The eye is an incredible structure. Despite being so small, the eye is actually made up of many completely different structures; structures very unique to the eye. Let’s go on a quick tour!
From front to back, the key anatomy of the eye includes the tear film, cornea, anterior chamber, iris, lens, vitreous and retina. The retina connects to the optic nerve to send vision information to the brain. Surrounding everything we have the eyelids, the conjunctiva, the eye movement muscles and the sclera.
These words all sounds confusing to you? Fear not, just read on! Let’s cover all the important structures of the eye.
Diagram of the Human Eye, Image by Rhcastilhos. And Jmarchn., CC BY-SA 3.0, via Wikimedia Commons / modified from original
Tear Film
Many people may not actually think of the tear film as a “structure", but it does serve an important function for the eye: it keeps our eye from drying out.
The tear film consists of three different layers:
- The bottom-most layer is a mucus layer that binds the tear film to the cornea below.
- The bulk of the tear film is an aqueous or watery layer (and is what we most often think of when we think of tears). These tears come from a gland in the corner of our eye called the lacrimal gland.
- On the top of the tear film is an oil layer. These oils are produced in tiny glands called meibomian glands in the eyelids. This oil layer protects the tear film from evaporating away.
Keeping the eye from drying out keeps the conjunctiva and cornea healthy. In addition, the tear film helps support these cells by supplying oxygen and nutrients.
Conditions involving the tear film:
- Dry eye disease. With dry eye, the tear film just isn’t adequate (either not enough tears are produced or the tears are evaporating away too quickly) and the eye dries out (as the name implies).
- That’s really about the only condition...But it’s a very broad condition!
Cornea
Below the tear film is the cornea. And now we are on to the actual “structures".
The cornea is divided into different layers. From front to back, this includes the epithelium, stroma and endothelium.
Layers of the cornea; Image by StemBook (CC BY 3.0) / modified from original
The epithelium is the “skin" on the cornea. It protects the rest of the cornea from bacteria and other pathogens. It also allows our eye to feel comfortable. The cornea is actually one of the most sensitive structures in our body. This is why it is very uncomfortable to touch your eye. But the epithelium prevents this sensitivity from being overwhelmingly painful. This becomes readily apparent if you develop a scratch on the epithelium.
The stroma provides the structural framework for the cornea. It consists of multiple flat layers of cells arranged in a way to be completely transparent!! (a very necessary property for vision). This layer allows the cornea to focus light into our eye. And the cornea is actually responsible for 2/3 of the total focusing power of the eye.
Finally on the back of the cornea is a layer of cells called endothelium. These cells work like water pumps constantly pumping water out of the cornea. This prevents the cornea from becoming swollen and hazy.
Conditions involving the cornea:
(note: there are many! Here are some more common ones)
- The cornea is the structure treated with the popular laser eye surgery lasik. Lasik changes the shape of the stroma to correct vision.
- Keratoconus. This condition causes progressive weakening of the cornea causing it to change shape.
- Basement Membrane Dystrophy. The epithelium develops a weak attachment to the stroma. This can cause the epithelium to bunch up on itself blurring vision and / or slough off causing recurrent painful scratches on the eye.
- Fuch’s Endothelial Dystrophy. The endothelial cells thin out over time and the cornea is no longer able to pump water out. This leads to swelling and blurry vision.
Anterior Chamber
After passing through the cornea, light enters the anterior chamber of the eye. This space exists between the cornea and the lens and is filled by a water-like liquid called aqueous humor.
This aqueous humor is produced in a structure behind the iris called the ciliary body. It flows from the ciliary body through your pupil and out through the cornea at the angle between the iris and the cornea.
Flow of Aqueous Humor within the eye; http://www.nei.nih.gov/photo/eyedis/index.asp, Public domain, via Wikimedia Commons
The aqueous humor works similar to the tear film in that it is provides nutrients and oxygen to the lens and cornea. But but nature of it being a water-like liquid, it also contributes to the pressure of the eye.
Conditions involving the anterior chamber:
- The big one is glaucoma. If the pressure is too high in the anterior chamber, this can lead to loss of vision. If the aqueous can’t drain out of the angle easily enough (or if the iris blocks the drain), the pressure of the eye will go up.
- During ICL surgery, a special lens is placed within the anterior chamber to correct vision and get out of glasses.
Iris
Within the anterior chamber is the iris. This is the colored part of our eye. The black hole in the middle of the iris is called your pupil.
The main role of the iris is to regulate how much light enters our eye. When we are in a dark environment, the iris dilates to allow more light into our eye. When we are in a bright environment, the iris constricts to limit how much light enters our eye.
Conditions involving the iris:
- Iritis. Which is simply inflammation of the iris. This can occur for a variety of reasons, but one hallmark of this condition is sensitivity to light. When light is shining in the eye, the iris wants to constrict but is irritated - this causes discomfort.
Lens
Next up is the natural lens.
The lens provides the remaining 1/3 of the focusing power of the eye.
When we are young, this lens also can change its focus and allow us to read up close.
Remember the ciliary body? It doesn’t just produce aqueous. It also attached to the natural lens and controls the shape of it. When we want to read up close, the ciliary body contracts and the lens changes shape to bring those up close objects into focus. When we want to look off in the distance the ciliary body relaxes and the lens returns to its baseline state.
How the eye focuses up close, image by MikeRun, CC BY-SA 4.0, via Wikimedia Commons
Conditions involving the lens:
- Presbyopia. Starting in our mid-40s, the lens becomes less flexible. It has a harder time changing shape. This impairs our ability to read up close and causes us to need reading glasses, bifocals or progressive glasses.
- Lens replacement surgery or refractive lens exchange is a procedure which replaces the lens with an artificial lens to correct vision and get out of those reading glasses, bifocals or progressive glasses.
- And finally, we can’t leave the lens without talking about cataracts. Gradually the lens becomes cloudy and impairs our vision. When this happens, this lens is called a cataract. Cataracts are corrected by removing the cataract with surgery and replacing it with an artificial lens.
Dense cataract; image by Imrankabirhossain, CC BY-SA 4.0, via Wikimedia Commons
Vitreous
The vitreous is the largest structure in the eye. It is the jelly-like substance in between the lens and the retina.
Similar to the aqueous humor, the vitreous humor is also a source of nutrients. But unlike aqueous humor, when our eye is growing, the vitreous supports the structure of the eye.
Conditions involving the vitreous:
- Posterior Vitreous Detachment. Gradually over time, the vitreous changes from a jelly-like substance to a liquid-like substance. This causes the vitreous to pull away from the retina and clump up on itself. This is notable for creating floaters in your vision.
Retina
We’ve passed through all the transparent structures of the eye and finally made our way to the retina.
The retina receives light and transforms that light into electrical nerve signals to send to our brain. How cool!
The retina; Image by BruceBlaus. When using this image in external sources it can be cited as:Blausen.com staff (2014). "Medical gallery of Blausen Medical 2014". WikiJournal of Medicine 1 (2). DOI:10.15347/wjm/2014.010. ISSN 2002-4436., CC BY 3.0, via Wikimedia Commons / cropped from original
Within the retina are specialized cells called photoreceptor cells. When light hits these photoreceptors, it causes an electrical change in these cells. These cells then send this information through the optic nerve to the visual processing center of our brain. There are two main types of these cells: rods and cones.
- Cones work best to detect color and fine resolution. These cells are concentrated in the very center of our retina, called the macula, where they are used to distinguish the fine details in our vision.
- Rods work best to detect light and don’t do colors all that well. These cells are located out in the periphery of the retina and help allow us to see in more dimly lit environments.
As mentioned, the macula is the center of our retina. And within the center of the macula is the fovea. When you look at something, you are using your fovea. This is the most important part of the retina.
Conditions that involve the retina:
(note: again, there are many! Like the cornea, there are specialists who devote their lives to the retina; but here are the more common and important ones.)
- Retinal tear and retinal detachment. A tear can develop in the retina. If this tear is left untreated, fluid from the vitreous can get underneath the tear and cause the retina to detach from the back of the eye.
- Macular degeneration. Underneath the retina is a blood-supply layer called the choroid. But macular degeneration can result if this choroid develops abnormal blood vessel growth and bleeding into the retina.
- “Strokes" of the retina. In addition to the choroid, the retina is also supplied by arteries and veins running throughout the retina. Blockages of these arteries can prevent the retina from receiving essential nutrients causing retinal cells to die. Blockages of these veins can cause lots of bleeding and swelling of the retina. (note: while these sound like strokes and are often colloquially referred to as strokes of the eye; they actually have a different cause than true strokes of the brain in most cases)
- Membrane of the macula. A thin membrane can start to grow over the macula. This membrane can cause the macula to wrinkle distorting and blurring your vision.
- Diabetic retinopathy. Diabetes can cause the blood vessels within the retina to become irregular. This can cause swelling and bleeding of the retina. The more severe the diabetes, the greater the damage to the retina.
Optic Nerve
Alas, we are on the final stop on the visual pathway tour of the eye - the optic nerve. Light processed from the retina travels through the optic nerve on its way to the brain. (fun fact: the optic nerve is actually part of the brain itself)
Conditions that involve the optic nerve:
- Glaucoma. While in most cases glaucoma is more of an issue related to the pressure in the eye, the direct effects of glaucoma can be seen in the optic nerve. With damage from glaucoma, the optic nerve thins out. This causes gradual loss of peripheral vision.
- Optic Neuritis. Conditions that affect the nervous system can affect this optic nerve. One common condition is multiple sclerosis. This condition causes neurological dysfunction and one common nerve it affects is the optic nerve. This causes vision loss and pain when moving the eye around.
- Optic nerve “strokes". Similar to how the retina can be damaged due to blood supply issues, the same can happen at the optic nerve as well.
Now that we’ve covered all the visual pathway structures of the eye, let’s move on out to learn the parts of the eye that support those structures. First up are the eyelids.
Eyelids
The eyelids serve two primary functions:
- Protect the eye. What is your reaction if an object comes toward your eye? You close your eyes. Whatever is coming towards your eye has to hit your eyelid first before it can get to your eye. The eyelid is working to protect the eye.
- Replenish the tear film
- Each blink of the eyelid works like a squeegee to smooth out the tear film on the surface of the eye. A smooth tear film equals clear vision.
- Tiny glands within the eyelid called meibomian glands create the oils which coat the surface of the tear film to keep our eye from drying out.
Conditions that involve the eyelids:
- Ptosis or droopy eyelid. This can be due to a large variety of causes; some mechanical, some neurological.
- Styes or chalazia. These occur from blockages in the meibomian glands. As the oils can no longer flow out properly, they build up and form uncomfortable bumps called chalazia. If it gets infected from skin bacteria, it becomes more painful and becomes a stye.
Conjunctiva
This is the white part of our eye. It extends from underneath our eyelids to our cornea.
The conjunctiva serves an important role in protecting our eye from the outside world. And it’s flexible enough to allow us to move our eye around.
Conditions that involve the conjunctiva:
- Conjunctivitis or pink eye. This is an infection of the conjunctiva. Most of these infections are caused by a virus but you can have bacterial causes as well.
- Pterygium. With exposure to irritants such as ultraviolet light, dust and wind (in general, being outdoors without sunglasses), the conjunctiva can transform and grow onto the cornea causing a pterygium.
Muscles Of The Eye
Directly below the conjunctiva and attached to the sclera are the muscles which control eye movement. These six muscles allow us to look in all directions as well as allowing rotate our eye when we tilt our head.
Conditions that involve the muscles of the eye:
- Strabismus or “lazy eye". More often occurring in children, strabismus is when one eye doesn’t line up properly with another eye. While not necessarily an issue with eye muscles, the fix often involves moving the position of eye muscles to correct the alignment.
- Diplopia or double vision. If some of these muscles are suddenly prevented from functioning, typically due to a neurologic issue, the eyes will be misaligned and you will see two separate images in your vision.
- Thyroid eye disease. The muscles of the eye can become enlarged with hyperthyroidism or Graves’ disease. This can push the eye forward and cause the cornea and conjunctiva to be more exposed - causing it to dry out more.
Sclera
Finally we’ve gotten to the main structural support of the eye: the sclera. This gives the eye the shape of a globe and most parts of the eye are attached in some way to the sclera.
Conditions that involve the sclera:
- There really aren’t many common must know conditions that affect the sclera.
Summary
So there you have it - the complete anatomy of the eye. Each part of the eye is completely unique and different from the others. A truly fascinating structure.
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