Eye Anatomy: Parts Of The Eye
The Parts Of the Eye and Their Functions
The eye is a complex organ with many parts — including the cornea, pupil, iris, retina and optic nerve. Light enters the eye and is focused by the cornea and lens, so it lands directly on the light-sensitive tissue, the retina. Signals from the retina are sent to the brain and interpreted as images.
The eyes sit in the orbit — bony sockets layered in fat tissues, providing protection and aiding function. The orbit also contains muscles and nerves that attach to the eyeball and help the eyes to move in sync.
Six muscles are responsible for moving the eye in every direction. These muscles are called extraocular muscles. They attach all around the eyeball, moving it up, down and to the sides, and even rotating it.
Working together, the eyes provide depth of vision and allow us to have a field of view of around 200 degrees.
The eye can be understood best by looking at it in layers, from the outermost layer to the innermost layer.
The outer layers of the eye serve many functions, one of the most important being protection.
The conjunctiva covers the inside of your eyelids and then wraps back to cover the white part of the eyes. This is why it is impossible for contact lenses to get lost inside the eye! The conjunctiva folds back and becomes the outer covering of the white part (sclera) of the eyeball.
In addition to keeping contact lenses out of the eyeball, the conjunctiva keeps the eye moist and lubricated and helps to fight eye infections. When someone gets “pink eye” — conjunctivitis — the conjunctiva has become swollen and irritated, usually due to an irritant, allergies or an infection.
Just below the conjunctiva is the episclera. The prefix “epi” means above. This layer lies above the sclera, which we will discuss next. It contains blood vessels that help support the sclera, providing oxygen and nutrition.
When you see the whites of someone’s eyes, you are looking at the sclera. It is a tough and fibrous layer that provides protection and shape to the eyes. The muscles that move the eyeball — extraocular muscles — attach to it.
The cornea at the front of the eye begins where the sclera ends. This clear tissue covers the colored part of the eyes — the iris— and the opening where light enters the eyeball — the pupil.
The five-layered cornea is responsible for most of the eye’s focusing power due to its dome shape. Some people’s corneas have a shape that requires correction for astigmatism with glasses or toric contact lenses.
In addition to focusing light, the cornea also plays a protective role, serving as a barrier against particles and germs. The cornea is also very sensitive — with free nerve endings that are 300 to 600 times greater than the skin! Finally, the cornea protects the eyes by filtering out a good deal of the ultraviolet radiation from the sun.
The middle layers of the eye are dynamic structures with multiple functions, from helping to control the amount of light that enters the eye to providing the inside of the eye with oxygen and nutrients through a blood vessel network.
What color are your eyes? This color is due to the iris. The iris of the eye controls how much light gets into the eyes. It is made of muscles that react to light by making your pupil larger or smaller.
The dark circle inside the iris — the pupil — is where light enters the eye. When you are in bright light, the iris makes the pupil larger, and when you are in dim light, the iris makes the pupil smaller.
The lens is positioned just behind the iris so that light entering the eye through the pupil goes through it. The clear, crystalline lens of the eye helps to focus light. The lens becomes thicker so you can see clearly when you look close up, which is known as accommodation.
As you get into your mid-40s, the lens begins to harden, resulting in the onset of presbyopia. This is when most people will start to need bifocal and multifocal contact lenses and glasses. The lens can become cloudy with age, forming what is known as a cataract.
Anterior Chamber and Posterior Chamber
The anterior and posterior chambers are located in the front part of the eye, from the cornea to the front of the crystalline lens. The area from the cornea to the iris is called the anterior chamber. The area from the iris to the front of the lens is called the posterior chamber.
A transparent and watery fluid called aqueous humor is constantly being made in the posterior chamber. It is made by a structure called the ciliary body, which is located behind the iris. The aqueous humor flows through the pupil into the anterior chamber and then drains out where the iris meets the cornea.
If the production and drainage of aqueous humor are not properly balanced, too much pressure can build up. Too much pressure in the eye is one of the major risk factors for a condition called glaucoma, which can damage the nerve — the optic nerve — that leads from the eye to the brain. This damage can lead to progressive loss of peripheral vision.
Ciliary Body and Ciliary Muscle
The ciliary body produces aqueous humor. It also includes the ciliary muscle. The ciliary muscle causes the crystalline lens to thicken when someone looks up close.
This thin layer of blood vessels helps to maintain the outer layers of the retina (the inner light-sensitive tissue in the back of the eye), providing oxygen and nutrients and carrying away waste. It is located between the sclera and the retina and is part of the uvea. The uvea is made of the iris, the ciliary body and the choroid.
The inner layer of the eye is where the action takes place — at least, your ability to see the action depends on this layer.
Between the back of the lens and the retina is a substance that fills the back two-thirds of the eye called the vitreous humor. It is clear and jelly-like and provides support to the inside of the eye while maintaining its round shape.
The innermost lining of the back of the eye is called the retina. This tissue is the location of cells called photoreceptors — specifically rods and cones. When light travels through the cornea and lens and past the vitreous humor, it lands on the retina and stimulates the rods and cones, which transform this light into electrical signals.
Macula and Fovea
The macula lutea is a part of the retina with a very high concentration of photoreceptor cells. It is located centrally and provides detailed vision and much of our color vision.
The fovea centralis is a pit in the macula with a high density of cones. It gives us our sharpest vision.
When the photoreceptors transform light into electrical signals, these signals are carried to the brain by nerve fibers that bundle together to form the optic nerve. More than one million nerve fibers make the optic nerve that sends messages to the brain.
The brain interprets the electrical signals carried by the optic nerve as images — and you experience all the shapes and colors of the world around you!