The world that surrounds us is completely filled with stimuli that give us information about everything we need in order to survive. Including smells, sounds, textures and general information.
Our brain would be completely overwhelmed if it had to process and understand them all using just a single organ.
Because of this, humans have several organs specifically dedicated to processing and translating all the information we receive from the outside world into electrical impulses that are transmitted to our brain and allow it to send back the proper response to whatever comes our way.
This, naturally, is what helps us survive avoiding dangers, enjoy life through sensorial experiences and learn about the world and adapt accordingly to evolve as species.
While all the senses and their related organs are extremely important, we often consider the sight and its related organ, the eye and its parts, as probably the most crucial and the one we rely on the most.
Deaf or mute people frequently adapt to their disabilities easier than blind people, as we have learned since we were born to first rely on what we see, when we do not know how to talk, and we don’t have the experience necessary to properly understand other people’s words or other thing’s textures.
In this article, we are going to talk about the eye and its principal parts, and how they work to allow us to see the world around us. So let’s get to it:
The visual perception or sight
Before we start talking about the eye –the organ involved in the visual perception-, we would like to talk a little bit about the visual perception, also called sight.
To put it simply, as there are some pretty technical aspects involved in the visual perception, the sight is how our eyes perceive, process and communicate to the brain the light that bounces on the objects that surround us.
All the aspects of the physical objects that we see, including the color, shape, texture and opacity are variations on how the light bounces on them. Since different materials reflect light in different ways, we can tell them apart just by looking them.
As this article is about the parts of the eye and not the sight itself, we are not going to get into much detail, but one pretty good example of what we mentioned above is color.
Color is based on the wavelength of the light emitted by objects –it can be their own light, as it happens with light bulbs or it can be light reflected from the sun or artificial illumination- as a part of our eyes interpret it and communicate it to the brain.
There are wavelengths that are so tiny or so big that our eyes can’t perceive them –as it happens with infrared or ultraviolet lights-, and some animals with different eyes can perceive different spectrums or none at all.
Everything we see and how we see it through our eyes is a direct consequence of how our brain interprets the signals send by all the parts of the eye. So let’s go one by one, shall we?
1. Outer Part of The Eye: Tear layer
The tear layer is the outermost part of the human eye, and it is also probably the simplest.
It consists of a thin and transparent layer of tears that is designed to protect the eyeball and all of its components from external factors should they come in contact with the eyeball, such as dust or other tiny particles.
This layer is made of tears and thus shares their consistency and composition: salty, moist and clear.
It serves as lubricant for the eyeball to be able to move around without being hurt by the eyelids or the inside part of the eye cavity. It is also the first part of the eye that the light touches.
2. White Part of The Eye: The sclera
The clera is the white part of the eye, and it is one of the outer layers of the eye.
It is made from collagen and elastic fiber and has four main layers:
- Lamina fusca.
The main function of the sclera is to give they eye its spherical shape and maintain all the inner organs in place, as it is made from a somewhat solid mix of elastic fiber and collagen and it serves as kind of a shell for other organs of the eye.
Because of this, the sclera is not directly involved in the visual perception and it does not process any outside stimuli.
One of the most widely accepted theories about why the human sclera is so big that it allows the iris –the colored part of the eye- to be entirely seen when the eye is wide open, is that humans evolved this way to be highly social beings.
Contrasting with animals, whose sclera is often colored as the eye or is tinier compared to the iris, humans have a big sclera to easily identify in what direction a person is looking.
Through this social mechanism, the human species have been able to develop deeper connections when looking into each other’s eyes and can use just their sight to point at places.
Thanks to its white color, the sclera is also used to detect some illnesses in humans, as they turn yellow when the liver is failing or might even turn black in rare cases of extremely severe liver or kidney failure, for example.
3. The cornea
The cornea is the second layer you will see if you look at an eye diagram, and one of the most important parts of the eye. It is transparent and dome-shaped and covers the front part of the eye.
The cornea is mainly made out of albumin, a soluble protein.
Since its main function is transparency, it does not have any blood vessels, so it receives nutrients and protection from infections from the tears and a fluid only present in the eyes called aqueous humor.
The cornea is formed by 5 layers with individual functions. These layers are:
- Epithelium: the outermost layer of the cornea, the epithelium helps with the absorption of nutrients and oxygen contained in the tears to be distributed to other parts of the cornea and serves as a barrier to protect the inner parts of the eye from external particles.
- Bowman’s membrane: It is a membrane located between the epithelium and the stroma. It is made from collagen.
- Stroma: It is the thickest layer of the cornea and gives it strength, form and elasticity. It is made from collagen and water.
- Descemet’s membrane: It is a membrane located between the stroma and the endothelium.
- Endothelium: It is the innermost and thinnest layer of the cornea and its main function is to keep the cornea clear by pumping out fluids that leak from the eye and into the stroma.
The cornea’s main function is to provide the eye with focusing power, being responsible for more than 65% of it.
The cornea bends or refracts the incoming light and focuses it on the retina to be processed and transmitted to the brain in the form of electrical impulses.
It also filters out dangerous UV light that comes from the sun, protecting the retina and other organs from being damaged by it.
4. The anterior chamber
The anterior chamber is a space inside the eyeball that separates the cornea from the iris –the colored part of the eye- and it has the sole function of being filled with aqueous humor, a fluid that nourishes the cornea instead of the blood that would make it opaque.
5. The Colored Part of The Eye: The iris
The iris is probably the most iconic part of the eye. It is the circular and colored part of the eye and can come in several tones of brown, blue, green and, in rare cases, even gray, and it contains a small black spot in its center: the pupil.
It is composed of two separate parts:
- The stroma of the iris: It is made from fibrovascular tissue that surrounds the pupil and transports blood to the iris; in dark-colored eyes, it contains pigment granules, and it is connected to a muscle that helps dilate or contract the pupil to adjust to light conditions.
- The epithelial layer: It is an extremely thin layer of only 2 cells of thickness that contains pigmented epithelial tissue, and is what gives the iris its color.
The main function of the iris is to control the amount of light that enters the pupil and the inner organs of the eye.
The pigmented epithelial tissue contained in the iris and connected to little muscles behind it helps protect the pupil from big amounts of light by covering it –contraction of the pupil- and it allows more light to enter when we are in the darkness to see better –dilatation of the pupil-.
While it is not a real function, the iris also determines the color of the eyes of a person and somewhat influences how much light can someone be exposed to before suffering injuries. This is the reason why people with blue or green eyes are more sensitive to the sunlight.
6. The pupil
The pupil is the black round spot at the center of the iris.
While it is not actually black –it actually has no color, since it is a hole and not a physical part-, it appears black because the tissues inside the eye absorb the light, and no light means no color.
As we mentioned before, the pupil is only a hole where light passes from the outside to the retina, and it has no function except this -allowing the light to pass-.
How much light passes depends on the iris, how the light is refracted and concentrated depends on the cornea and the lens and how the light is interpreted depends on the retina.
7. The lens
The lens is a clear structure that, along with the cornea, contributes to bending the light before it is projected on the retina for processing.
The lens is made from proteins called crystallins and water. As the cornea, it lacks blood vessels for maximum transparency.
Even when the lens requires very low energy to function, since it has no blood vessels it obtains the nutrients it needs from glucose.
The lens is formed by several layers with a firm nucleus and a soft cortex.
The lens’ main function is to refract light from the outside and into the retina.
It also helps with the focusing power of the eye by changing shape, becoming thicker to see objects that are closer and thinner to see objects that are farther –this movement is called accommodation-.
The lens naturally loses its ability to change shapes with time, which is one of the main factors why people over fifty or sixty years old have more problems selectively focusing objects at different distances.
Also, it tends to become yellow with time, which is called cataracts –one of the most common sight-related diseases among old people-.
8. The retina
The retina is arguably the most important part of the eye when it comes to processing outside stimuli.
The retina is the innermost light-sensitive part of the eye and it is responsible for perceiving the two-dimensional image that is projected on it and translating it to electric impulses for the brain to process.
The retina is composed of several complex layers of photoreceptor cells –ten, to be precise- with very particular individual functions.
While we can’t explain every one of them and their functions, we can talk about the retina in general.
The light-sensitive particles called photoreceptor cells detect particular aspects of the light that enters the retina, such as color or brightness, and then process the information and send it to the brain as we are going to detail next.
The main function of the retina, as we said before, is to process the light that enters it after being properly refracted and filtered through the cornea, the iris and the lens.
Since the visual perception is a very complicated process for the brain to understand on its own, the retina helps define several characteristics of the things we perceive through sight to present a more unified and understandable image.
It does this by using two main photoreceptor cells:
- The cones: The cones are little photoreceptor cells found inside the retina that become excited with bright light and create patterns that are processed by the retina to understand the shape, color and brightness of objects in well-lit environments, such as outside during daytime.
- The rods: The rods are also little light-sensitive particles found inside the retina and they have the same function as the cones, but are used in dim light situations, such as during night when there is only poor illumination. They also react to light and create a pattern that the retina process to understand the shape, color and brightness of objects.
While it would be impossible to fully explain how the retina uses the cones and rods to create understandable images for the brain to use, it can be summarized by saying that the patterns that the rods and cones create when they become excited by being exposed to light are comprehensive images that contain the key aspects of the objects we are seeing, such as color, texture, distance, opacity, brightness, shape and tridimensional location.
After the patterns are created, the retina translates them to electrical pulses that are sent to the brain via the optical nerve.
It is interesting to mention that the image sent from the retina is inverted and is only corrected later, in a process called decussation.
9. The choroid
The choroid is the main vascular layer of the eye and it helps provide the retina with nutrients and oxygen.
The choroid is formed mainly by connective tissue and blood vessels, is located between the sclera and the retina and it is composed by four layers:
- Bruch’s membrane: the innermost layer of the choroid.
- Choriocapillaris: a layer of capillaries.
- Sattler’s layer: a layer of blood vessels of medium diameter.
- Haller’s layer: the layer that contains larger blood vessels.
The choroid has two main functions.
One of them is to provide oxygen and nutrients to the retina that are transported through the blood vessels contained in the choroid.
The other function is to prevent uncontrolled reflection of light inside the eye that would result in confusing visions, and it is achieved thanks to the melanin contained in the choroid.
Since albino humans have a lack of melanin, they often have low vision, but in exchange they are better at seeing at night.
10. The optic nerve
The optic nerve is the main nerve that connects the eye to the brain, and it is located behind the eyeball.
The main function of the optical nerve is to transport the electrical impulses produced by the retina after processing the patterns created by its cones and rods and transporting the information to the brain.
The visual cues that travel through the optical nerve are what allows the brain to interpret the pattern, and they include every aspect of the objects that we perceive when we see them: contrast, brightness, color, texture, spatial location, shape and opacity, among many others.
The secondary function of the optical nerve is to also transport the visual impulses that control two of the most important neurological reflexes:
- The accommodation reflex: Remember when we talked about accommodation in the section of the lens?Well, for the lens to change shape to enable our eyes to focus on specific things, there needs to be a visual impulse that tells the brain how far or near the object we are trying to focus on is.The optical nerve sends these impulses to allow the brain to control the lens and adapt to what we are trying to focus on.
- The light reflex: And now, do you remember when we talk about pupil contraction and dilatation made by the little muscles in the iris?This is also made possible by the optical nerve. It transports the visual impulses that tell the brain that there is too much or too little light so the brain can control those little muscles and contract or dilate the pupils to adapt to the light conditions.
11. The extraocular muscles
The extraocular muscles are not involved in the process of the visual perception, but they control the physical movement of the eyeball to allow us to look in different directions, since the internal parts of the eye cannot move around.
They are six and control the ways that the eye moves, and there is an additional one that controls the eyelids to allow us to blink.
12. The eyelids
The eyelids are an external part of the eye that have the primary function of protecting the eyeball from big particles and external adverse situations, such as sand, dust, bigger particles or even very intense light that would damage the tear layer and the cornea.
While they can be controlled at will by the extraocular muscle we mentioned above to protect us in emergency situations, they are also involuntarily controlled by our brains so we can blink automatically whenever we need to without having to think about it.
13. The eyelashes
The eyelashes are placed on the eyelids and help them protect the eyeballs from physical particles. They are made from little individual hairs that grow over follicles.
Since they are sensitive to pressure, they also serve to perceive incoming objects before they get too close to the eyeball, causing us to close our eyes involuntarily to protect the eyeball from damages.
14. The tear ducts
They are formally called nasolacrimal ducts, but they are commonly known as tear ducts. They are little canals placed at the edge of the eye that is closest to the nose, and they help transport tears from the lacrimal sac to the eyes and from the eyes to the nasal cavity.
Tears help our eyes remain lubricated to allow comfortable movement and to replenish the tear layer that serves as the first protection for the eyeballs after the eyelashes and eyelids.
When there is an excess of tears, the same ducts transport them to the nasal cavity to avoid the blurry vision that excess tears can cause –as it happens when you cry or have allergies-.
The eye is a magnificent organ that allows to perceive the signals around and it is safe to say that mankind would not be as evolved as it is today if we did not have eyes.
While the process involved in the visual perception –which is called visual pathway, by the way- is incredibly complicated and involves several dedicated parts of every organ in the eye, it could be resumed in a simple manner like this:
The light bounces on objects and reflects towards our eyes, and the way that light is reflected sort of imprints on it the qualities of color, texture, tridimensional location, etc. of the object.
Then, the light is filtered through our cornea and lens to bend it to allow us to focus. The iris cells cover the pupil to adapt to how much light passes through it.
Finally, the light excites light-sensitive particles in the retina that create a pattern that is sent to the brain to understand through the optic nerve.
The other parts of the eye help us keep our eyes clean and nourished so this process is not interrupted.
Now you know roughly how we perceive objects and we would like to suggest you to investigate further if you are interested in the more specific and complex sub processes that happen in every component of the parts of the eye.