Anatomy and Structure of the Human Eye (With Diagrams)

Edmund has spent the last ten years working in clinical research. He has written many articles on human anatomy and physiology.

REVIEWED BY
Bhavin Shah, Neurodevelopmental and Behavioral Optometrist specializing in myopia management, Central Vision Opticians

Parts of the Eye and How We See

The eye is the organ responsible for vision, which allows us to see and experience the world around us.

This article explores the anatomy of the human eye, looking at the different structures and their functions. The diagrams show cross sections of the human eyeball; as we journey through the different parts, refer to them to better understand their functions.

Eye Diagram

Structures of the Human Eye

Our eyeballs are roundish organs cushioned by fatty tissues. They each sit in a bony socket inside the skull that helps protect them from injury. Each of our eyes contains the following structures.

Sclera

The sclera is the outermost layer of the eyeball. It is the white (and opaque) part. The muscles responsible for moving the eyeball are attached to it at the sclera.

Cornea

At the front of the eyeball, the sclera becomes the cornea—the transparent, dome-shaped part of the eyeball. Light rays from the outside world first pass through the cornea before reaching the lens. Together with the lens, the cornea is responsible for focusing light on the retina.

Choroid

The choroid is the middle layer of the eyeball located between the sclera and the retina. It provides nutrients and oxygen to the outer surface of the retina.

Anterior Segment

This refers to the front third of the eye. The anterior segment contains both the anterior and posterior chambers.

Anterior Chamber

The space between the cornea and the lens is known as the anterior chamber. It is filled with fluid called aqueous humour.

Aqueous Humour

Aqueous humour is a transparent and watery fluid circulating in the anterior chamber. It provides oxygen and nutrients to the inner eye and exerts fluid pressure that helps maintain the shape of the eye. The aqueous humour is produced by the ciliary body.

Posterior Chamber

The posterior chamber is a larger area than the anterior chamber. It's located opposite the anterior chamber at the back of the lens. As the ciliary body produces aqueous humour, it flows first into the posterior chamber and then into the anterior chamber.

Posterior Segment

This refers to the back two-thirds of the eye. The posterior segment contains many structures, including the vitreous humour, retina, choroid and optic nerves.

Vitreous Chamber

Located in the posterior segment, this holds the vitreous humour.

Vitreous Humour

Vitreous humour is a transparent, jelly-like fluid that fills the vitreous chamber. It exerts fluid pressure that keeps the retina layers pressed together. It maintains the shape of the eye and keeps images sharp and in focus on the retina.

Iris

The choroid continues at the front of the eyeball to form the iris. The iris is a flat, thin, ring-shaped structure sticking into the anterior chamber. This is the part that identifies a person’s eye colour.

The iris contains both circular muscles going around the pupil and radial muscles radiating toward the pupil. When the circular muscles contract, they make the pupil smaller. When the radial muscles contract, they make the pupil wider.

Pupil

The pupil is the hole at the centre of the iris located in front of the lens. Whenever more light needs to enter the eyeball, the muscles in the iris contract like the diaphragm of a camera to increase or decrease the size of the pupil.

Lens

The lens is a biconvex, transparent disc made of proteins called crystallines. It is located directly behind the iris and focuses light on the retina. In humans, the lens changes shape for near and for distant vision.

Ciliary Body

The choroid continues at the front of the eyeball to form the ciliary body, which produces aqueous humour. The ciliary body also contains the ciliary muscles that contract or relax to change the shape of the lens.

Ciliary Muscles

The ciliary muscles are located inside the ciliary body. These muscles continuously change the shape of the lens for near and distant vision.

Zonules

The zonules, also known as suspensory ligaments, are a ring of small fibres that hold the lens suspended in place. They connect the lens to the ciliary body and allow the lens to change shape.

Retina

The retina is the innermost layer lining the back of the eyeball and the light-sensitive part of the eye. The retina contains photoreceptors that detect light. These photoreceptors are known as cones and rods.

Cones enable us to detect colours, while rods help us to see in poor light. The retina contains nerve cells that transmit signals from the retina to the brain.

Fovea

The fovea is a small depression in the retina near the optic disc. The fovea has a high concentration of cones. It is the part of the retina where visual acuity—the ability to distinguish shapes and details of objects at different distances—is greatest.

Optic Nerve

The optic nerve is located at the back of the eyeball. It contains the axons of retina ganglion cells (nerve cells of the retina) and transmits impulses from the retina to the brain.

Optic Disc

Impulses are transmitted to the brain from the back of the eyeball at the optic disc, also called the blind spot. It's called the blind spot because it contains no photoreceptors; hence any light that falls on it will not be detected.

Eye Muscles

The muscles of the eye are very strong and efficient; they work together to move the eyeball in different directions. The main muscles of the eye are the lateral rectus, medial rectus, superior rectus and inferior rectus.

Central Artery and Vein

The central artery and vein run through the centre of the optic nerve. The central artery supplies the retina, while the central vein drains the retina. In the diagram, the artery is shown in red, and the vein is in blue.

Tear Duct

This is a small tube running from the eye to the nasal cavity. Tears drain from the eyes into the nose through the tear duct. This is why teary eyes are usually accompanied by a runny nose.

Sources

• Vision Basics: How Does Your Eye Work?
• Eye Anatomy
• The Ocular Circulation

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