Sight is a complex process. More of the brain is dedicated to vision than to hearing, taste, touch, and smell combined. In this article, we explain the anatomy of the eyes and how they let us see.
Here is a
- Light passes through the cornea, a dome-shaped structure. The cornea bends the light to help the eye focus.
- The iris allows some of this light to enter the pupil.
- Light passes through the lens. With the cornea, the lens focuses the light onto the retina at the back of the eye.
- The retina converts the light signal into electrical impulses.
- The optic nerve carries the impulses to the brain, which processes the signals and produces the image.
To understand how this happens, we start by looking at the anatomy of the eye.
Below is a fully interactive 3D model of the eye. Explore the model using your mouse pad or touchscreen to understand more about the eye.
The only part of the eye that people can see is the front. The rest is inside the eye socket, or orbit. Muscles connected to the eyeball allow the eye to move according to the direction of the person’s gaze.
There are three main types of tissue in the eye:
- refracting tissues that focus light
- light-sensitive tissues
- support tissues
Below, we look at each of these types.
Refracting tissues focus incoming light onto light-sensitive tissues to give a clear, sharp image. If tissues are the wrong shape, misaligned, or damaged, vision can be blurry.
The refracting tissues include:
This is the dark spot in the center of the colored part of the eye. The colored part is called the iris. The pupil expands and shrinks in response to light.
In bright light, the pupil constricts to protect the sensitive retina from damage. In low light, it dilates. This allows the eye to take in as much light as possible.
This is the colored part of the eye. It has muscles that control the size of the pupil and the amount of light that reaches the retina. In this way, it is similar to the aperture on a camera.
After it travels through the pupil, light reaches the lens. This is a transparent, convex structure. The lens can change shape, helping the eye focus light accurately onto the retina. With age, the lens becomes stiffer and less flexible, making focusing more difficult.
This is a muscular ring attached to the lens. As it contracts or relaxes, it changes the shape of the lens. This process is called accommodation.
The cornea is a clear, dome-like layer that covers the pupil, iris, and anterior chamber. This chamber is a fluid-filled area between the cornea and iris.
The cornea, like the lashes, eyelids, and tear fluid,
The cornea is densely populated with nerve endings and is highly sensitive. It is the eye’s first defense against foreign objects and injury. Because the cornea must remain clear to refract light, it has no blood vessels.
Vitreous and aqueous fluid
Two fluids circulate throughout the eyes to provide structure and nutrients. Vitreous fluid is thick and gel-like and is present in the back of the eye. It makes up most of the eye’s mass.
Aqueous fluid is more watery, and it circulates through the front of the eye.
These include the retina and the optic nerve.
The retina is the innermost layer of the eye. It contains
Photoreceptor cells in the retina contain protein molecules called opsins that are sensitive to light.
The two primary photoreceptor cells are called “rods” and “cones.” When these sense light, they send electrical signals to the brain.
Cones are present in the macula, the central part of the retina. The retina contains around
Cones help people see in typical light conditions and distinguish colors. There are different types, depending on the color that they are sensitive to. These roughly correspond to:
Red and green cones mostly occur in the center of the fovea, while the blue ones are mostly around the outside.
Rods generally exist around the edges of the retina. They
The optic nerve is a thick bundle of nerve fibers that transmits signals from the retina to the brain. Thin retinal fibers called ganglion cells carry light information from the retina to the brain.
The ganglion cells leave the eye at a point called the optic disc. Because there are no rods or cones here, it is also called the “blind spot.”
Different kinds of ganglion cells register different types of visual information. For instance, some are sensitive to contrast and movement, others to shape and detail. Together, they carry all the necessary information from our visual field.
The brain gives depth perception by coordinating the signals from both eyes.
The signals generated by the retina end up in the visual cortex, a part of the brain that processes visual information. The visual cortex brings together impulses from both eyes to create images.
There are many support tissues in the eye,
People commonly call this the whites of the eyes. It is fibrous and supports the eyeball, helping it keep its shape. It is attached to muscles that can move the eye in almost any direction.
The conjunctiva is a thin, transparent membrane that covers the sclera and lines the eyelids. It does not cover the cornea. Tear glands, each about the size of an almond, provide fluid that helps lubricate the eye and protect it from microbes.
The uvea is the middle layer of the eyeball. It supplies blood to the eye. The iris is part of the uvea, along with the ciliary body and the choroid. The ciliary body contains capillaries, which secrete aqueous humor. The ciliary muscles are connected to zonular fibers. Together, they help adjust the shape of the lens for viewing short or long distances.
A wide range of health issues can affect the eyes. They may involve:
- genetic factors
- features a person is born with
- other health conditions
Here are some examples:
- Achromatopsia: Also known as color blindness, this genetic condition affects the cone cells. A person has difficulty distinguishing between certain colors.
- Age-related macular degeneration:This causes blurry vision in the center of the visual field. It can lead to vision loss.
- Amblyopia: This begins in childhood and is often called “lazy eye.” One eye does not develop fully because the other, stronger eye dominates.
- Anisocoria: This occurs when the pupils are an unequal size. It can be harmless, but it may indicate a more serious medical problem, such as a stroke.
- Astigmatism: The cornea or lens is incorrectly curved, so light does not focus properly on the retina.
- Cataracts: This is a clouding of the lens. It can lead to vision loss.
- Chalazion: A lump forms in the eyelid due to a blockage. It can resemble a stye, but it does not stem from an infection.
- Conjunctivitis: Also called pink eye, this involves an infection of the conjunctiva, which covers the front of the eyeball.
- Detached retina: The retina can come loose, possibly due to an injury. It requires urgent treatment.
- Diabetic retinopathy: High blood glucose levels can lead to retinal damage and degeneration. This can lead to vision loss.
- Diplopia: This is also called double vision, and it can stem from several conditions, some of which can be serious.
- Floaters: Specks can seem to drift across the visual field. This is very common and usually harmless, but it can also indicate a condition that needs urgent attention, such as retinal detachment.
- Glaucoma: Pressure builds up inside the eye and damages the optic nerve. It can lead to vision loss.
- Mydriasis: Both pupils dilate or contract in an unusual way.
- Myopia: Also known as nearsightedness, myopia makes it difficult to see things clearly at a distance.
- Optic neuritis: The optic nerve becomes inflamed, often due to an overactive immune system.
- Strabismus: The eyes point in different directions.
- Uveitis: The uvea becomes inflamed, causing swelling and redness in the eye. It needs urgent attention.
A person should see a doctor if any of the following occur:
- any sudden changes, including a sudden increase in floaters
- severe pain and redness
- severe sensitivity to light
- loss of vision, double vision, or other vision changes
- an injury that affects the eye or eye socket
Vision is a complex process involving different parts of the eye and the brain.
Many people have vision problems. Some, such as cataracts, are highly treatable but can lead to vision loss without treatment.
Eye-related symptoms can indicate a serious health condition. Blurry vision or sudden vision changes, such as an increase in floaters, are signs that a person needs medical care. Anyone with concerns about their eyes or vision should see a specialist called an ophthalmologist.