The Perfect Design of the Eye
Considering the eye's complex structure and special function, it occupies only a very small volume of our body. Like a precious artifact kept in a safe, it is guarded by the skull to withstand injury from outside blows. The superbly designed protection is proportional to its vital purpose.
Within their sockets, the eyeballs rest upon a protective cushion of fat, 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. Collectively, these surrounding bones and tissues are termed the orbit.
Beside the tight protection, the eyes are ideally 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.
Imagine for a moment if our eyes were fixed somewhere on our knees or worse, our ankles. Since we could see nothing more than the path we were walking upper parts of our bodies, especially the head, would keep banging into unseen obstacles. Given such a mismatched anatomy, many routine tasks like eating or using tools would become problems in their own right. There would be countless difficulties, had our eyes been situated anywhere else than exactly where they are.
The head is the optimum location where the eyes can be maintained in health and safety. You can move your head quickly, with instant reflexes at the right time, so as to protect your eyes from the slightest contact with any harmful material.
Your eyes are also situated at a perfect position on the face. Were they anywhere else—under the nose, for example—it would be a literally uphill struggle to provide the same safe viewing angle, not to mention the aesthetic appearance.
The eyes' position achieves its aesthetic ideal by means of symmetry. They are separated from each other by the approximate width of an individual eye. This golden proportion is surrendered, and the expression lost, when the two eyes are closer or further apart.
The eye, together with all its attributes, is a glittering proof that the human being was created by God. To better understand this proof, let us now take a closer look at the eye's components. Once again, we'll see the theory of so-called evolution's helplessness in trying to explain away the eye's form and function.
The eyes are the body's windows to the outside world. With the help of a specialized system, these windows' protection and maintenance are perfectly maintained. Eyelids, the most important part of this system, undertake the double function of protecting the eyeball from harmful contacts, and also keeping the cornea (the transparent membrane covering the front of the pupil) and the conjunctiva (the delicate mucous membrane lining the eyelid's inner surface), both at a constant moisture level. During sleep, when the eyes' surfaces are not exposed directly to the air, veins on the conjunctival layer inside the eyelid feed needed oxygen onto the eyeball.
The skin of the eyelid, which can cover the eyeball firmly and completely when necessary, 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, allowing for easy accumulation of blood to the area. If this skin were any thicker and fattier, shutting and opening the eyelids would be a troublesome undertaking.
Without being aware of it, people blink their eyes thousands of times a day. These mostly involuntary movements make it possible for the eyes to preserve themselves from intense light and external particles. This unconscious operation, which most people take for granted, is actually an important blessing.
(Figure 1.1). A cross section of the eyelid. Glands inside the eyelid produce tears, and also secrete a lubricant that covers the eyelashes, allowing them to curl upwards, broaden the visual perspective and become aesthetically attractive.Did the eyelashes or eyelid get the idea to secrete such an oil? Of course not! Every aspect of the eye was designed by the Lord of everything, God.
1 -Meibomian Glands
What if this reflex were not automatic? Human beings would remember to blink only after detectably large amounts of dirt had already accumulated in their eyes, which would lead to infection. Due to this incomplete half-way cleaning, vision would be blurred. The task of blinking would have to be consciously remembered, all through the day.
While opening and shutting, the eyelid perfectly fits the convex shape of the eye, contacting the entire surface of the eye's outside layer. Did the eyelid not fit the eyeball's curve so precisely, it would be impossible to remove dust particles remaining in the unreachable corners of the eye enclaves.
During blinking, an oily lubricant is pumped from the special meibomian glands inside the eyelid. This liquid eases the sliding of the eyelids while keeping them from sticking to themselves when they fold up..2
During sleep, it is important for the eyelids to be closed. If the eyelids did not cover our eyes, sleeping would be painful and awkward. A darkened room would always be required, without which, catnaps, siestas, or even sleeping under a bright moon would be unthinkable.3 Eyes would be defenseless, exposed to outside dangers.
To show how irreplaceable the eyelids really are, let us consider the complete reverse of what the case is now. If we had no eyelids, all of mankind would lose their eyesight in a very short time. The cornea, which forms the eye's upper layer, would dry out, costing the eye immediate loss of function. The tiniest dust particle lodged in the eye would create serious problems from infection, thanks to the bacteria on its surface. The eye, left unprotected from even the gentlest impact, would be in constant danger of going blind.
As a real-life example, take lagophthalmos, an illness in which sufferers cannot close their eyelids completely. Infection, along with drying of the cornea, becomes inevitable. Chronic cases can result in permanent eye damage. Since the eyelids cannot be fully shut and the cleansing fluids are not available, the patient needs to constantly cleanse and disinfect the eyes. Even then, an eye that remains partly open all night collects aerial dirt and dust by the morning.4
An Early-Warning System
The eye is protected with the help of a built-in early-warning system. Whenever danger threatens, nerves activate to engage the eyelids, stimulate the muscles that close the lids.
Different types of muscle groups specialize in eyelid movements, whose closing takes three forms:
Blinking is a property of vertebrates that possess eyelids and live in contact with the atmosphere. In humans, the rate of blinking is between ten to twenty per minute depending on activities such as reading and intense concentration, and conditions like the rise of air humidity—all of which reduce the rate. Grief, a rise in temperature and intense light all accelerate blinking. Thus the hygiene of the eye is maintained by automatically adjusted rates of blinking, freeing us from worry over when to close our lids.
(Figure 1.2). The eyelids are automatically triggered whenever contact is made with the cornea, eyelashes, forehead or eyebrows. As in an early warning system, the alert is sent down nerve paths to activate the eyelids. This diagram shows just a few of the special nerves constituting this early warning system—an example of God's flawless art of creation.
Reflexes are involuntary and rapid responses to various stimuli. The reflex mechanism that activates eyelids when necessary protects the eye against external threats. Stimuli that create reflexes include contacting the cornea, the eyelashes or even the forehead.
A close inspection of the neural network controlling eye reflexes reveals the immaculately fine design of its architecture. For every reflex described above, different impulses are routed from different neural paths. The eye's peripheries are stuffed by such early-warning nets (Figure 1.2).
The brain, evaluating these fast-traveling warnings, dispatches neural impulses to the relevant muscles, routing them without ever making a single error along the chaotic network. Within a thousandth of a second, the warning signal reaches the brain and returns as a command, by which the eyelid closes in time to protect or cleanse the eyeball. The process of identifying the emergent danger and creating different reflexes by means of signals traveling along different neural paths is extremely complex.
For survival, man needs to be informed, with no interruptions, of his ever-changing environment. To satisfy this condition, blinking occupies only a very brief time without disrupting the continuity of perception. Any longer blinking time might cause serious dangers—while driving on the highway, for instance, and not noticing a suddenly appearing truck in time to swerve.
Acknowledging the Granted
Blinking is an involuntary action that is executed thousands of times every day. No one struggles to blink, nor does anyone contemplate why we blink when we do. We take this irreplaceable perfection for granted.
A person can best realize the value of healthy eyes when he awakens in the morning with his eyelids stuck together and filled with sticky mucus. These symptoms belong to an illness called blepharitis, which turns the eyes into a virtual Petri dish—a breeding ground for germs. Blepharitis, an inflammation of the eyelids, initially emerges as swelling and redness of the margins of the eyelids, but advanced cases can lead to small abscesses and ulcers in the eyelid.
Of course, there are many other eyelid illnesses. One of the more common ones is caused by the weakness of the muscles that raise the upper eyelid. As a result, one or both eyelids remain lowered, giving the face a bored, sleepy expression. These tiny muscles' incomplete functioning, also narrows the sufferers' angle of vision, making them see less than they should.5 It is incredible that the cells making up these muscles, which can be seen only through a microscope, are tirelessly in action all through our lives, and almost entirely beyond our control.
We don't need to suffer from a painful illness to understand what a blessing health is. Those who believe constantly thank our Creator for their health. When confronted with disease, they simply ask help from God and then face it with the grace and confidence that the Qur'an requires.
Any blessing you have is from Allah. Then when harm touches you, it is to Him you cry for help.(Surat An -Nahl, 53)
Tears: The Perfect Eye Drop
(Figure 1.3). A diagram of the channels through which tears are released. With its superior abilities, the tear is a miracle in itself. The perfect systems that produce and discharge tears, combined with the delicate balance in its production, provides us with solid evidence, with no room for coincidence.
1- Lacrimal gland (tear gland)
Many people assume that tears are just the salty fluid shed when they cry. But actually, it's a very unusual liquid, with various ingredients serving different special functions.
Primarily, a teardrop protects the eye against germs. The eye is disinfected by lysozyme, a germicidal enzyme, able to kill microbes and tear apart many types of bacteria. The mighty lysozyme is actually stronger than some chemicals used for disinfecting whole buildings—yet miraculously, such a strong substance does not cause the slightest damage to the eye.
It's worthwhile to pause and reflect on this surprising evidence. How can such a powerful substance not harm the most delicate organ? The answer is clear: The tear's powerful disinfectants are created to serve perfectly under the eye's chemical system. The perfect harmony existing at every level and in every aspect of creation is evident in the eye as well.
No artificial disinfectant with similar effects can be applied to the eye. Nor is there any manmade substance that can replace tears—a situation that raises some questions evolutionists cannot answer. How did systems such as the eye and tears, working together in complete harmony, come about at the same time? Clearly, blind coincidences could never have created such perfect structures in the human body. But to illustrate how far away evolutionists are from science and logic, let us for a moment assume the impossible: That coincidences are able to bring about something.
Considering that billions of other disinfectant substances exist, how did accidental processes synthesize one so powerful, yet which causes the eye no harm? Before the trial-and-error "evolution" of such an ideal liquid, how did the eye protect itself? The eye can function only if its present chemical structure and the chemical makeup of tears are working together. Consequently, we must add that this simultaneous cooperation includes the functions of the brain, as well as all the other body parts and processes.
For a moment, imagine that the eyes appeared suddenly, by accident, in an organism complete with all its organelles, tissues, liquids, glands and extensions including the relevant vision center of the brain. That would still be far from being enough for the eyes to function. They require the body's digestion system, the liver and bone marrow for the essential chemical and subsystems that support them. If such systems haven't yet evolved, then the accidental appearance of complete and perfect vision is pointless, since it cannot function. In short, it is not possible for any single portion of the eye to have evolved coincidentally. The eye and all its components were created by God:
Say: 'Have you thought about your partner gods, those you call upon besides Allah? Show me what they have created of the earth; or do they have a partnership in the heavens?' Have We given them a Book whose Clear Signs they follow? No indeed! The wrongdoers promise each other nothing but delusion.(Surah Fatir, 40)
To continue observing this miracle of creation, let's take an in-depth analysis of a teardrop's content.
(Figure 1.4). In the system that produces and ejects tears, a superior design is involved. This diagram indicates the ducts through which tears are emptied into the eye and those through which tears drain. If tears are a fluid that evolved coincidentally, why are there systems dedicated just to producing and ejecting them, in the eyelids and the bone of the skull? If tears evolved coincidentally, how did the ducts come to be? Just like water fittings beneath the ground, these ducts are within the bones, so as not to diminish the beauty of the human face. These are all examples of flawless creation.
98.2 percent of it is water, the rest being urea, found in the same proportion as in the blood plasma and, in lesser proportions, glucose, salts and organic substances,6 of which lysozyme constitutes only a small fraction. In other words, tears are a special liquid that contains different substances in different proportions.
Among the various components of the teardrop, a thin film of fat secreted by glands slows the teardrop's evaporation. This thin film, yet another amazing detail, rules out our eyes' drying out prematurely.
So who has coated the teardrop with a protective fatty layer that retards the effects of evaporation? How could such a specialized formula come about?
Tears are also secreted in accurate quantities, just enough to protect the cornea from drying and to maintain the eyeball's characteristic slipperiness. Thus, when the eyeball rotates, there is no uncomfortable friction between its upper layer and the conjunctiva inside of the eyelid.
If tears were produced at a lower rate, then friction between eyeball and eyelid would create never-ending pain. People suffering from a drought of tears experience a constant burning sensation, as if sand were in their eyes. Their eyes swell and turn red. In the advanced stages of the illness comes the inevitable blindness.
Once any irritating stimulus—foreign particles such as dust, for instance—contacts the eyeball's surface, tear production increases automatically. More lysozyme is secreted for antiseptic purposes, while more sheer liquid is secreted to help in quick disposal of the foreign element.
The fact that the tear glands are equipped with an accurate balance mechanism that controls secretion in precisely the necessary amounts, by itself, is enough of a miracle to refute the claims of coincidental evolution.
No sensible person imagines that a small bottle of eye drops, stamped with its production date and factory, can compose itself via a series of accidents. There must be someone who invented the drop's formula, manufactured the product and packaged it. Anyone who thinks otherwise would have his sanity questioned. Teardrops, possessing features that are superior, are produced with unique chemical ingredients in delicate proportions. There are also the glands that secrete them, sensor-based systems to control secretion, and sensitive canals through which they are ejected. Taking these into account, it's not logical to claim that tears came about coincidentally and were—again accidentally—located in the eyes. Every human being past and present has had tears, which do not differ from person to person. It is Almighty God Who created the eye as a complete whole, for every person, as yet another of God's flawless creations.
The Fine Art of Defense
By now, it's clear that the eye's sensitive structure is granted VIP protection. But it's vital to keep in mind the aesthetic perfection in which this maximum security is realized. The eyes might have been encased in a thick, rough, armor-like shell, but creation presents a more pleasant aesthetic view, with the bone circling the eye, and with eyelids, eyebrows and eyelashes. The result is only one of the countless examples of the unequalled beauty to be found in the creations of God.
He is Allah – the Creator, the Maker, the Giver of Form... (Surat Al-Hashr , 24)
(Figure 1.5). Thanks to its three sets of muscles, the eye is capable of moving in all directions.
The eyelashes attached to the outer edges of the eyelids protect the eye from outside dust and larger particles. When lost or cut, they grow again from the same roots. An eyelash stops growing when it reaches its previous length.
Eyelashes are straight and soft with slight curves towards their tips. This shape is not just attractive, but optimally practical. It is no coincidence, of course, that eyelashes have adopted this unique shape. They attain their curved, elastic form with the help of the greasy secretion from sebaceous glands (known as glands of Zeis) inside the eyelids.7 Without this suppleness, the lashes would be rough as bristles and tend clump together annoyingly with every blink.7
The eyebrows' function is to block the sweat draining down from the forehead into the eyes. The brows also save the eye from reflected or direct sunlight by blocking and obscuring the rays from above. Third, they are one of the most distinctive elements of the human face, beautifully completing the eye's visual appeal.
Say: 'Who is the Lord of the heavens and the earth?' Say: 'Allah.' Say: 'So why have you taken protectors apart from Him who possess no power to help or harm themselves?' Say: 'Are the blind and seeing equal? Or are darkness and light the same? Or have they assigned partners to Allah who create as He creates, so that all creating seems the same to them?' Say: 'Allah is the Creator of everything. He is the One, the All-Conquering.'(Surat Ar-Ra'd , 16)
Muscles Unvisited by Time
Muscles surrounding the eye are among the most active in the body, making possible some one hundred thousand movements a day. Over a lifetime, the average human performs literally billions of eye movements—even while asleep. Despite this heavy, never-ending duty, the eye muscles never complain of fatigue. In fact, few people are even aware of the muscles in their eyes, regardless of their lifestyle or age, which have no effect on the muscles at all.
(Figures 1.6 ve 1.7) The eye muscles, as seen from the front and from behind.
1- The extraocular muscle that moves the eye up
Surrounding each eyeball are six muscles: One pair each for horizontal, vertical, and side-to-side oblique movements (See Figures 1.6 and 1.7). Each muscle in a pair moves the eyeball in an opposite direction. But this is no ordinary partnership that tolerates imperfection. Each member of the pair, as well as all three groups, must work together in perfect coordination so that both eyes turn to the object of interest, such that its image falls on both retinas. If even one of these twelve muscles, in six groups, is not sufficiently precise, focusing becomes a problem and you see double. (To get an idea of how difficult the result becomes, simply press gently against the side of one eye with your finger and try to view any nearby object.)
Apart from the effect of double vision, when the harmony between the muscles is gone one's facial expression is distorted as is the case with squinting.
If the eyes did not possess such muscles, they would remain motionless like a pair of frozen glass buttons. The face would have an unchanging, uncommunicative expression, without any meaning or message. To see anything, we would have to aim the head directly in the direction of the object, costing us much mobility and flexibility in the course of our daily lives.
Conjunctiva: Lifetime Care
(Figure 1.8) A diagram of the eye muscles, seen from the side. They are designed to allow eye movement in every direction. Such a special structure cannot have developed coincidentally, by itself. The eye was flawlessly created by God.
In addition to the tears lubricating and disinfecting the eyeball round-the-clock, the eyes have another liquid maintenance system that secretes greasy liquid to smooth the eye's some hundred-thousand-a-day rotational motions against friction and external particles.
The eyeball consists of many tissue layers one atop the other. The conjunctiva membrane's job is to lubricate the eyeball's surface layer. Conjunctiva is situated between the inner surface of the eyelid and the eyeball, together with another tissue called sclera (commonly known as the white of the eye). This is a firm, transparent membrane that covers about five-sixths of the eye's surface. Both of the membranes are composed of living cells and fed by tiny, invisible veins—a fact that demands attention.
The section of the conjunctiva coating the anterior portion of the eyeball is very movable, easily sliding back and forth over the front of the eyeball it covers.
During secretion from the tear glands, the conjunctiva plays an instrumental role. Covering the two surfaces where teardrops function—the inner surface of the eyelid and the outmost layer of the eyeball—and activating tiny mucus glands embedded within it, conjunctiva supplies tears with the lubrication necessary for smooth, slippery rotation and blinking.
Whether it's a hinge or a car engine, no mechanical device with movable parts can run efficiently without regular lubrication. Forget the grease and oil and soon the engine will burn out. But with the eyeball making approximately hundred thousand movements per day, you don't need to do a thing. Lubrication is provided automatically by the system just described.
If that system were absent or even interrupted temporarily, each movement of the eye would cause unbearable pain. Yet thanks to God's flawless creation, a healthy person will never have such difficulties.
Cornea: The Window of the Eye
The eye is a round sphere, except for the small raised bump at the front, where it receives light. Surrounding this sphere is the sclerotic layer—white as milk, hard and tough, protecting the eyeball's internal tissues. The white area of the eye surrounding the colored iris in the center is only the visible part of this layer.
Suppose that the white of the eye was not hard and tough, but much softer, like jelly. Were this the case, the eye's internal layers would not be protected. Also, any external substances that entered the eye would adhere to the eyeball, becoming difficult to remove and causing potential damage. However, tear drops easily clear the eye of any foreign particles thanks to the fact that the white of the eye is fairly hard.
The structure of this hard white tissue changes suddenly at the center, when it approaches the bulging spot at the front of the eye. This structure, the cornea, is made up of a transparent layer permeable to light. Despite being a continuation of the sclera or white of the eye, it is distinctly separate and possesses a completely different structure (Figure 1.9). If the eyeball were compared to a building, the white of the eye would be the marble exterior; and the cornea would be its single round window.
The reason for the cornea's small size is quite simple: If the eye were completely covered by the thin tissue making up the cornea, it would be effectively defenseless, and would almost certainly wind up blinded.
However, if the white of the eye were to cover the eye completely, including the transparent layer, then light would be unable to penetrate and enter, thus making it impossible for the eye to see. How is it that two distinctly different tissues, lying along the same layer and continuous with one another, are clearly separated by a circular border? Who drew this border?
The cornea's function is to focus (or refract) incoming light, thus allowing it to pass through the lens towards the retina at the rear of the eye. This process refracts some two-thirds of the light needed to focus on an object, while the remaining third is processed by the lens.
In order for objects to be seen clearly, it's crucial that the cornea be always transparent. This is vital because even one drop in it causes misty vision, while alertness is equally important: The eye must be able to detect even the smallest dust particle that may enter.
The cornea owes its perfect transparency to the delicate arrangement of fibers inside it. Any interference will stain the cornea and cloud vision.
Think of the importance of objective in photography—for the eye, the cornea is equally important. So clear that it cannot be seen from a distance, it is one of the most sensitive parts of the body.
The cornea is made up of countless nerves and lymph vessels which, however, do not disrupt vision. The slightest movement around the cornea triggers reflexes that command the eyelids to close. Thereupon, the eyelids swiftly eject anything which may have stuck to the cornea and protect against possible damage by closing over the eyeball.
The cornea is like a window, behind which the eye operates. It is possible, for instance, for wind to blow a sand grain or wood chip into the eye and scratch the cornea. But thanks to its built-in self repair system, the cornea can repair itself.
During the day, the cells composing the cornea are fed with glucose from the tear fluid and, since the cornea contains no blood vessels, with oxygen from the air. During sleep, however, when outside oxygen cannot penetrate the closed lids, the cornea is supplied by the capillaries on the inner surface of the eyelids.
If this precise balance in the cornea were never maintained, we would always have misty vision and never know the meaning of clear sight. Safe to say, the world would be a very different place, looked at it through unclear eyes. It's amazing to think how much this thin layer of tissue does for us.
The cornea is completely isolated from the body, making it easier for surgeons to transfer it from one patient to another. A new body does not reject the cornea, because antibodies in the bloodstream never reach it.
(Figure 1.10) The cornea is the eye's window to the outside world. Its transparency is similar to a window's, the only difference being that the cornea is biological, composed of tissue, whereas windows are made of glass. It is God Who allowed a piece of human tissue to become clearer than glass.
An intensely transparent layer, the cornea allows some 98% of light to pass through, thus approaching the transparency of window glass (Figure 1.10) . Of particular note is that the cornea is a living tissue, made up of cells and constantly fed with glucose and oxygen.
How can a living part of the body be so utterly transparent? How did it acquire this transparency? Even though we are looking through countless capillaries and vessels, how is it that we still see the world so very clearly?
From the divisions of one single cell came all the cells in our body, including the ones in this delicate, transparent living layer of the eye, in the rigid bones, in the kidney tissues and in the blood. What is the power that, with the division of a single cell, can create two structures as entirely different as rock-hard bone and a crystal clear cornea? How did the cells differentiate from one another to that extent? Do they possess the faculties of planning and decision-making to carry out these plans?
Cells, made up of inanimate and unconscious atoms, do not possess such faculties, of course. It is God Who inspires the cells what to do, to form various organs and perform a multitude of tasks.
That the fibers and the nerves making up the cornea are so sensitive again evidences the superior creation. Thanks to a complex early-warning system, this extremely delicate layer summons the eyelid to its defense in the event of danger. But how does that happen? Can the cornea cells really have developed their own life-support system to stay alive, and then made an agreement with the brain for the eyelids to guard them?
Another miraculous aspect of the eye lies in the shape of the cornea. The focusing of light requires calculation, not to mention experience in the field of optics. However, this very complicated process is carried out flawlessly by corneal tissue, which came into being in the mother's womb through the simple splitting of a few cells. Every cornea is angled so as to allow light to enter directly into the retina. Does the cornea have the intelligence to predict this angle, or did each cornea cell attain this knowledge individually? One conclusion is certain: No calculation this complicated was solved through a series of coincidences.
Many other details—besides the cornea's shape that focuses light on the retina, its extraordinary structure providing a clear vision through its fibers, the conjunctiva and vessels of the lymphatic system feeding it, its early warning system—are all flawless, synchronized mechanisms that couldn't have come into existence coincidentally.
The cornea has a most superior design, which can have been created only by a uniquely superior intelligence, whose Owner is God.
O man! What has deluded you in respect of your Noble Lord? He Who created you and formed you and proportioned you and assembled you in whatever way He willed. (Surat Al Infitar, 6-8)
Fluids in the Eye
The inside of the eye is divided into three sections. Of the two chambers toward the front of the eye, the first lies between the back of the cornea and the iris. The rear chamber, on the other hand, is a small gap between the iris and the lens. A wide space beyond the eye's center and the lens, often referred to as the dark chamber, is filled with a clear, colorless fluid known as the vitreous humor or the "glassy fluid."
This jellylike fluid is enclosed in a sac between the lens and the retina and holds the retina in place. The back chamber (between the iris and the lens), and the front chamber (between the iris and the cornea) are also filled with a watery fluid. Produced by the ciliary body, this fluid feeds both the cornea and the lens, for neither has access to oxygenated blood vessels.
To nourish the components of the eye, this fluid contains a large number of chemicals and minerals, including salts, sugars and disinfecting substances drawn from the blood vessels and then mixed into the fluid through microscopic pumps in the ciliary body.
This fluid, which gives life to the eye, doesn't remain stationary, but is constantly circulating in a manner similar to the basic flow of water in the oceans, in which the colder water flows deeply below, while warmer currents flow closer to the surface.
Along with delivering nutrients and disinfectants, this fluid also expels waste matter in an exceptionally delicate, microscopic manner. Another of the fluid's functions is maintaining internal pressure, so as to keep the eyeball distended and stable.
Pressure within the Eye
The eyeball can be considered to be a sphere with restricted flexibility. The gelatinous fluid the sphere contains gives it a certain amount of internal pressure, determined by the quantity of the aqueous humor—which in turn is produced by the ciliary body. After being secreted, first it flows into the back chamber, then through the pupil into the front chamber, before being absorbed by tissues between the back of the cornea and the iris. If the rates of production and absorption become unbalanced, this can affect the eye's internal pressure.
When these two rates are equal, however,—that is, when the amounts of the produced and absorbed aqueous humor are equal, due to the continuous flow of fluid—the volume of fluid within the eye does not change. But if the production increases while the flow of absorption is reduced or somehow obstructed, pressure within the eye builds.
To recap: This fluid is produced at a discrete quantity, and the same amount of excess is absorbed. More importantly, this process is constant, ongoing in every human eye.
In this respect, the eye is similar to an aquarium that's filled at one end while it empties at the other: If the flow of water is blocked, it will overflow. However, if the water from the source is cut off, then the aquarium will dry up. Likewise, the amount of liquid contained in the tanks in many industrial and chemical plants, is maintained with the use of computerized control systems. These systems, demanding highly delicate measurements and calculations, are programmed and supervised by specialized engineers. Any disorder in the system can lead to catastrophe.
To ensure the balance in such a small volume as the fluid within the eye, measurements and calculations need to be even more delicate and precise. The slightest inaccuracy, even smaller than mere millimeters, would result in blindness. In a healthy eye, however, these calculations and the cycle of fluid in the eye remain balanced throughout a lifetime. That the fluid exists is a miracle, but the fact that this very fluid is carefully produced and accurately absorbed is an even greater miracle that one should reflect on deeply.
But what if the sensitive balance of eye fluid is disrupted, as in an overflowing aquarium? When the fluid is not absorbed properly or the production of fluid is increased unnecessarily, the result is a quite painful condition known as glaucoma, marked by abnormally high pressure within the eyeball. This causes intense discomfort and sometimes loss of vision. The eyeball inflates like a balloon ready to burst, and the smallest impact can rupture it.
As with most other bodily processes, it's natural not to be aware that your eye fluid is constantly being secreted into, and absorbed out of, your eye—until you read this book. Some people, however, learn about the presence of this fluid the hard way, by developing glaucoma. Like any critically ill person, they realize how much of a blessing good health is and, as a last resort, turn to God.
You differ from those in such a situation, in that you learned of this miracle by reading this book, rather than through developing the disease and suffering the pain. But this doesn't mean you'll never experience pain in your life. If God desires it, He may impose such a condition or even a more painful one on you at any time, so that you may remember the value of good health and be thankful. But the truly acceptable way is to turn to God without waiting for an illness—to be grateful to Him, and to remember and glorify Him at all times.
What will those who invent lies against Allah think on the Day of Rising? Allah shows favour to mankind but most of them are not thankful. (Surah Yunus, 60)
The Iris: A Light Regulator
Placed behind the cornea, the iris protects the retina from unnecessary illumination. Muscles placed on either side let the iris change the diameter of the pupil, according to light intensity (Figures 1.11 and 1.12). One of these muscles contracts the pupil in bright conditions, while the other group, radiating from the pupil like the petals of a daisy or the spokes of a wheel, expands the pupil in darker conditions. In this way, the amount of light entering the eye is kept constant.
If this were not the case, and if the pupil size weren't regulated according to the changing amount of light, our eyes would then take much longer to adjust to even the slightest changes in light, making us unable to see for longer periods of time.
There are two reasons for the dazzling sensation we experience upon moving from a bright environment to a darker one. First, in the dark, the retina's sensitivity increases. Secondly, it takes a moment or two for the iris muscles to activate. When suddenly we move from a dark environment to a bright one, the pupils remain wide for a short instant. But within 0.04 to 0.05 seconds, the pupils contract with the help of the iris muscles; which is maximized in a tenth of a second.
If this interval were any longer, we would spend a considerable period of time unable to see. But thanks to our eyes' perfect structure, we can see our surroundings in changing light with minimal discomfort.
The iris also contains pigmented cells that give the eye its distinctive color. Just as the skin, the iris's color depends on the type and amount of pigment. Light-skinned people tend to have blue, hazel or light gray eyes, whereas dark-skinned individuals typically have dark brown or black eyes.
What we call the pupil is actually an opening at the center of the iris and can rapidly expand or contract to adjust the intensity of light entering the eye. Generally, both eyes receive the same amounts of light, but any change in the amount entering one eye will affect the pupil of not only that one eye, but the other as well.
(Figure 1.11) The iris, which controls the amount of light entering the eye, and its surrounding muscles.
The amount of light entering the eye can be multiplied nearly thirty times according to how wide the pupil is. The change in the amount of light produced by a flash camera in 0.1 seconds, for example, causes the pupil to instantly adjust its size and admit less light.
Upon light's entering the eye and hitting the retina, nerves transmit a signal to the brain. The brain is not only informed of the light's existence, but also 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.
At first glance, the line of communication between the iris muscles and the brain seems like a normal biological link in the body. But when analyzed in detail, this link can be seen for the miracle it really is.
The measurement of outside light intensity, the immediate relay of signals to brain, and the brain's consequent adjustment of the iris muscles to regulate the light entering the eye is a complicated process which is amazingly conducted in the brain of every person who has ever lived, with the exception of the congenitally blind. This is nothing short of a miracle, and a way for us to comprehend our Creator's power and knowledge and realize His true measure. It is the responsibility of humans to give thanks to God, Creator of the universe, and also to indulge ourselves in acts which will please Him. In one verse of the Qur'an, God describes those who ignore His signs as wrongdoers:
Who could do greater wrong than someone who is reminded of the Signs of his Lord and then turns away from them, forgetting all that he has done before? ... (Surat Al-Kahf , 57)
(Figure1.12) The iris controls the amount of light entering the eye by adjusting the size of the pupil. It contracts in strong light (a), thus reducing the amount of light entering the eye. In dimmer light, the pupil expands (b) to allow more light to enter. Thanks to a complex and advanced system, the amount of light entering the eye is calculated and the pupil size adjusted, in a tenth of a second. Is it really possible that a collection of atoms came together coincidentally to create such a perfect system?
Adjusting to Brightness and Dark
(Figure1.13) A diagram of the muscles that are triggered upon orders from the brain, and can either contract or relax to change the pupil size. This way, a constant intensity of light always enters the eye. The second diagram to the right depicts the same muscles, but magnified.
You can test for yourself all the details about the eye we have described up to this point. When you first enter a dark room, it's difficult to distinguish different objects within. This is because at that moment, your retina's level of sensitivity is very low. But this sensitivity can multiply itself by a factor of ten times in less than a minute, allowing your retina to respond to gleams only a tenth as powerful as before. In twenty minutes, the retina can adjust itself 6,000 times, and in forty minutes, nearly 25,000. The eye can increase its sensitivity to a maximum of between 500,000 and 1,000,000 times. This factor is adjusted automatically, according to the surrounding brightness in the environment.
In order for the retina to register an image, it must determine the dark and light spots upon the object being viewed. For that reason, sensitivity must be adjusted so that the receptors respond always to the brighter points, not the darker ones.
Imagine, for example, that you're stepping out into bright daylight, having just sat through a film at the cinema. Everything you look at, even spots that normally appear dark, will seem unusually bright and because of low contrast you will see a lot of light colors. This is inadequate vision, of course, and fixes itself once the retina adjusts itself so that its receptors are not overstimulated by the darker spots in your field of vision. When you walk into a darkened room, now your retina's sensitivity is very low and therefore, even the brighter spots on objects cannot stimulate it. But once your retina adjusts to the dark, the bright spots do register. The retina can adjust to extreme light and dark. And even though sunlight is 30,000 times brighter than moonlight, your eye is able to adjust and see in environments illuminated by either source of light.8
The Lens: The Eye's Focusing Mechanism
The lens, situated immediately behind the iris and the pupil, breaks down incoming beams of light and focuses them on the retina. Made of protein fibers, the lens is transparent, hard but slightly elastic and yellowish in color. Similar to a magnifying glass, the center of the lens is convex in structure.
With the aid of muscles surrounding it, the lens is able to change shape, allowing it to adjust itself according to the angle light comes in, ensuring it is always directed onto the retina. When you look at a point close to your eyes, muscles flex your lens into a more convex position. But when you view a distant point, the muscles relax, stretching the lens into a flatter configuration and thus clarifying the images of distant objects.
Like the cornea, the lens contains no blood vessels, and so it is nourished by the eye fluid.
Interestingly, the lens never stops growing throughout a human's life, although the rate of growth does slow down with age, leading to loss of its elasticity. Certain cell layers become isolated from the rest of lens and are consequently deprived of food and oxygen; a process which eventually kills these cells. The lens begins to harden. It becomes more difficult for it to curve itself and, as more and more cells die, it loses its ability to adapt itself to viewing nearby objects. This is why the elderly so often find themselves reading the newspaper at arm's length and using glasses to support their farsighted vision.
(Figure 1.14) Fibers connected to the muscles responsible for expanding and contracting the lens. Sensitive adjustments made by these fibers allow incoming light to be focused in on the retina at the proper angle.
One should reflect on the fact that the eye lens doesn't maintain its capabilities for an entire lifetime. Just like other organs in the body, the lens of the eye can't survive the aging process and loses its originally perfect structure. It is a sign, God's way of reminding us that we are getting old. We are reminded of such facts as that life upon Earth is only temporary and that our human bodies will perish one day. Only those who truly use their minds can see God's such warnings wherever they look.
The lens in the eye works in a way similar to the lens in a camera. To get the clearest picture, it is necessary to adjust the camera lens either manually or automatically so as to focus light upon the film, depending on the distance. When you look at an advanced camera close-up, you'll see that when focusing, the lens revolves around its own axis. While this process takes place, the picture in the camera's view finder becomes blurred.
Even though the functioning of the eye was imitated in the construction of camera lenses, the eye's lens is countless times more developed. In particular, its dimensions are smaller than a camera lens. The lenses used in cameras reached their present level of technology after years of research. Scientists have still not succeeded in making an optical system as perfect as the eye.
Your eyes do not frequently break down, the way a camera does, and have no need of maintenance. Cameras are produced by expert technicians in special factories, using many different materials—plastic, metals, glass, etc.—according to engineers' designs. The eye, on the other hand, forms in the mother's womb as the result of the division of a single cell.
If you tie a camera atop your head and run or walk while filming, the resulting image will bear traces of shaking and slippage. Yet as you walk your eyes, which register images just like two cameras fixed to your head, never make you feel uncomfortable. There is never any shaking or slippage in the images you see.
Another question that may come to mind is why the muscles forming the lens seek to make light fall upon the retina. No one ever thinks, "I must make the light entering my eye fall onto my retinal layer so I can see properly." Most people are quite unaware of their retinas and lenses. Yet the whole day through, these tiny organs perform functions requiring unimaginable calculations. In order for the lens to do such things by itself, it needs to know the task of the retina, what vision entails, the structure of the brain, and the purpose served by photons. Only in this way can it focus the light falling upon it onto the retina.
Naturally, neither the lens nor the cells comprising it have any will of their own. The lens, cornea, iris, retina, their cells and the muscles around them, and the brain all carry out their functions in ways inspired by God, and by His will.
The retina receives the beams of light refracted by the cornea and the lens, and constructs the image we see. This image is then sent to the brain in the form of electrical signals (Figure 1.15).
The retina serves exactly the same purpose for the eye as the film does for a camera. In the same way that photographic film lies behind the lens, the retina lies at the back of the eyeball and there forms an image of the object being focused on.
Once a camera has recorded an image, the film is moved onto the next unexposed empty frame space so that another photograph may be captured. The retina, on the other hand, receives countless images every second, but doesn't have to change or be replaced, because the retina is capable of renewing itself. It displays and uses countless images throughout an entire lifetime without deteriorating or breaking down.
The retina is composed of eleven separate, microscopically thin layers (Figures 1.16 and 1.17). Images fall on the ninth layer, an area almost 1 millimeter wide. It's quite amazing to consider that entire kilometers of landscape can be focused down upon this tiny point. No one should forget that his whole world is recreated within this tiny area; that thanks to that area, he has perceived the existence of everything he has ever seen; and that ultimately, that point is nothing more than a tiny concave layer of cells.
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; a ratio of nearly 20 rods to every cone.
But the only difference between these two cells is not their shape or their number. Each type of cell has a different method of perception. Rods can respond to even the weakest beams of light. For the cones to respond, however, more powerful light is needed.
Rods can respond by forming only a black-and-white image, depending on the light received from the objects. They are designed to function even in environments where light is minimal. However, they do not perceive the details or colors of the objects.
When we are observing the stars at night, or trying to find our seat in a darkened movie theater, we succeed thanks to the images generated by the rod-shaped cells in our retina. We are able to make out objects' shapes, but not their colors. This is why, as the saying goes, "In the dark, all cats are gray"—in the dark, all objects seem to be black and gray in color. 9
A little earlier, we mentioned that the rods and cones convert light waves into electrical energy. This conversion process is a most complicated one, but how does it take place? How, why and by what logic does a mere cell convert light energy to electricity? How did the cell first acquire the knowledge to complete such a process? How did it acquire its unique structure to carry out this process? Taking into account that these cells are divided into separate groups according to their function of perceiving shape and color, how did they allocate separate tasks to themselves in the first place?
By itself, on its own, a cone-shaped or rod-shaped cell is of no use. Were it not for their excellently organized placement across the retina, the network of nerves connecting them with the brain, components of the eye such as lens and cornea directing light towards them, or the fine capillary vessels feeding them, not even several thousand of these cells would allow us to see. Moreover, were there no brain to interpret the signals sent by these cells, there would be little reason for the presence of these cells at all. This system, with all its parts, must have been present from the moment mankind first appeared on this planet. It's not possible for certain parts of this system to have developed at later stages, because in the meantime, man would be unable to see. The first human's retina was no different from the retinas of humans living today.
It is a miracle enough for just one single cell to convert light into electrical energy. But there is an even greater miracle—millions of these cells, all working together for a common purpose. It is clear that these cones and rods, together with other components of the eye and the brain, were created by God. It is God Who created humans with a flawless design. As God tells us in a verse, there is no other god besides Him:
He is the Living – there is no god but Him –so call on Him, making your deen sincerely His.Praise be to Allah, the Lord of all the worlds. (Surah Ghafir, 65)
The Four Perceptions of the Retina
The retina is capable of interpreting four different properties of vision: Contrast, color, light and shape.
Under darker conditions, the rod cells are able to perceive more light than do the cone cells. Thanks to the rods, we can see at twilight, for instance. In brighter conditions, however, the cone cells come into play. This is why the eyes of nocturnal animals have a large amount of rod cells.
Cone cells play a large part in perceiving the shape of objects. The area of most acute vision of shapes is the fovea centralis, which has the highest concentration of cone cells.
The ability to differentiate between areas that are not clearly separated, but have slightly different amounts of illumination, is extremely important. Loss of ability to distinguish contrast is common in a number of illnesses, 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.
As mentioned earlier, it is in itself a miracle that the retina can convert light into electrical energy. But the miracles do not end there. The method by which images formed on the retina are sent to the brain is just as extraordinary. The retina doesn't transmit a picture to the brain as a whole. First the retina breaks up the picture, and then these pieces are reassembled in the brain. The left-hand side of an image ends up on the right-hand side of the retina, and vice-versa. The pieces are transmitted separately in less than a tenth of a second, to be interpreted in the brain. What's been described here is a brief summary of what actually takes place in the retina.
The better to understand these miracles, let's examine the process in closer detail. To see an object, the light energy entering the eye must first be converted into nerve impulses. Beams of light cause a physical stimulation, which triggers chemical and electrical reactions. This chain of reactions, ending with a vision of the object, depends on a Vitamin A-based pigment called rhodopsin, found in the rod cells.
Light striking the retina bleaches the rhodopsin. As a result of this bleaching, a chemical substance forms that stimulates the nerve cells. Rhodopsin loses its property in bright light, but reforms again in darkness.
When you enter a movie theater, for example, at first you will be unable to see clearly, because at that moment, there is not enough rhodopsin present in your eyes. Once more rhodopsin is produced, your vision clears. You won't be able to see clearly until enough rhodopsin is produced; but once the rhodopsin balance is maintained, you'll find it easier to distinguish objects in the dark.
Once you leave the cinema and walk back out into the sunlight, however, rhodopsin breaks down rapidly, sending many signals to the brain at once. Objects in your vision become unusually bright, making it difficult to see. In bright light, rhodopsin breaks down faster than it is synthesized. That's why your vision seems defective for a while. Again, rhodopsin is why your eyes are dazzled by the sunlight and the snow. Once most of the rhodopsin is deformed, fewer impulses are transmitted to the brain; the eyes have become light-adapted. 10
Rhodopsin, when needed, is produced at just the right amount. It works in conjunction with the other parts of the eye, allowing us to see easier in the dark. But who first decided to produce this substance? Did eye cells, unable to see in the dark, spontaneously gather and decide to make a substance that enhances vision in the dark and breaks down in brighter light? Supposing that they did so, then who designed rhodopsin's physical and chemical structure? And how did the eye cells gain all the genetic information they need to work with rhodopsin?
There are far more details to the process of seeing than we've described in these few paragraphs. But rhodopsin by itself is an accurate demonstration of what a miraculous system the eye truly is. Clearly, its cells didn't develop rhodopsin on their own. The eye, with its delicately calculated system, is a creation of God.
The Primary Colors
As we mentioned earlier, the cones within the retina are those cells that perceive colors. There are three separate groups of cones, each of which reacts to certain specific wavelengths of light—namely, blue, green and red.
These are the three primary colors found in nature. Other colors come about through the varying combination of these basic three. For example, if you were to mix red and green light, we would get yellow. The pigment cells work following the same principles: When the cones sensitive to red and green light are alerted to an equal degree, you perceive the color yellow. If the cones sensitive to red, green and blue are alerted to an equal degree, we see white. When the cones that perceive all three colors are alerted at differing degrees of intensity, then it is possible to see any other color in existence. But our knowledge in this field of chromatics is pretty much limited to the above, and is currently nothing but a theory. It is still unknown, for instance, how the brain decodes the signals sent from the retina.
As you can appreciate, the process of color separation is very complicated. But as an aid to understanding it, consider an example from modern technology. Color television screens work in a manner similar to the eye's color separation system. On the screen, colors of different wavelengths are placed very close together, such that a magnified photograph of the screen would show that the TV picture is made up of miniscule red, green and blue dots. When we draw back a little distance from the screen, these colors merge to create the various shades we're used to seeing.
To assemble the pictures we all see with our eyes, a large number of complicated color adjustments are constantly effected. The intensity of signals sent by millions of cone cells must be delicately adjusted, then decoded by the brain. What's more, this is not a process that takes place in the bodies of only a few for short periods of time. Every human perceives billions of images over a lifetime, and color adjustments are made for every single one.
Acuity of Vision
Whether the sight be a speck of dust or a vista from the summit of a mountain; any vision—from thousands of kilometers to a few millimeters in size—eventually focuses upon a yellowish spot, only one millimeter square, called macula lutea.11
At the central point of the macula, only about 0.4mm wide, the retina thins and contains a slightly depressed area called the fovea centralis. At the fovea's center, the sensory layer is composed entirely of cone-shaped cells. As mentioned earlier, cone cells can differentiate between visual details. Here, therefore, at the point where vision is at its clearest, the colors, shapes and depth of vision are concentrated. Outside the fovea, visual acuity can drop by up to 1,000%.
When you examine an object carefully, your eyeball's active muscles move and adjust themselves so that light can be concentrated upon the fovea.
Someone with maximum visual acuity can discern, from ten meters away apart, between two bright points as big as a tip of a needle, separated by only a few millimeters.
The Choroid: A Vein of Life
Between the sclera and the retina lies the dark-brown vascular coat of the eye known as the choroid. It is composed of blood vessels—millions of capillaries—through which the cone and rod cells are fed.
By itself, the choroid is effective evidence that the theory of evolution is incoherent and laughable—additional proof of the miracle of creation.
Without the choroid, which feeds every cell in the retina, the eye would lie completely useless. It's not possible for such a layer to evolve over time, simply because most other components of the eye could never survive without it, however miraculous they may be in themselves.
As we have pointed out repeatedly, the eye is composed of countless different sections and layers that include the cornea, sclera, iris, pupil, lens, eyelid, nerves connecting the cornea to the brain, and countless other structures. All of them can work together only as a whole—they are simply too specialized and interdependent to have evolved on their own. In order for the eyes to see, all those other structures and tissues must be present at the same time, working in complete and perfect synchronization.
This observation renders completely irrelevant the evolutionary theory that humans reached their state today through a series of coincidental mutations. Such a perfect organism cannot have come about by means of any power other than creation. The choroid layer feeds the retina, in an unrivalled example of God's artistry of creation.
[He is] the Originator of the heavens and Earth. When He decides on something, He just says to it, "Be!" and it is. (Surat Al-Baqara, 117)
The Paint of the Retina
So that it can stimulate the cone and rod cells, light entering the eye passes first through two layers—one of which is the melanin layer, containing a black pigment. Melanin absorbs any light passing through the retina, thus preventing it from reflecting back and away. Without the melanin layer, light would scatter itself around inside the eye, and no coherent images could be formed. In other words, the retina is lined with black pigment called melanin—just as the inside of a camera is black—to lessen the amount of reflection.
So that it can stimulate the cone and rod cells, light entering the eye passes first through two layers—one of which is the melanin layer, containing a black pigment. Melanin absorbs any light passing through the retina, thus preventing it from reflecting back and away. Without the melanin layer, light would scatter itself around inside the eye, and no coherent images could be formed. In other words, the retina is lined with black pigment called melanin—just as the inside of a camera is black—to lessen the amount of reflection.
So if we asked the same question for the eye, what would the answer be?
How can the structure of the eye, far superior to a camera, possibly have come about by means of a series of coincidences? Quite the opposite is true—the eye was created by a superior mind.
It's interesting how some individuals will marvel at the technology behind a simple camera, but still insist that the eye was not similarly created. Easily fooled by the forgeries of Darwinism, they utterly deny the true Creator.
To prove the flawlessness of His creation, God has left a number of lessons for us humans to dwell upon. For example, the importance of the melanin layer is truly dramatized in a disease called albinism. Sufferers of the condition lack normal pigmentation, with the result that light reflects all around inside the eye, especially under bright conditions. This brings with it an uncomfortably bright vision.12
(Figure 1.21). The visual field is the area that you perceive when looking straight ahead and not moving your eyes. From each eye, the two nasal fields of vision (towards the nose) overlap when the eyes look straight ahead. The images from both eyes are combined in the brain, resulting in perfectly stereoscopic vision..
1. The visual field
The total angle that the eye can take in without moving the head is called the visual field. As you can prove for yourself, it is at its widest at the edges, and narrowest towards the center (Figure 1.21). This field prevents the prominence of the nose from interfering with our vision.
What if the visual field did not narrow towards the center? If this were the case, the nose would become an immovable obstruction to our vision. We would be forced to look at our noses all day, constantly. But thanks to this distinction, given to us by God, the nose causes us little discomfort on a day-to-day basis.13
The Identity of the Eye
Fingerprints are a popular means of identifying people. And just as with fingerprints, the pattern of every person's iris is different, thanks to the varying arrangements of connective tissues, tissue fibers, muscle lines, blood vessels, rings, color, and stains within the iris.
Every one of the billions of humans on the planet possesses a different eye pattern. No pair of eyes are the same, not even on the same individual.
2. Jillyn Smith, Senses and Sensibilities, Wiley Science Edition, New York, 1989, p. 55
3. "Bell's Palsy," Neurology Channel, September 26, 2003; www.neurologychannel.com/bellspalsy/treatment.shtml
4. Daniel Vaughan, MD, Taylor Asbury, MD, General Ophthalmology, translated by Unal Bengisu,LANGE Medical Publications, California, 8th edition,p.144
5. "Drooping Eyelid (Ptosis)," Medical Content Reviewed by the Faculty of the Harvard Medical School, Health A to Z; http://www.intelihealth.com/IH/ihtIH/WSIHW000/9339/9845.html
6. Daniel Vaughan, MD, Taylor Asbury, MD, General Ophthalmology, translated by Unal Bengisu, LANGE Medical Publications, California, 8th edition, p. 77-78
7. Jillyn Smith, Senses and Sensibilities, Wiley Science Edition, New York, 1989, p. 55
8. Arthur C. Guyton, Textbook of Medical Physiology, Harcourt International Edition, 10th edition, 2000, p. 583
9. Jillyn Smith, Senses and Sensibilities, Wiley Science Edition, New York, 1989, p. 62
10. Ibid., p. 63
11. Arthur C. Guyton, Textbook of Medical Physiology, Harcourt International Edition, 10th edition, 2000, p. 573-574
12. "Albinism," March 1, 2002; http://www.wcs.edu/phs/academics/faculty/cousineau/publish/Albinism/Albinism.htm
13. Meliha Terziolu, Fizyoloji Ders Kitabi (Textbook of Physiology), vol. 1, Cerrahpasa Tip Fakultesi Yayinlari, Istanbul, p. 492