, many difference causes of diplopia originate in the brain.

Dry eye is caused when the eyes don’t produce enough tears, or the tears are poor in quality. The eye is dependent on the flow of tears to provide consistent moisture and lubrication so it can maintain vision and comfort. Tears are made up of water (to moisturize), oils (to lubricate), mucus (to spread evenly), and antibodies and special proteins (to protect or improve the resistance of the eye to infection). The tears are secreted by special glands that are located around the eye, so when there is an imbalance in the tear system, a person will experience dry eyes. Other causes of dry eye are when the tear film is dried out due to dry air from environmental conditions, lupus, scleroderma, and Sjogren’s syndrome.

Glaucoma is an eye condition that causes damage to the eye’s optic nerve. It is caused by increased pressure in the eye because this intraocular pressure can damage the optic nerve. Glaucoma causes the eye to slowly lose vision, and if left untreated it can cause total and permanent blindness within a few year times.

The other eye conditions and diseases are blepharitis, corneal abrasion, diabetic retinopathy, hyphema, keratitis, optic neuritis, pterygium, retinal detachment, retinitis, stye, chalazion, and uveitis. Blepharitis is a common eye condition and is an inflammation of the eyelids near the eyelashes. It causes the eyelids to be reddened, itchy, somewhat swollen, and scaly-looking at the base of the eyelashes. The causes of blepharitis are usually an excess growth of bacteria that is normally found on the skin, blockage of the eye’s oil glands, and some occasional allergies. A corneal abrasion is a scratch on the eye’s cornea. It can be caused by many situations such as being poked in the eye, getting foreign matter (dirt, sand, ash, etc.) trapped under the eyelid, chemical burns, rubbing your eye harshly, dirty or ill-fitting contact lenses, and certain types of eye infections.

Diabetic retinopathy is an eye condition that affects people with diabetes who have high blood sugar over a long period of time. Too much blood sugar can destroy/damage the blood vessels in the back of the eye, which then prevents the retia from receiving the correct amount of nutrients it need in order for it to remain vision. Over some time, the weakened blood vessels may overgrow the retina or bleed, which will then in turn affect vision. Another eye condition related to the bleeding in the eye is hyphema. Hyphema (bleeding in eye) is caused by trauma to the eye. The trauma causes bleeding in the anterior chamber (between the cornea and the iris) of the eye.

Keratitis is inflammation or infection of the cornea. It usually occurs after germs enter a corneal abrasion. The next eye condition is optic neuritis (also known as papillitis) which is characterized by the inflammation and deterioration of the optic nerve but mostly the optic disc. Optic neuritis is usually caused by an overactive immune system, and normally results in painful vision loss in one eye. Pterygium is a thickened conjunctival mass normally on the inner part of the eyeball, however it can cause vision problems if it covers a part of the cornea.

A very serious eye condition related to the retina is retinal detachment. This happens when the retina comes loose from the back of the eye and separates from the tissue around it. Under these conditions, the retina cannot work properly so it is possible to permanently lose vision if the separated retina is not repair promptly. This is caused most commonly due to trauma and diabetes. Another eye condition related to the retina is retinitis. Retinitis is the inflammation or infection of the retina. It can be a long-term genetic condition or the result from a viral infection.

Two very similar eye conditions are chalazion and stye. Both involve the formation of a bump due to a certain reason. However, chalazion occurs when an oil-making gland gets blocked and it swells into a bump, while stye occurs when bacteria infect the skin on the edge of the eyelid, which creates a tender red bump. The difference between the two is that chalazion is not caused by an infection.

Lastly we have uveitis, which is also known as iritis. It is the condition where the colored part of the eye (iris) becomes inflamed or infected. Uveitis develops quickly and usually only affects one eye. It causes pain in the eye, blurred vision, reddened eye, and small or unusually shaped pupil among other things. Uveitis can be caused by blunt trauma to the eye, sarcoidosis, Reiter syndrome, Lyme disease, tuberculosis, herpes simplex, bacteria, and viruses.

There are many eye treatments used today to help treat eye conditions and diseases. Contact lenses and glasses are some of the most commonplace treatment that corrects refractive errors such as nearsightedness, farsightedness, and astigmatism. Then there is LASIK (laser assisted in situ keratomileusis) in which a doctor cuts a flap in the cornea with a tiny saw. A laser reshapes the surface of the cornea, which improves nearsightedness; however it can also be used to correct farsightedness and astigmatism. Another treatment used to treat nearsightedness is photorefractive keratectomoy (PRK). In this treatment a doctor rubs off the surface of the cornea and then uses a laser to improve nearsightedness. The cornea later on heals and grows back. A treatment similar to LASIK is LASEK (laser epithelial keratomileusis), however in LASEK there is not flap cut in the cornea. Instead, the very top layer of cornea cells is pulled off, which allows the laser to reshape the cornea. To treat dry or irritated eyes is artificial tears. They’re eye drops that have a similar composition to natural tears. Cyclosporine eye drops (also known as Restasis) are immune-suppressing eye drops which help with dry eye caused by keratoconjunctivitis sicca. A treatment most often used to treat diabetic retinopathy is laser photocoagulation, a treatment in which the doctor uses a laser to burn blood vessels in the retina that are leaking or growing unusually. Lastly there is cataract surgery. This procedure removes the cloudy cataract from the lens and replaces it with a manmade lens.