Anatomy of the eye: structures, characteristics and measurements

05/08/2025

The eye is a complex and fascinating organ that allows us to see and perceive the world around us. It is incredibly intricate and made up of various structures that work together to capture, focus, and transmit light to the brain. Below, we explain the most important parts of the eye, their main characteristics, and their approximate dimensions.

Cornea – Transparent outer layer that focuses light (1)
 

The cornea is the transparent, curved outer layer of the eye. Its main function is to protect the internal structures and to help focus light onto the retina.

• Diameter: 11–12 mm
• Thickness: 0.5–0.6 mm in the centre, up to 0.8 mm at the edges

Sclera – White, protective structure of the eyeball (2)

The sclera is the white part of the eye, a fibrous and tough layer that provides structure and protection.

• Diameter: approximately 24 mm
• Thickness: thinner at the front and thicker at the back

Conjunctiva – Protective and lubricating membrane of the eye (3)

The conjunctiva is a thin, transparent membrane that covers the sclera and the inner surface of the eyelids. Its function is to protect and lubricate the eye.

Pupil – Opening that regulates the entry of light (4)
 

The pupil is the circular opening that regulates the amount of light entering the eye, adjusting to different lighting conditions.

• Diameter: between 2 and 8 mm (depending on the light)

Iris – Coloured part that controls the size of the pupil (5)

The iris is the coloured part of the eye that surrounds the pupil. Its function is to control the size of the pupil and, therefore, the amount of light that enters.

• Diameter: approximately 12 mm

Lens – Natural lens that focuses at different distances (6)

The lens is a natural, transparent, and flexible structure located behind the iris. It is responsible for focusing on objects at different distances.

• Diameter: 9 mm
• Thickness: 4 mm

Retina – Light-sensitive layer that initiates vision (7)

The retina is a light-sensitive layer located at the back of the eye. It converts light into electrical signals that travel to the brain.

• Thickness: around 0.5 mm
• Diameter: similar to that of the sclera, about 24 mm

Optic nerve – Communication pathway between eye and brain (8)

The optic nerve connects the retina to the brain. Its function is to transmit visual signals so they can be processed and understood.

Other structures of the eye

In addition to these main parts, the eye also includes:

• Extraocular muscles, which allow the eye to move.
• Lacrimal glands, which produce tears to keep the eye lubricated and free of particles.

Understanding the anatomy of the eye is essential for understanding how vision works. Each structure plays a fundamental role in the visual process, from the entry of light to its interpretation by the brain. Taking care of our eyes and understanding how they function is key to maintaining good visual health throughout life.

Dr Silvana Belotto, ophthalmologist at the Barraquer Ophthalmology Centre