chapter 5 continuation on head

The EYE

What are eyes?

Your eyes are the sensory organs that allow you to see. Your eyes capture visible light from the world around you and turn it into a form your brain uses to create your sense of vision. Your brain doesn't have sensory abilities of its own. It needs your eyes (and other senses, like hearing and touch) to gather information about the world around you.

Most people are born with two eyes. Working together, they give you a field of view about 200 degrees wide and 135 degrees tall. When your eyes work together correctly, they give you depth perception and 3D vision. They also give you color vision.

It's also important to remember that sight and vision aren't necessarily the same thing, even though many people — including eye care specialists and healthcare professionals — use those terms interchangeably. Sight is what your eyes do. Vision is the entire process that starts with sight and ends with your brain processing what your eyes see into a form your brain can use and understand.

Your eye is made up of many structures that work together so you can see.

Cleveland Clinic is a non-profit academic medical center. Advertising on our site helps support our mission. We do not endorse non-Cleveland Clinic products or services. Policy

Function

How do your eyes work?

Everything your eyes do starts with light from the outside world. Your eye structure lets light enter and pass through a series of clear components and sections, including the cornea, aqueous humor, lens and vitreous humor. Those structures bend and focus light, adjusting how far the light beams travel before they come into focus.

The focus needs to be precise. If it isn't, what you're looking at appears blurry. Your eye has muscles that can make subtle changes to the shape of your eye, moving the focus point so it lands correctly on the retina.

When light lands on the cells of your retinas, those cells send signals to your brain. The signals are like coded messages describing everything they can about the light. That includes the color, how intense it is and any other relevant details. Your brain decodes and processes the signals and uses them to "build" the image you see.

Anatomy

How do eyes work?

Human eyes are complex, and it takes many parts working together correctly for you to see.

Eye anatomy

The parts of your eye include the:

Cornea. This protects the inside of your eye like a windshield. Your tear fluid lubricates your corneas. The corneas also do part of the work bending light as it enters your eyes.

Sclera. This is the white part of your eye that forms the general shape and structure of your eyeball.

Conjunctiva. This clear, thin layer covers the sclera and lines the inside of your eyelids.

Aqueous humor. This is fluid that fills a space called the anterior chamber. The pressure of the aqueous humor helps maintain your eye's shape.

Iris. This part contains the muscles that control the size of your pupil. It's also responsible for eye color. The iris can be brown, blue, green or hazel (a blend of brown, yellow and green).

Pupil. This is the black circle inside the iris. It's like an adjustable window to the inside of your eye. It widens and narrows to control how much light enters your eye.

Lens. This focuses light that enters your eye and directs it to the back of your eye.

Vitreous humor. This clear, gel-like fluid fills the space between the lens and retina. It helps your eye hold its shape. It's also sometimes known simply as "the vitreous."

Retina. This thin layer of light-sensitive cells at the back of your eyes converts light into electrical signals. It contains rods (which help you see in low light) and cones (which help you see colors).

Macula. This small area of your retina is key to your vision. It's responsible for the center of your visual field. It also helps you see color and fine details.

Optic nerve. This connects your retinas to your brain. It's like the data cable that carries signals from your eyes, with connection points linking to multiple brain areas.

External muscles. These control your eye's position, alignment and movement. They also contribute to your eye's shape, which is part of your ability to switch your vision's focus between near and far objects.

Conditions and disorder

What are the common conditions and disorders that affect your eyes?

The types of conditions that can affect your eyes vary depending on the specific part(s) involved. That's because your eyes include a variety of tissue types. It has muscle, connective tissue, nerves, blood vessels and more.

Some of the different types of eye conditions include — but aren't limited to — the following:

Refractive errors. These are problems with how you see because light isn't coming into focus on your retinas correctly. Refractive errors can take many forms, such as focusing too soon (nearsightedness) or too late (farsightedness). They can also involve distortions in your sight, like with astigmatism.

Corneal disorders. These are conditions that affect the cornea itself. They can happen for many reasons, ranging from congenital conditions (which you have at birth) to conditions that don't develop until later in life.

Retinal disorders. These conditions can happen because of problems that affect the retina directly. They can also be secondary effects of another disease, like how lattice degeneration can lead to a retinal detachment.

Optic nerve-related conditions. These affect the nerve that links the eyes and brain. Examples include optic neuritis and optic atrophy.

Age-related eye disorders. These conditions are more likely to happen as you get older, especially after age 65. They range from minor concerns like age-related loss of near vision (presbyopia) to serious concerns like cataracts. Some age-related eye diseases, like macular degeneration or glaucoma, are severe enough to cause permanent vision loss.

Your eyes are also susceptible to more general conditions and issues. Examples of these include:

Infections.

Injuries and trauma.

Cancer.

Congenital malformations.

What are some common signs or symptoms of eye conditions?

Signs and symptoms of an eye condition can vary greatly. One reason for that is the many different parts that affect or contribute to your vision. A common example of this is how a metabolic and circulatory condition like Type 2 diabetes can lead to vision loss over time.

Some symptoms affect the surface of your eye only. Others affect the inside of your eye. Some key types of eye symptoms include:

Eye surface issues.

Eye appearance/alignment.

Eye function and sight.

Eye surface issues

These symptoms affect your eyes' surface or the area immediately surrounding them. They include:

Irritation.

Red eye.

Watery eyes.

Discharge, ooze or "goop" coming from your eyes.

Eye appearance/alignment

Changes in the color of the sclera (such as blue or yellow sclera) or noticeable bleeding into the conjunctiva.

Reflex or appearance changes affecting the iris or pupil (like leukocoria, coloboma, etc.).

Eye misalignment (strabismus), including inward (esotropia), outward (exotropia), upward (hypertropia) or downward (hypotropia).

Eye function and sight

Symptoms from eye-related conditions can also affect your sight itself. The eyes are often a sense you rely on heavily, so sight- or vision-related symptoms are often easier to notice.

Some of them involve changes or disruptions in how or what you see. Examples include — but aren't limited to — the following:

Trouble seeing clearly or properly: Blurred vision from refractive errors, or specific vision issues like night blindness or color blindness.

Decrease or loss of vision: Sudden vision loss or low vision and blindness.

Disrupted vision: Double vision or visible auras (bright haze or splotches in your field of view) like from ocular migraines.

Light sensitivity (photosensitivity or photophobia): This is when bright light causes you pain or discomfort severe enough that you try to avoid brighter surroundings and prefer dimmer areas or light levels.

How the link between your eyes and brain influences symptoms

It's also important to remember that many causes of eye symptoms — especially sight/vision changes or eye movement control symptoms — may not be due to an eye condition. Some might happen due to a condition elsewhere in your body. An example of this is yellowing of the sclera when you have jaundice.

The familiar saying is that "the eyes are a window to the soul." But from the medical perspective, they're also like a window to your brain. Eye-related symptoms are a key way for healthcare providers to find brain conditions and issues. That's why vision changes can be telltale indicators of brain-related issues like concussions or strokes.

What are some common tests to check eye health?

Many tests can detect conditions that affect your eyes directly or that cause eye symptoms. The most important of them is an eye exam. Regular eye exams can detect many eye conditions or concerns before you ever have symptoms. And eye exams can help prevent long-term vision damage or issues when you have other conditions like Type 2 diabetes.

Other common tests include:

Color blindness test.

General imaging tests, like ultrasound or magnetic resonance imaging (MRI).

Glaucoma tests.

Intraocular pressure test.

Retinal imaging tests.

Slit lamp exam.

Visual acuity test.

Visual field tests.

There are many tests your eye care specialist or other healthcare provider may recommend depending on your symptoms and the suspected cause(s). They may also recommend tests for other body systems that might influence or cause eye symptoms. Your specialist or provider is the best source of information about test options, what they recommend and why.