Human eye

The eye is the visual system organ that provides sight or vision; one of the most important senses for the human body. 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 human eyes are so complex and detailed it is amazing to even comprehend. They are positioned at the location that is most comfortable for vision, and 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 which we would not be able to see. The movement of the eye is accomplished by a variety of parts. Each muscle and nerve in the eye has its own job to help us see and function.

Jonathan Sarfati speaks to the exquisite sensitivity of the eye. The retina can detect a single photon of light, and it’s impossible to improve on this sensitivity! More than that, it has a dynamic range of 10 billion (1010) to one; that is, it will still work well in an intensity of 10 billion photons. Modern photographic film has a dynamic range of only 1,000 to one.

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

To suppose that the eye with all its inimitable contrivances for adjusting the focus to different distances, for admitting different amounts of light, and for the correction of spherical and chromatic aberration, could have been formed by natural selection, seems, I freely confess, absurd in the highest degree.

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

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.

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
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. 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.

Whites of the Eyes
Human beings show the whites of their eyes while primates and most other animals do not. Since humans use their eyes as part of their body language and communicate some information simply by showing where they are looking, the white of the eye is very important. Evolutionists presume it developed because of cooperation between humans though it would seem less important for cooperation as for communication of attention. In cartoon animations of animals, the animals are often given eye whites probably to make them seem more human. Kevin Anderson points out in an editorial that reconstructions of Lucy often have white sclera (the white of the eye), even though there is no fossil evidence one way or the other. Interestingly some domesticated dogs show whites of the eyes, as if humans bred for that characteristic in dogs.

Cornea

 * Main Article: 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.

Retina
The retina is 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.

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.

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.

Receptor cells

 * Main Article: Photoreceptor cell

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.

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 less light than do the cone cells, and are responsive to as little as one photon of light. 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. 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.

Design Features
The human eye is an incredibly complex optical processing structure. The retina of the eye, which images the light, is made up of about 120 million rods which are sensitive to black, white, and grays, and about 6 million cones which are the color recepters. The Foveola, part of the Fovea, is the region of greatest sharpness of vision, containing about 15 thousand cones. The eye is more or less equivalent to a 126 megapixel camera. .

Vision sharpness is measured by grating acuity and vernier acuity. Grating acuity is the distance at which you can see that two bars have a space in between, and is most related to the ability to see the difference between C and O. Vernier acuity is the ability to perceive if two line segments align perfectly or if there is an offset as you go from one line to another. Vernier acuity is related to the ability to see the difference between D and O. The normal human eye can perceive a grating accuracy of between 60 and 30 seconds of arc. Its vernier acuity is about ten times better (five to ten seconds of arc), as if the eye system were designed to especially recognize continuous lines in our surroundings.

Stereoscopic Vision
We have not one camera, but two, made to work together to provide stereoscopic vision that lets us judge by eye the distance to nearby objects. This requires good acuity but also a combination of "wiring" and "programming" that let us match what is seen by one eye with what is seen by the other.

When fixed on an object at a distance of two meters (6.5 ft), eyes eight centimeters apart differ in their alignment by 2.3 degrees. At 1.5 meters, the difference is 3 degrees, showing that perception of eye alignment must be much better than .7 degrees or we could not be sure of the distance. Juggling requires fast processing and very accurate alignment and mapping between two multiple megapixel eyes (as well as hand coordination), but most people can learn to juggle with a little practice.

Image Processing Power
A TV screen changes its pictures thirty times per second, which is faster than our eyes respond, and so we see motion rather than individual frames. This speed of change (or flicker rate) gives a general idea of the processing power of the human optical system. If we say that the human eye can process 10 frames per second, our image processing system must deal effectively with about a billion responses per second with high accuracy. Stereovision compounds the difficulty for the processing system that we were born with, yet it functions so well that we are rarely aware of glitches or errors in information transfer.

Sarfati comments further. Another amazing design feature of the retina is the signal processing that occurs even before the information is transmitted to the brain, in the retinal layers between the ganglion cells and the photoreceptors. For example, a process called edge extraction enhances the recognition of edges of objects. Dr John Stevens, an associate professor of physiology and biomedical engineering, pointed out that it would take ‘a minimum of a hundred years of Cray [supercomputer] time to simulate what takes place in your eye many times each second.’1 And the retina’s analog computing needs far less power than the digital supercomputers and is elegant in its simplicity.

Pixel Wiring by Chance Unlikely
It is difficult to imagine how the correct wiring between eye pixels and the brain could simply develop. If we think in terms of hooking a digital camcorder to a television monitor, only certain patterns of connections will form an image on the monitor. Random connections with more than just a few pixels (five or ten) would tend to be incorrect almost all of the time. David Stoltzmann calculated that with 7 pixels the percentage of the random permutations which have 7 pixels correctly connected would be .0074%. The number of wrong wirings possible for an 8 x 8 pixel retina is greater than the estimated number of particles in the universe. Since evolution works by random trial and error, even if a one pixel eye worked, more pixels and correct wirings must be added randomly. If 3 pixels out of twelve are correct, how can "a mutation-driven process isolate the 3 correct pixels from subsequent variations?" It seems that evolutionary theory needs to map out how this kind of development could have proceeded.

Cornea Transparency
The cornea remains transparent because it has no blood vessels. Researchers found that the cornea is heavily stocked with a protein that halts blood vessel growth. Other processes help also, such as the enzyme that helps to degrade organelles inside lens cells so that they cause less blocking and fogging of vision. It was mentioned that in mice, failure of the enzyme leads to cataracts (white fogging of the lens).

Disease
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.
 * 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.


 * 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.

Infections
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.
 * Pinkeye

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.
 * Red eye

Treatment
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.
 * Refractive Surgery

Improving Vision
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. Glasses are the most common form of eye-wear 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.
 * Glasses and Contact Lenses