The eye is a miracle as it is. Even though we have a rough understanding of its basic anatomy, we are confronted with a much more complex miracle when we venture into the intricacies of its anatomy and the physiology of the act of seeing. We have theories about the many details – such as the perception and representation of mental images and their storage in the memory – but we still do not exactly know how the act of seeing works.

One of the most mysterious layers of the eye, the retina has an elaborate structure with a slew of functions. Not every eye surgeon dares touch the retina, which houses the most sensitive and special cellular layers. It is a three dimensional, crescent-shaped structure located in the back of the eye and is made up of ten super thin layers of cells that span the exterior part of the eye abutting the veins and its interiors.

Retinal layers

The ten layers that form the retina are:

1. Pigment (coloring matter) layer
2. Layer of rods and cones
3. External limiting membrane
4. Outer nuclear layer comprising rod and cone cells
5. Outer plexiform layer
6. Inner nuclear layer
7. Inner plexiform layer
8. Ganglion layer
9. Nerve fiber layer
10. Inner limiting membrane

These layers are incredibly sensitive and elaborate, and studying their intricate structure give a sense of awe.

Light for sight

Light first arrives at and penetrates through the cornea, the living glassy layer at the outermost layer of the eye. It then goes through the frontal fluid (aquesous humor) and the aperture called the pupil (pupilla). It hits the internal wall of the retina (the inside of the crescent) after passing through the lens in the eye and then the optic fluid filling up the chamber in the back. This alone is an interesting fact because it is much later that the light that gets to the retina reaches the layer of sensitive cone and rod cells, which perceive light. As these cone and rod cells are lined one after another for their protection, light reaches this outer layer of the retina after the ganglion cells, retinal layers and nuclear layers. Such an alignment leads to a reduction of acuity in the peripheral regions of the retina.

We have theories about the many details – such as the perception and representation of mental images and their storage in the memory – but we still do not exactly know how the act of seeing works

The central pit (fovea centralis)

The inner layers at the center of the retina, on the other hand, are drawn to the sides to prevent any loss in visual acuity. Resembling a pit, this section is much thinner than the periphery of the retina, so the layers that are likely to obstruct the passage of the light, and hence reduce visual acuity, are aligned specifically to allow light to directly hit cone and rod cells. Besides, cone cells, which are in charge of exact, colored sight, exist in this region, whereas rod cells, which are in charge of rough and uncolored (black and white) sight, do not.  The central pit where visual acuity is at its highest is for keen, colored, and exact sight. Why then is the rest of the retina not created for acute sight and why is this small section equipped with this ability?

As it turns out, if the entire retina had the ability to see keenly then it would not be possible for the eye to focus on a spot and accurately distinguish it from surrounding objects. If we could see the entire page of a book at a glance, for example, the lines would mix up in our brain. We would not be able to understand what we are reading. We normally start reading from the top of the written page and continue line by line as we focus on and take in each word. Our brain then can focus and perceive a single word accurately by restricting keen perception of the surrounding area.

The retinal pigment layer

The color black is known to absorb, not reflect, light. Thanks to such absorption, the layer made up of black pigments (melanin) or dyes prevents the reflection of light, which is crucial for visual acuity. This black substance functions like the black dye in the bellows of old cameras. If there were not any layer to absorb light, light would scatter off the wall inside the eyeball, thereby obscuring the sharpness between light and dark spots, which is essential for the formation of a clear image, and producing a blurry image due to the overall illumination of the retina.

People who lack this melanin pigment as a result of a genetic defect (Albinism disorder) have white hair, and they are oversensitive to light because the colored iris layer of the eye does not contain the melanin pigment, which refracts light. When an albino person enters a bright area, the light that hits the retina is reflected in all directions through the pigment-lacking retina and the white surfaces of the rigid layer underneath (sclera). Therefore, a ray of light that would normally stimulate a few cones or rods is scattered everywhere, stimulating all or most of the light receivers. As a result, visual acuity in albinos can only be between 20/100 and 20/200, even with the help of the best optical correction, which is a low value compared to 20/20 in normal sight. The joke that rabbits do not wear glasses because they eat carrots is based on the high concentration of vitamin A in this pigment, or black dye, layer of the eye. Indeed, vitamin A is a crucial factor for the health of these pigments.

Layer of rods and cones

Composed of 127 million light-sensitive receivers (photoreceptors), the retina is 0.2 mm thick at the yellow spot (macula lutea), where the image forms most clearly, and 0.1 mm thick at the edges of this area. The 120 million cylindrical rods in the retina are in charge of black and white sight (at twilight), and the 7 million tapered cones, of colored, colored and exact sight. The more common cylindrical rods are 50 µm (microns) in length and 1-5 µm in thickness. The less common cones are 40 µm (microns) in length and 3-5 µm in thickness.

The concentration of the cones increases toward the center of the retina and decreases toward the edges, to be outnumbered by the rods. In the cytoplasm of the cones and rods are stored substances that are sensitive to light (photosensitive) which break up when light contacts them and produce electricity in the cones and rods. In rods this chemical is called rhodopsin. In cones, on the other hand, are three substances corresponding to the colors red, green, and blue that are sensitive to the wavelengths of colored light. To be more exact, there are three separate cone cells that include one of these three substances. Chemically, the photosensitive substances in the cones are a little different from rhodopsins.

The destruction of rhodopsin by light energy

Rhodopsin, along with the color substances, fills up about 40% of rods and cones. They are made up of a protein called scotopsin and a molecule called retinene that is derived from vitamin A. When light energy is absorbed by rhodopsin or the color substances, rhodopsin starts to fade in as fast as one trillionth of a second. The underlying reason for this is that the electrons in the retinene (vitamin A) part of rhodopsin is activated by light, which alters the shape of the retinal molecule at a mind-boggling speed (one trillionth of a second). Extremely complicated and precise chemical and physical changes then take place. It is very difficult to monitor all of these biochemical changes, and they require specialization [1].

The regeneration of the rhodopsin destroyed by light

Rhodopsin destroyed by light is regenerated in the dark. Only in the twentieth century did we manage to partially identify the mechanism by which molecules with very specific geometric shapes decay at incredible speeds only to be regenerated later. It is wondrous that such knowledge and power are present in the cell allowing the light energy to destroy the molecule and the regeneration process is launched. The circulation between destruction and regeneration of the molecule continues throughout a lifetime [2]. Vitamin A is assigned a crucial task in the generation of rhodopsin in the dark. All-trans retinal is converted in the retina into all-trans retinol, which is then converted into 11-cys retinol with the help of an isomerase enzyme. Finally, 11-cys retinol is converted into 11-cys retinal, which in turn combines with scotopsin to form rhodopsin. Vitamin A is present both in the cytoplasm of rods and in the pigment layer of the retina. In this way, vitamin A is kept in reserve to be used for generations of new retinal. If, on the other hand, there is an excess of vitamin A in the retina, the excess amount is converted into retinal, by which the amount of light-sensitive pigment in the retina is lowered.

Night blindness

Night blindness appears in anyone who suffers a serious deficiency of vitamin A. Because there is a lack of vitamin A to be converted into retinal, rhodopsin amounts decrease dramatically. This disease is called night blindness as it is noticeable only in the dark or at night and does not affect sight during the day, due to the reduction of light for proper seeing. In daylight, however, cones can still be stimulated despite a similar decrease in color pigments. Night blindness typically only appears in people that have a low vitamin A diet because huge reserves of vitamin A are normally stored in the liver to be used for the eyes.

The human retina contains 400,000 light receptive cells per square millimeter. For comparison, this number is 680,000 in the retina of the owl, which needs to perceive even the slightest glow when hunting in the night. As another example, with 397,000 such cells the amount in the cat’s retina is almost the same as ours.

An average of 130 sight cells in the retina are connected to a ganglion (nerve node) cell. Constituting the nerve of sight, or the optic nerve (nervus opticus), every nerve fiber is connected to a ganglion cell. It takes about 15-60 seconds for the retinal optic nerves to adapt from dark to light, while it takes as long as 30-45 minutes to adapt from light to dark. The visual range of optic cells, i.e. the lowest and highest amount of light the eye can perceive, is between 10^-7 and 10⁶ nanometers. Light that is below or above this range is invisible to us because of its insufficient or overwhelming wavelength.

All the colors we see in the world are named according to the wavelengths absorbed and reflected by optic cells. The spectrum of the optic cells lies between red and violet, which is the limit of visible light for humans. Light with a wavelength of 400 nm is perceived as violet, while light with a wavelength of 760 nm is perceived as red. Our eyes cannot see infrared and ultraviolet light. Some animals, however, are known to see light beyond these limits. For example, we know that bees can see certain shades of ultraviolet light, which helps them find flowers to pollenate.

Clearly, it is not an easy job to elaborate on the divine art manifested in such a small area as the retina. Complicated structures and reactions that require specialization even to comprehend keep taking place smoothly every moment we look around. At least we can be thankful for this amazing gift of vision given to us free of charge so that we can recognize the universe.

Notes

[1] It causes the cys form of the molecule to change into the all-trans form. Although the all-trans form has the same chemical structure as the cys form, its chemical structure is different in that it is a flat rather than angular molecule. With the impact of light photons, the position of the molecule in space changes, yet its chemical composition remains the same. Because the position of the reactive regions of all-trans retinal no longer fit in with the reactive regions on scotopsin protein, the molecule is pulled off. The product that forms at that moment is batorhodopsins, which is a partly destroyed combination of all-trans retinal and scotopsin. An extremely unstable compound, batorhodopsin turns into lumirhodopsin in nanoseconds, into metarhodopsin I in microseconds, into metarhodopsin II in about one millisecond, and finally into decomposed products, scotopsin and all-trans retinal much more slowly (in seconds).

[2] The first step in the reformation of rhodopsin is the recycling of all-trans retinal into 11-cys retinal, which requires ATP energy and is catalyzed by retinal isomerase enzyme. Once created, 11-cys retinal combines with scotopsin to form rhodopsin.

Sight is one of the most miraculous things for which we can never be thankful enough. Our eyes serve as windows connecting our inner life with the outer world. In “Retina the Mind-Boggler,” Dr. Serranur explains the intricacies of the eye’s physiology. Despite many theories and scientific advances, we still don’t completely know how sight works. Scientists do know that the retina plays a crucial role in this very complex structure – so important, in fact, not every eye surgeon dares touch it.

Stem cell research has expanded our understanding of biology and medicine to once-unimaginable levels. Among the treatments for which the initial findings are promising are paralysis, Parkinson’s, diabetes, and even blindness. Despite these groundbreaking discoveries, many are not sure about how far we should go in genetic programming. Dr. Yorulmaz elaborates on this dilemma in “Embryonic Stem Cells.”

The question of how to handle refugees has been a critical issue throughout human history. Despite many regulations, international legal conventions, and technological progress, refugees still pose intractable questions to governments around the world. Last summer, when Dr. Sophia Pandya was visiting Greece to observe the situation of refugees there, the country suffered one of the deadliest wildfires of the twenty-first century, in which a hundred people died and more than a thousand buildings were destroyed. Greece has been the crossroads for many refugees, including, yes, many Turkish citizens, especially in the last few years. The Erdoğan regime’s clampdown on the Hizmet Movement and many others who oppose his autocracy has forced tens of thousands of Turkish citizens to flee their homes and seek asylum in Europe, the US, and elsewhere. Hundreds of thousands of people have been detained in these purges; thousands more are waiting their turn, as their freedom to travel has been illegally curtailed by the Turkish government. What is their crime? Association with a group that opposes autocracy and supports democracy. For many fleeing injustice, Greece has become the last chance for freedom. What Dr. Pandya saw there was representative of what the Hizmet Movement has been doing for decades: Turkish refugees reached out to victims of the wildfires and distributed food and other aid. This was their way of responding to the hospitality of Greek people, who opened their homes to refugees when, as described in this issue’s lead article, they were forced to flee “dark-spirited night demons who were overwhelmed by hatred, rancor, and enmity, and dejected souls growling with wrath.”

How can animals in the wild hunt during the night as if it were day? A remarkable biological feature acts as a pair of “natural” night vision goggles.

Animals can move and even hunt in the pitch-dark of the night. How do these animals see comfortably in the dark? How are their eyes different from ours?

Anatomically, the eye consists of three layers called the Tunica fibrosa (outer layer), the Tunica vasculosa (mid layer), and the Tunica nervosa (retina). The outer layer of the eye is composed of the sclera and cornea; the middle section of choroidea, corpus ciliare, and the iris; and the inner section of the retina.

Light rays reflected off objects first pass through the eye’s translucent layer (the cornea), then into the black round part located in the front of the eye (pupillae, or pupil), and later go through the lenses. The colored section around the pupil, covered by smooth muscles, is in charge of regulating the amount of light entering the eye. The real and inverted image of an object is projected on the retina by the refraction of light through the lens; the image is transmitted to the optical center of the brain as a result of nerve cell stimulation via the optical nerves. Sight happens as a result of these causal chains.

Now, there is a twist to all this: the optical center in the brain accepts spots that do not emit light as black. For humans, this means a loss of sight. Black objects are not visible because they absorb and keep all the light reaching them.

How could these animals, without night vision goggles, see in pitch blackness despite having the same optical mechanisms as humans?

The answer to this question was hidden in an anatomic structure (tapetum lucidum) located in the eyes of some vertebrates. This special structure, lacking in humans, monkeys, squirrels, birds, red kangaroos, and some other mammals, is found in equines, ruminants, and many carnivores. This structure is located in the cytoplasm of the choroidea layers in between the innermost light sensitive ocular layer and sclera, and it acts like a biological reflector. With its crystalline composition of varying colors from golden yellow to white, tapetum lucidum is a wonder of creation. The eyes of these animals shine when a light source is projected due to the aforementioned structure.

The primary task of this formation is to reflect the light which is projected to the rear section of the eye again without absorbing it. This happens due to the crystalline makeup. Thus, lower light levels are enhanced by the repeated reflection in the eye, and sight is enabled for animals. The second job of Tapetum lucidum is to elevate the sensitivity of the retina to light in order to transmit signals with no stimulation strength to the optical center.

This reminds us of night vision goggles. Night vision cameras are electro-optical devices that strengthen the light that is present. Light enters this device through the lens and hits the charged cathode, which has a lot of high energy. The energy load hits the phosphorous screen where the image is focused after passing through the vacuum inside the charger. The image is an enhanced picture on the phosphorous screen and it is not visible directly through the object. Tapetum lucidum however, is such an artistic work that it cannot even be compared with the night vision systems of technology. While the lifespan of a night vision system is only 2,500–4,000 hours (104–167 days), animals with Tapetum lucidum can benefit from this for a lifetime. The newest night vision systems, with a maximum optic range of 30-120 meters, cannot provide sight under light conditions one fourth of the moon’s illumination strength, whereas animals with Tapetum lucidum can see objects hundreds of meters away in much dimmer light.

This isn’t the only example of how masterfully and diverse eyes can be. Damseflies have over 30,000 simple eyes, called ommatids; eagles can see their prey from thousands of feet above them.

By the principle that “certain things are appreciated best in their absence,” if we imagine a dark night in which we are unable to see anything and cannot even step a foot safely, we can perhaps understand that Tapetum lucidum is a great blessing for these animals.

Reference

Veterinary Ophthalmology (2004) 7, 1,11–22. Comparative morphology of the tapetum lucidum (among selected species), F. J. Ollivier,* D. A. Samuelson, D. E. Brooks, P. A.Lewis, M. E. Kallberg and A. M. Komáromy.

With respect to action and bodily endeavors, we are no more than weak animals or helpless creatures. The realm at our disposal is so limited that our fingers can touch its circumference. Our weakness, impotence, and indolence are so great that even domesticated animals are influenced by them. If any domesticated animal is compared with its undomesticated counterpart, great differences will be observed.

But as passive, recipient beings who need to pray and petition, we are worthy travelers allowed to stay for a while in the guesthouse of this world. We are guests of a Generous One Who has put the treasuries of His infinite Compassion at our disposal, and subjugated to us His peerless works of creative power and His special servants. Also, He has prepared for our use and pleasure such a vast arena of things that its radius is as far as sight or even imagination can reach.

If we rely on our physical and innate abilities, taking the worldly life as our goal and concentrating on its pleasures, we will suffocate within a very narrow circle. Furthermore, our bodily parts, senses, and faculties will bring suit and witness against us in the Hereafter. But if we know that we are guests and so spend our lives within the limits approved by our Generous Host, we will lead a happy and peaceful lives and reach the highest rank. We will be rewarded with an everlasting life of bliss in the Hereafter, and all of our bodily members and faculties will testify in our favor.

Our wonderful faculties have not been given to us so that we can use them in this trivial worldly life; rather, they have been given for our eternal life. We have many more faculties and senses than animals do, but the pleasure we derive from our physical lives is much less than what an animal derives. Every worldly pleasure bears traces of pain, is spoiled with past sorrows and fears of the future, and the pleasure’s ultimate disappearance. Animals, on the other hand, experience pleasure without pain, enjoyment without anxiety, and have no past sorrows or anxiety about the future. They enjoy comfortable lives and praise their Creator.

We have been created in the best pattern. If we concentrate on this worldly life, we are far lower than a sparrow, although we have far more developed faculties than any animal. In another treatise, I explained this fact in the form of a parable, as follows:

A man gives his servant 10 gold coins and tells him to have a suit made for himself out of a certain cloth. He gives another servant 1,000 gold coins and sends him to the market with a shopping list. The former buys an excellent suit of the finest cloth. The latter acts foolishly. He neither notices how much money he was given nor does he read the shopping list. Thinking that he should imitate his friend, he goes to a shop and asks for a suit. The dishonest shopkeeper gives him a suit of the very worst-quality cloth. The unfortunate servant returns to his master and receives a severe reprimand and a terrible punishment. Anyone can see that the 1,000 gold coins were not given for a suit, but for a very important transaction.

In the same way, our spiritual faculties, feelings, and senses are much more developed than those of animals. For example, we can see all degrees of beauty, taste all varieties of a food item’s particular tastes, penetrate the many details of visible realities, yearn for all ranks of perfection, and so on. Animals, on the other hand, with the exception of a particular faculty that reaches a high state of development according to its particular duty, can realize only a very slight development, if any.

Our senses and feelings, which have developed a great deal owing to our mind and intellect, require that we have many faculties. Our many needs have caused us to evolve different types of feelings, and to become very sensitive to many things. Also, due to our comprehensive nature, we have been given desires turned to several aims and objectives. Our senses and faculties have greatly expanded because of the diversity of our essential duties. Furthermore, since we are inclined and able to worship, we have the potential to realize all kinds of perfection.

Obviously, such rich faculties and abundant potentialities cannot have been given to us for an insignificant, temporary, worldly life. They were given to us because our essential duty is to perceive our obligations, which are directed to endless aims; to affirm our impotence, poverty, and insufficiency via worship; to study by our far-reaching sight and penetrating understanding; to bear witness to creation’s glorification of God; to discern and be grateful for the All-Merciful One’s aid sent in the form of bounties; and to gaze, reflect upon, and draw warning from the miracles of His Power as manifested in creation.

O world-worshipping one charmed by the worldly life and ignorant of the meaning of your nature as the best pattern of creation! Once I saw the true nature of this worldly life in a dream, as follows:

I was on a long journey. My God had caused me to set out on this journey. He gradually gave me some of the 60 gold coins he had allotted to me. This went on for some time. After a while, I arrived at an inn that provided some entertainment. I gambled away 10 gold coins in one night of entertainment and frivolity. When it was morning, I had no money for the provisions I would need at my destination. All I had left was pain, sorrow, and regret left by sin and illicit pleasure.

While in this wretched state, a man said to me: “You have lost everything, and so deserve to be punished. Moreover, you will go on to your destination with no money. But if you use your mind, the door of repentance is open. When you receive the rest of the money, keep half in reserve and use it to buy what you will need at your destination.”

My selfhood did not agree, so the man said: “Save a third of it then.” Still, my selfhood balked. The man insisted: “Then a quarter.” I realized that my selfhood would be unable to abandon its addictions, so the man turned away in some indignation and disappeared.

At just this moment, I found myself on a train traveling at a high speed through a tunnel. I was alarmed, but there was no escape. To my surprise, I saw very attractive flowers and tasty-looking fruits alongside the track, hanging out from the sides of the tunnel. I foolishly attempted to pick some of them. But all around them were thorns that, because of the train’s speed, tore at my hands and made them bleed. What I tried to hold on to them, they slipped from my grasp.

Suddenly an attendant came beside me and said: “Give me 5 cents, and in return I’ll give you as many flowers and fruits as you want. Otherwise, with your hands all cut up, you will lose a 100 instead of 5. Besides, there is a punishment for picking them without permission.”

Depressed by this condition, I looked out the window to see when the tunnel would end. But there was no end in sight. The tunnel’s walls had many openings into which passengers were being thrown. Suddenly I caught sight of an opening just opposite me with a gravestone on either side. When I peered out, I made out my name, Said, written in capital letters on the gravestones. I gave a cry of bewilderment and repentance.

Unexpectedly, I heard the voice of the man who had advised me at the inn, asking : “Have you come to your senses?” I replied: “Yes, but I am in despair and there is nothing I can do.” He told me to repent, and trust in God, to which I replied that I would. Then I woke up and I found myself transformed into the New Said; the Old Said had gone away.

I will now interpret some aspects of this dream: The journey is your life, a journey from the incorporeal world of eternity, passing through the stages of mother’s womb, youth, old age, the grave, the intermediate world, Resurrection, and the Bridge. The 60 gold coins are the 60 years of an average lifetime.

I was 45 when I had this dream. Only God knows when I will die. A sincere student of the Qur’an showed me the true path so that I might spend half of the remaining 15 years for the Hereafter. The inn, as I came to understand, was Istanbul. The train represents time, and each railroad car is a year. The tunnel is the worldly life. The thorny flowers and fruits are illicit pleasures and forbidden amusements that make the heart bleed with the idea of separation at the very moment you reach for them. The disappearance of pleasure increases sorrow, and besides, being unlawful, cause one to suffer punishment.

The attendant on the train had said:

“Give me 5 cents, and in return, I will give you as many flowers and fruits as you wish.” This means that the permissible tastes and pleasures, obtained in lawful ways, are enough for ones satisfaction. There is no need to pursue illicit ways.

You can interpret the remaining details for yourself.

Adopted from Bediuzzaman’s Twenty-third Word, Second Chapter

There was a peaceful silence and was pouring light upon light

Things were being done with the precision of clockwork;

Centuries-old confusions disappeared one after the other.

All were respectful to each other and behaved in sincerity;

Water of life was flowing through taps of porphyry.

Holy ones were as if on parade with faces bright with faith

As they walked, they gave strength to our hopes.

People were busy with founding a new world;

The earth was so luminous as if to compete with the sky.

When those heroes with penetrating sight passed by me,

I came to understand that mankind were in the time promised.

I saw young men with clear signs on their faces and concluded

They were those promised to come toward the end of time

I continued my way without stopping, nor becoming tired;

I met those who had absorbed light from the ‘grand fountain’.

Some, full of gratitude, were walking arm-in-arm;

Finally all reached their destination, which is the Pure Path.