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.

“Honey, you already fasted last year for three days, didn’t you?” replied her mother, hardly able to hide her smile.

“Mom! I don’t mean a ‘kid’s fasting’. I am talking about the real stuff,” she said raising her soft voice.

“It is wonderful that you want to fast but you need to grow up some more. Why don’t you first try your ‘kid’s fast’ again tomorrow, fast until noon, then have your lunch. I am sure even angels will look down at you with admiration. They will say ‘Wow! Such a brave girl! She is only five years old and yet she already fasts for half of the day.'”

Rolling her eyes, she protested to her mother once again:

“Mom I am not a baby anymore! My sister is fasting, my older brother is fasting, you are fasting, and dad is fasting. Everybody is fasting except for my baby brothers and I. Please mom, I can do it, I want to do it, please…”

“Dear, you are a very strong girl, I know, but you see your stomach is still very little. Your hands, your mouth, you are still a little person. You already know that children don’t need to fast until they are a lot older. Maybe next year, or the year after you can try to fast a day or two, and when you become a teenager, you’ll be fasting the entire Ramadan. But this year you’ll only be doing the kid’s fast.”

As she looked at her little daughter hoping that her words would end the discussion, her heart was overflowing with such a joy and happiness, in the midst of compassion. How proud she was! She uttered a silent little prayer inside, “My little angel, O my dear Lord, please keep the love of worship and devotion overflowing from her heart her entire life with a child-like purity and sincerity, just like now.”

As soon as the little girl’s dark eyes caught her mother’s lips moving, she jump off of her seat. She must have had thought that it was a green light, and that her mother was about to say “yes.”

“Great!” she screamed, her face was enlightened with an excitement. Hardly grasping a breath, she continued: “Then you can wake me up for the early morning breakfast before dawn. Then I can have enough food and water and then be prepared for a day of fasting. I am so happy. And then…”

Her mother and father looked at each other, somewhat reluctant, and then her father interrupted her. “Dear, it seems like you are not listening to what your mother is saying. I know you are a brave girl, and your mother told me that your friends have been calling you ‘fearless’ since the day you challenged them and climbed all the way up to the top of the power pole on the street. I also know you love God, and you want to begin your journey as an obedient servant, and you are excited about your first full fasting, but you are missing something my dear…”

Ready to protest again, her little fingers swayed back and forth over her fork and spoon, and she asked: “What? What is it that I am missing?”

“You also have to be patient. God willing, next year you could try to do one full day of fasting.”

About to burst into tears, she mumbled with a barely audible voice:

“I am fasting tomorrow, whether or not you wake me up for the early breakfast. I am ready and I am fasting tomorrow!”

Her older sister sitting next to her couldn’t help hearing her murmuring and she whispered into her ears: “Oh, you are more stubborn than a goat!”

As she left the table and went back to her room, she wasn’t sure who the winner of the argument was. She climbed up to her bunk bed and closed her eyes tight. She murmured: “Tomorrow will be my first real fasting!”

As soon as she opened her eyes in the morning, she jumped out of bed, almost falling down as she climbed down, and ran to the window. It was already very bright outside. Was it already morning? Had she missed the dawn? Did everybody oversleep? She ran to her parents’ room, screaming:

“Mom, dad, wake up! We overslept, we overslept!”

She saw that her father had already gone, and her mother was about to finish her daily reading of the Qur’an.

“Honey,” said her mother. “I thought we had reached an agreement last night.” She wasn’t able to get any answer from her daughter, so she continued: “Our Creator is so merciful that He never asks us to do anything that is more than we can bear, it is not only children, but many adults are also excused from fasting. For instance, do you remember when I had your baby brother, I didn’t fast for a while, or when your grandmother got very sick and had to take so many medications, she skipped some days of fasting. When you become old enough and your body gets stronger, I know you will be super cool with your fasting, but now I don’t want you to rush into something before you are ready and lose your enthusiasm for later.” She was ready to continue but when she saw the tears in her little angel’s eyes, she stopped and hugged her: “Okay, okay young lady, let’s make an agreement then. You can try and keep your fast as long as you can but it is a summer day, it will be long and very hot so when you get really thirsty, hungry or tired, you can take a few minutes break, then you can continue your fasting. If you forget that you are fasting and eat or drink something, that’s okay, that means angels are trying to help you with the blessings from heaven. And don’t forget, no matter how you do, I am proud of you.”

Finally all the shine and happiness came back to her eyes and the little girl answered: “Deal mom!”

A couple of hours later, she decided to go outside to play. It was really warm outside. She looked around to see if any of her friends were around but she could only see a few shadows from the distance. First she thought about running over to play with them, but then she felt a little tired. Instead, she sat down on the brick pavement.

” I guess I am tired, and it is not even noon yet!” She bent over her stomach and began to chat: “Hey there! How do you feel? Are you hungry?” The only response from her little stomach was a rumbling voice. She smiled and patted herself on the stomach: “Listen, we are growing up today and learning to be patient. Think about all the poor people in the world, they cannot find food most of the time, now do you understand them better?” It seemed like her stomach was convinced and stopped rumbling, she was so proud of herself. With a big smile on her face, she started to watch people passing by on the street.

All of a sudden she heard her favorite sound of summer. A street vendor was calling out: “Snow cones! I have snow cones! Snow cones with all the flavors!” She forgot about everything and quickly checked her pockets. Yes! She had enough money to buy whichever flavor she liked. She rushed to the snow cone vendor and shouted: “Hey, wait! Wait for me! May I have a chocolate flavored snow cone please? You know what, give me the one with lemon and strawberry flavor as well.”

The vendor was a dark skinned young man. As he handed her snow cones wrapped with blue papers, he greeted his favorite customer with a big smile: “Look at you! You have grown up a lot since last year.”

She giggled and handed the money. She was hardly able to hold all three snow cones in one hand, the weather was so warm that they started to melt and drip all over her hands. She didn’t seem to mind, the coolness of the icy drops helped her cool down a little, but it also made her feel her thirst even more. In a hurry as she was trying to unwrap the blue paper from the first one, she noticed the vendor’s dark face, now even darker due to walking under the sun trying to sell all the snow cones and his lips were cracked probably because of thirst. She felt so selfish to have all these three snow cones for herself. A little shy and hesitant, she extended one of them to the man and said: “Please have one of them, three is too much for me anyway.” The man answered: “You are a generous girl, but thank you, I am fasting.” Suddenly, time stopped and she remembered that she also was fasting. “Oh ok,” she said nodding her head goodbye to the vendor and headed back to the brick she was sitting on a few minutes ago. With her stomach rumbling, she mumbled: “So am I, so am I!”

As her vendor friend disappeared around the corner, she didn’t know how to feel. She looked around, there was no one watching her. Well, nobody would think that she was fasting anyway. Her stomach started rumbling again, her lips felt dryer than ever, her hands were covered with the cool-sweet snow cone drips. She took a deep breath, stood up, and began to run towards to children playing in the distance. As she was running her heart was pounding, the ground under her feet felt like it was slipping away and her lips were constantly moving: “I am fasting, I am fasting. My dear God, I promised you, I am fasting today.” As she reached the children, she was gasping for the air and her legs were shaking, she could hardly call out to them: “Hey guys, do you want some snow cones?”

As she walked away from the children enjoying the cool sweet snow cones, her stomach had stopped rumbling, her lips were still dry but wide open with a big smile. Her heartbeat had slowed down and felt like it could fit into it the entire universe. She took her every step with such confidence, she felt like a big oak tree. Her fingers were still sticky and wet, and her lips were still moving: “I am fasting today, I am fasting. I am fasting for the sake of My dear God.”

***

“Excuse me, could you speak a little louder?” With the kind voice of the moderator, she was pulled back from her childhood. It was an interfaith group meeting where people were discussing various faith related topics including their stories in regards to how they shaped them as adults. She must have zoned out in the midst of the last question.

“Oh I’m sorry!” she said, clearing her throat nervously and a little embarrassed. “Where were we?”

As her group-mates smiled at her, the moderator answered:

“So it was your turn. Would you care to share with us your first memory of when you felt the presence of God in your life? It might be very difficult to pin-point, maybe in your teenage years or later? It’s okay if you can’t recall the exact moment, we are just causally sharing.”

She quickly glanced at the wondering faces of her friends. After 20 some years, she could still feel the cold-sugary drips on her hands. With a smile, she took a deep breath before she started to tell her story….

“That’s okay, I know exactly what moment it was.”

Ayse Kubra Coskun, MD, is currently serving as Vice President of Boston Dialogue Foundation.