Put a finger lightly on your closed eyelid and press for a second. The firm dome you feel underneath, right over the colored part of your eye, is the cornea.
It is about the size of a dime and thinner than a credit card, and it is the only tissue in your body you look through rather than at. It is also doing more optical work than the lens deeper inside the eye.
Why it is clear when nothing else is
Almost every other tissue is opaque. Three things keep this one transparent.
The first is that it has no blood vessels. Blood supply would scatter light, so the cornea takes oxygen directly from the air through the tear film and nutrients from the fluid behind it. That is also why a contact lens has to let oxygen through.
The second is the arrangement of its collagen. In the thick middle layer, the fibers are laid down in sheets with remarkably even spacing, and that regularity lets light pass rather than bounce.
The third is water control. The cornea will swell and cloud if it takes on too much fluid, so a single layer of cells on the back surface pumps water out continuously, all day, for your whole life.
Five layers, five jobs
- Epithelium. The outer skin of the cornea. It is only a few cells deep, it renews itself constantly, and it is packed with nerve endings, which is why a tiny scratch hurts out of all proportion to its size.
- Bowman's layer. A tough sheet of collagen under the epithelium. It does not grow back once it is breached, which is part of why deeper injuries can leave a mark.
- Stroma. The bulk of the cornea, roughly nine tenths of its thickness. This is the ordered collagen that provides both clarity and structural strength.
- Descemet's membrane. A thin, resilient sheet that acts as a base for the cells behind it and as a barrier against infection pushing inward.
- Endothelium. The single layer of pumping cells on the back surface. You are born with a set number, they thin out slowly with age, and they do not replace themselves.
The eye's main focusing lens
Most people assume the lens inside the eye does the focusing. It does the adjusting. The cornea does most of the bending, because the jump from air into corneal tissue is where light changes speed most sharply.
That makes corneal shape a major factor in your prescription. If the dome is too steeply curved for the length of the eye, light focuses in front of the retina and distance goes blurry. Too flat, and the focus falls behind it.
Astigmatism is a shape problem too. A cornea curved more in one direction than another, closer to the back of a spoon than to a section of a ball, gives you two focal points instead of one, and things look smeared at every distance.
It is also why laser vision correction works on the cornea. Reshaping a fraction of a millimeter of stroma changes where light lands.
What damages it
Being exposed, the cornea takes most of the abuse the eye receives.
- Scratches. A fingernail, a branch or grit under a lid can strip the surface layer. Common, usually healing well, and painful out of proportion.
- Infection. Bacteria, fungi, viruses and waterborne organisms can invade a compromised surface. Sleeping in contact lenses, topping up old solution or rinsing lenses in tap water raises the odds.
- Ultraviolet light. A long day on snow or water without eye protection can leave the surface sunburned, with pain and light sensitivity showing up hours later.
- Chemicals. Cleaning products and workplace splashes cause damage in seconds. Rinsing immediately with clean water matters more than getting to a phone.
- Dryness. A poor tear film leaves the surface rough, which blurs vision, stings, and over time leaves the cornea more vulnerable.
- Shape and structural disorders. In keratoconus the cornea thins and bulges into a cone over years. Inherited dystrophies can cloud it. Failing endothelial cells let it swell.
How it heals, and where it does not
The outer layer is one of the fastest healing tissues you own. A simple surface scratch often closes within a day or two, which is why an uncomplicated abrasion is usually managed with protection and time.
Deeper injuries are a different story. Once damage reaches the stroma, repair happens with scar tissue, which is not laid down in that neat, light-friendly pattern. A scar off to the side may never bother you. One in the center of your line of sight can blur vision permanently.
The pumping cells at the back do not regenerate at all. When too many are lost, through age, disease or surgery, the cornea holds fluid and hazes over, and a transplant may eventually be the option discussed.
Because the surface is so densely supplied with nerves, pain is a poor guide to how serious a problem is. A trivial scratch can be agonizing, and a worsening infection can feel manageable. Get it looked at rather than waiting to see.
Everyday care
Wear real eye protection for yard work, power tools and racquet sports. Wear sunglasses that block ultraviolet light outdoors, in winter as much as summer. Follow the wearing schedule and cleaning routine your eye doctor set for your contact lenses, and keep water away from them. If your eyes are chronically dry, get the cause looked at rather than reaching for whatever drop is nearest.
Common questions
Can the cornea be transplanted?
Yes, routinely. With no blood vessels involved, corneal grafts are less prone to rejection than most transplanted tissue, and often only one layer is replaced rather than the full thickness.
Does the cornea have any feeling?
A great deal. It is among the most sensitive tissue in the body, which is why an eyelash under the lid feels like a boulder.
When is corneal pain an emergency?
Treat any chemical splash as immediate, rinse first and get seen the same day. Severe pain, sudden blurring, a white spot on the colored part of your eye, or pain in a contact lens wearer that does not settle after taking the lens out all need same-day attention.
The cornea is easy to ignore until something goes wrong, and its most useful feature is that a trained examiner can see the whole thing directly at a routine visit. If it has been a while, or you have questions about how contact lenses are treating your eyes, you can search for a nearby eye doctor who accepts your vision plan.