Eye Health

Glossary of Eye Anatomy Terms

An Overview of Eye Anatomy

The human eye is an organ that detects light and sends signals along the optic nerve to the brain. Perhaps one of the most complex organs of the body, the eye is made up of several parts. And each individual part contributes to your ability to see.


The cornea is the transparent, dome-like structure on the front part of the eye. It gives the eye two-thirds of its focusing or refracting power. One-third is produced by the internal crystalline lens.

Much like a camera lens, the cornea helps to focus light coming into the eye onto the retina.

The cornea is also full of nerves that alert us to irritations that could potentially harm our vision and eye health. And the cornea is susceptible to injury. If you scratch your eye, you have actually scratched your cornea. Minor corneal scratches usually heal on their own, but deeper injuries can cause pain and sometimes corneal scarring.

A corneal scar can result in a haze on the cornea that impairs your vision. If you scratch your eye significantly, it's important to see an eye doctor. An eye doctor can view the cornea under a slit lamp biomicroscope.


The pupil is the hole or opening that is located in the center of the iris of the eye. The pupil controls the amount of light that enters into the eye. Pupil size is controlled by the dilator and sphincter muscles of the iris.

The pupil's job is very similar to a camera aperture which allows more light in for more exposure. At night, our pupils dilate to allow more light in to maximize our vision.

In humans, the pupil is round. Some animals have vertical slit pupils while some have horizontally oriented pupils. Pupils appear black because light that enters the eye is mostly absorbed by tissues inside the eye.


The iris is the colored part of the eye that controls the amount of light that enters into the eye. It is the most visible part of the eye. It lies in front of the crystalline lens and separates the anterior chamber from the posterior chamber.

The iris is part of the uveal tract—the middle layer of the wall of the eye. The uveal tract includes the ciliary body, the structure in the eye that releases a clear liquid called the aqueous humor.

Iris color depends on the amount of melanin pigment in the iris. A person with brown eyes has the same color of melanin pigment that a person with blue eyes. However, the blue-eyed person has much less pigment.

Crystalline Lens

The crystalline lens is a transparent structure in the eye—suspended immediately behind the iris—that brings rays of light to a focus on the retina.

Small muscles attached to the lens can make it change shape which allows the eye to focus on near or far objects.

Over time, the lens loses some of its elasticity. This causes the eye to lose some of its ability to focus on near objects. This condition is known as presbyopia and typically presents problems with reading, around 40 years of age.

A cataract is a clouding of the lens and is a common occurrence that comes along with aging. Fortunately, cataracts grow slowly and may not affect your vision for several years. By age 65, over 90 percent of people have a cataract. Cataract treatment involves removing the cloudy lens surgically and replacing it with an implantable intraocular lens.

Aqueous Humor

The aqueous humor is a clear, watery fluid located behind the cornea. It helps bring nutrients to the eye tissue. It is formed behind the lens and flows to the front of the eye to maintain the pressure inside the eye. Problems with the aqueous fluid can lead to issues involving the eye's pressure, such as glaucoma.

Vitreous Humor

The vitreous humor, which lies against the retina, makes up a large part of the eye. It is a jelly-like substance that fills the inside of the eye. Made mostly of water, the vitreous fluid gives the eye its shape. It is composed of water, collagen, and proteins and contains cells that help to maintain its clarity.

As we age, the vitreous humor becomes less firm. This change sometimes causes it to pull on the retina. If the force of the pulling becomes strong enough, the vitreous humor may actually separate from the retina. This is called a posterior vitreous detachment, as it normally occurs at the back (posterior) of the eye.


Located on the inside of the eye, the retina is the light-sensitive area located at the back of the eye that the lens focuses images upon, making vision possible. The retina is made up of 10 very thin layers. Within these layers are rods and cones that are used to detect color.

The retina is very fragile. A detached retina occurs when the retina is separated from the other structures of the eye.

It typically happens during contact sports or as a result of trauma. A retinal detachment is a serious injury that requires immediate attention by an eye care professional.


The sclera of the eye is better known as the "white of the eye." While we can only see the visible portion of the sclera, it actually surrounds the entire eye. It is a fibrous sac that contains the inner workings that make vision possible. It also keeps the eye in a rounded shape.

Scleritis is an inflammation of the sclera. It can cause intense eye pain, redness, and loss of vision for some people. It can also be associated with trauma or infection—more than half of scleritis cases are associated with an underlying systemic disease.


How Do Your Eyes Work? National Eye Institute website

The Eye. University of Michigan Kellogg Eye Center website.

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