Human eye

From CreationWiki, the encyclopedia of creation science

Revision as of 05:34, 3 April 2009

NSCS ProjectThis article is a work in progress by a student at North Sound Christian School.
Please do not edit the article until this banner is removed.

This is a girl with brown eyes; one of the most common colors of the eye.

Introduction

The eye is the organ of sight, in humans appearing externally as a dense, white, curved membrane, or sclera, surrounding a circular, colored portion, or iris, that is covered by a clear, curved membrane, or cornea, and in the center of which is an opening, or pupil, through which light passes to the retina.^[1] Sight is one of the most important senses, of the five humans obtain. We rely on our eyes for vision. Out of all the ways to communicate; eye contact is the most powerful. Without our eyes we would be living in a different world. The eyes are positioned at an area most comfortable for vision. The location of this area enables us to control and direct our bodies and limbs in an optimum way. The eyeballs rest upon a protective cushion of fat in the socket, are encircled with special tissues, and joined to the skull by six bony extensions. They are protected against external harm by the brow ridges, by the arch of the nose and the cheekbones. These surrounding bones and tissues are called the orbit. The brain works hand in hand with the eye. Without the brain we would not be able to see. The eye is a fascinating muscle that is made up of a variety of parts. Each muscle and nerve in the eye has its own job to help us see and function. The eye is so complex and detailed it is amazing to even comprehend.^[2]

External Structures

There are many structures that make up the eye. Each part is necessary for them to function. The eyebrow, eyelid, and the eyelashes are three parts that protect the eye.

Eyebrows

The eyebrow's function is to block the sweat draining down from the forehead into the eyes. The brows also protect the eye from reflected or direct sunlight by blocking the rays from above. Third, they are a distinctive element of the human face, beautifully completing the eye's visual appeal.

Eyelids

Eyelids are the most important part of this system. They have a double function of protecting the eyeball from harmful contacts, and also keeping the cornea and the conjunctiva both at a constant moisture level. As you sleep, the veins on the conjunctiva layer inside the eyelid feed needed oxygen onto the eyeball. The skin of the eyelid is far thinner than the skin on other parts of the body. The lower skin layer of the eyelid is very loose and lacks fat. This allows easy accumulation of blood to the area.

Eyelashes

Eyelashes are straight and soft with a slight curve towards their tips. The sebaceous glands inside the eyelids help them keep their shape by producing a greasy secretion. Without this suppleness, the lashes would be rough and clump together annoyingly every time you blink. Eyelashes are attached to the outer edges of the eyelids to protect the eye from outside dust and larger particles. If an eyelash is lost or cut it will grow again from the same roots.^[2]

Blinking is a natural reflex and is necessary to keep the eye moisturized. A human’s rate of blinking is between ten to twenty per minute depending on activities. The hygiene of the eye is maintained by automatically adjusted rates of blinking. Every time light enters the eye and hits the retina, nerves transmit a signal to the brain. The brain is informed of the light's existence and of its intensity. It immediately sends back a response as to how far the muscles around the pupil should expand or contract. The entire process of communication, calculation, and functioning, is over in less than a second.

Tears

This is a picture of a tear forming from the eye.

Tears are a liquid with various ingredients serving different functions; that protect the eye against germs. Tear drops are made up of 98.2 percent water, the rest urea.^[2] Lacrimal Glands are the glands which produce tears. The tears that you cry flow from your tear glands into your eyes through tiny tear ducts. These tear glands are located under your upper lids, and produce tears to form a thin layer over your eyeballs. Every time you blink the film spreads over your eyes to keep them moist and free of dust and other irritants. Reflex tears can also be called irritant tears. They produce extra tears when your eyes are irritated. This salty fluid is always flowing from the tear glands. All the used tears you cry drain down into two tiny openings on the brim of your upper and lower eyelids at the inner edge of your eyes, which lead to the tear ducts next to the bridge of your nose. From there, they are channeled into your nasal cavity where they are swallowed or blown out with other nasal fluids. ^[3]

Internal Structures

Cornea

Cornea is a clear outer part of the eye's focusing system located at the front of the eye. The cornea is the transparent convex tissue at the front of the eye (figure at right) that serves two specialized functions. First it forms, with the sclera, which is a protective physical barrier that shields the inner eye from the external environment. Equally important is its ability to protect itself from various types of damage, ranging from physical trauma and biochemical injury to infections by myriad pathogenic organisms, to the deleterious effects of long-term exposure to light itself. In protecting itself, the cornea also safeguards many underlying ocular structures from similar damage. Second, the cornea serves as the main refractive element of the visual system, directing incoming light onto the crystalline lens, which focuses it onto the retina. Refraction depends on the cornea acquiring transparency during embryonic development and maintaining it throughout adult life.^[4]

Other

• Pupil: The opening at the center of the iris. The iris adjusts the size of the pupil and controls the amount of light that can enter the eye.

• Iris: The colored part of the eye; regulates the amount of light entering the eye.

• Macula: The small, sensitive area of the retina that gives central vision; contains the fovea.

• Fovea: The center of the macula; gives the sharpest vision.

• Sclera: The tough white outer coat of the eye.

• Vitreous humor: The clear gel filling the inside of the eye.

• Optic nerve: The bundle of more than 1 million nerve fibers that carry visual messages from the retina to the brain.

• Lens: The clear part of the eye behind the iris that helps to focus light on the retina; allows the eye to focus on both far and near objects.

• Choroid: A layer of blood vessels that feeds the retina.

Retina photoreceptors

• Retina: The light-sensitive layer of nerve tissue that lines the inside of the eye and sends visual messages through the optic nerve to the brain. It is the innermost layer in the eye and is compared to as the film in a camera. This system of nerves sends an impulse through the optic nerve back to the brain. At the back of the retina are a number of rod-shaped and cone-shaped cells. These cells convert received light into electrical signals. Because of their shape as observed under a microscope, they are called rods and cones.^[5]

Cones and rods

There are 6,000,000 cones and 120,000,000 rods in the eye. That is a ratio of nearly 20 rods to every cone. Rods can respond by forming a black-and-white image. Cones perceive color. The rods and cones convert light waves into electrical energy. There are four perceptions that the retina can interpret: light, shape, contrast, and color. The rod cells are able to perceive more light than do the cone cells. Because of the rods we can see at twilight. In brighter conditions, the cone cells come into play. Cone cells perceive the shape of objects. The ability to differentiate between areas that are not clearly separated is extremely important. Loss of ability to distinguish contrast is common in a number of illnesses. This is a condition which can bother patients even more than loss of their acute vision. Color comes from the mind's interpretation of different wavelengths of incoming light. The retina separates the wavelengths, interpreting each as a different color. We actually see with our brains; and our eyes collect and deliver the information to do so. The retina is composed of ten layers: pigmented epithelium, photoreceptors; bacillary layer, internal and external limiting membrane, inner and outer nuclear, inner and outer plexiform, ganglion cells, and nerve fiber layer. Beneath the pigmented epithelium, the four layers include the sclera, large choroidal blood vessels, choriocapilarius, and Bruch's membrane.^[5]

The Primary Colors

Red, blue, and green are the three primary colors found in nature. All other colors come about through the varying combination of these three. These three separate groups of cones, each react to specific wavelengths of light. If the cones sensitive to red, green and blue are alerted to an equal degree, we see white. Color adjustments are made every time you see an image. ^[6]

Loss of Function

• Blindness

Vision problems can occur if the brain has trouble processing the information that was sent. The optic nerve sends pictures to the brain, and if it doesn’t form correctly the brain will not receive the message that is needed for sight. Blindness can be genetic or inherited, which means that this problem gets passed down to a kid from parents through genes. Blindness also can be caused by an accident, if something hurts the eye. That's why it's so important to protect your eyes. Some illnesses, such as diabetes, can damage a person's vision over time. ^[7]

• Cataract

Other eye diseases, such as cataracts, can cause vision problems or blindness, but they usually affect older people. A cataract is a clouding of the eye's natural lens, which lies behind the iris and the pupil. The lens focuses light onto the retina at the back of the eye. The lens also adjusts the eye's focus, letting us see things clearly both up close and far away. The lens is mostly made of water and protein. The protein is arranged in a precise way that keeps the lens clear and lets light pass through it. But as we get older, some of the protein may clump together and start to cloud a small area of the lens. This is a cataract, and over a period of time, it may grow larger, making it harder to see.

Cataracts are classified as one of three types:

A sub-capsular cataract begins at the back of the lens. People with diabetes, high farsightedness, retinitis pigmentosa or those taking high doses of steroids may develop a sub capsular cataract. A nuclear cataract is most commonly seen as it forms. This cataract forms in the nucleus, the center of the lens, and is due to natural aging changes. A cortical cataract, which forms in the lens cortex, gradually extends its spokes from the outside of the lens to the center. Many diabetics develop cortical cataracts. ^[8]

Infections

This is an example of what the infection pinkeye looks like.

• Pinkeye

Pinkeye is a very common infection and is highly contagious. Pinkeye is redness and swelling of the conjunctiva, the mucous membrane that lines the eyelid and eye surface. Another name for pinkeye is conjunctivitis. The lining of the eye is usually clear and if it becomes red and swollen infection has occurred. It usually is not serious and goes away in 7 to 10 days without medical treatment. This infection is caused by viruses or bacteria, dry eyes, chemicals, fumes, smoke, and allergies.

• Red eye

Red eye is a general term that includes not only pinkeye but also many other problems that cause redness on or around the eye. Pinkeye is the main cause of red eye. Red eye has other different causes, such as: infection from metal or insects, scrapes, and sores. ^[9]

Treatment

• Refractive Surgery

This eye surgery is designed and performed to reduce or eliminate refractive errors of the eye -nearsightedness, farsightedness and astigmatism. Refractive surgery can also "correct" Presbyopia (the normal age-related loss of near focusing ability) by eliminating the refractive error of one eye and intentionally making the other eye mildly nearsighted. Their are various different types of surgery, but one of the most common ones is LASIK (Laser-Assisted In Situ Keratomileusis). This is a procedure that involves creating a thin flap on the cornea (with a laser) and reshaping the underlying corneal tissue with a laser. ^[10]

Improving Vision

• Glasses and Contact Lenses

If you have vision problems, you can either buy contact lenses or eyeglasses. Contact Lenses are a great solution for people who do not wish to have surgery and do not like the appearance of eyeglasses. Millions of people wear these little disks that fit right on your eye and give you clear vision, without changing your appearance. The majority of people are able to wear contacts, but some are not due to high amounts of eye infections and extra sensitive eyes. There are several different varieties and types of contact lenses to choose from. Some allow you to keep them in for long period of time and others are disposable. However, if you keep contacts in for too long your eyes will become itchy, red, and your vision will be blurred. ^[11] Glasses are the most common form of eyewear used to correct or improve many types of vision problems. Refractive errors can include nearsightedness or myopia (difficulty seeing far away), farsightedness or hyperopia (difficulty seeing close up), and astigmatism (blurring due to an irregularly shaped cornea). This function is performed by adding or subtracting focusing power to the eye's cornea and lens. ^[12]

Evolution

Even Charles Darwin recognized that the eye was imminently complex and admitted that attempting to explain its origin through natural selection seemed absurd.

Nevertheless, Darwin believed this absurdity was merely illusory, and proceeded to provide an explanation for its evolution in his book,

References

1. ↑ [1] Dictionary.com
2. ↑ ^{2.0 2.1 2.2} The Perfect Design in the Eye By Harun Yahah
3. ↑ Tears Newtons Apple
4. ↑ Report of the Corneal Diseases Panel by the National Eye Institute.
5. ↑ ^{5.0 5.1} Anatomy, Physiology & Pathology of the Human Eye By Ted M. Montgomery
6. ↑ Wild About Healthy Vision by the National Eye Institute.
7. ↑ http://kidshealth.org/kid/health_problems/sight/visual_impaired.html
8. ↑ http://www.allaboutvision.com/conditions/cataracts.htmCataracts By Judith Lee and Gretchyn Bailey, MD
9. ↑ http://www.webmd.com/eye-health/tc/pinkeye-topic-overview Pinkeye, Web MD
10. ↑ [2] Refractive Surgery, Eye topics
11. ↑ [3]Contact Lens Guide
12. ↑ [4] Eye Glasses and Contact Lenses
13. ↑ The Origin of Species by Charles Darwin. J.M. Dent & Sons Ltd, London, 1971, p. 167.

External links

• Miracle of the Eye by Harun Yahya

A section of retina shows the thinner rods and wider cones.

Retinal photoreceptor

v • d • e Biology Organisms Animals • Algae • Amphibians • Bacteria • Birds • Crustaceans • Dinosaurs • Fish • Fungi • Humans • Insects • Mollusks • Plants • Protists • Reptiles Disciplines Agriculture • Anatomy • Baraminology • Biotechnology • Botany • Cell biology • Ecology • Epidemiology • Embryology • Entomology • Evolutionary biology • Genetics • Herpetology • Medicine • Microscopy • Physiology • Taxonomy • Zoology Systems Auditory system • Circulatory system • Digestive system • Endocrine system • Immune system • Integumentary system • Lymphatic system • Muscular system • Nervous system • Olfactory system • Respiratory system • Sensory system • Skeletal system Processes Gene expression • Metabolism • Parasitism • Reproduction • Symbiosis • Trophism • Resources Diseases • News • Scientists Categories • Portal