Human Eye Anatomy: Parts, Functions, and Diseases Explained with Diagrams

More than 70–90% of the information we perceive from the world comes through our sense of sight. In humans, the eyes are highly complex and delicate organs designed to capture light, convert it into electrical signals, and send those signals to the brain for interpretation.

Understanding the structure of the eye helps explain how vision works and what happens when diseases affect it.

What Is the Human Eye?

The eye is a spherical sensory organ approximately 2.5 cm in diameter and weighing 7–8 grams. Its primary function is to detect light and transmit signals through the optic nerve to the occipital lobe of the brain, where visual images are processed.

Each eye can move thanks to six extraocular muscles attached to the outer white layer of the eye (the sclera). These muscles work in coordination to allow smooth and precise movements.

The eye has two main fluid-filled chambers:

• Anterior chamber (contains aqueous humor)
• Posterior chamber (contains vitreous humor)

During development (embryogenesis), the retina forms from the diencephalon, the same embryonic structure that gives rise to the thalamus and cerebral hemispheres.

Layers of the Eye

The eye is composed of three main layers:

Layer	Structures Included	Main Function
Outer layer	Sclera, Cornea	Protection, light refraction
Middle layer	Choroid, Iris, Ciliary body, Lens	Nourishment, focus, light regulation
Inner layer	Retina, Macula, Fovea	Vision (image capture and processing)

Main Parts of the Eye

1. Sclera

• Commonly called the “whites of the eyes.”
• A tough, opaque layer that protects the eye and maintains its shape.
• Provides attachment for the extraocular muscles.
• Connects to the cornea at the front and to the optic nerve at the back.

🔗 Learn more: American Academy of Ophthalmology – Eye Anatomy

2. Cornea

• Transparent, dome-shaped structure at the front of the eye.
• Rich in nerve endings, making it extremely sensitive.
• Functions as the primary refractive surface, bending light toward the retina.
• Irregular curvature of the cornea can cause astigmatism.

3. Choroid

• A vascular layer located between the sclera and retina.
• Supplies oxygen and nutrients to the retina.
• Contains pigment that absorbs excess light to prevent blurry images.

4. Iris

• Colored part of the eye (blue, green, brown, etc.).
• Controls the size of the pupil, regulating how much light enters.
  • Contracts in bright light (miosis).
  • Dilates in dim light (mydriasis).

5. Pupil

• Dark, circular opening in the center of the iris.
• Adjusts automatically to control light entry.

6. Ciliary Body

• Connects the iris to the choroid.
• Produces aqueous humor.
• Responsible for accommodation (changing the lens shape to focus on near or distant objects).

7. Lens (Crystalline Lens)

• Transparent, biconvex, and flexible.
• Works like a camera’s zoom, changing shape to focus images.
• With age, the lens thickens and loses elasticity, leading to presbyopia (difficulty reading up close).
• Clouding of the lens results in cataracts.

8. Retina

• The innermost, light-sensitive layer.
• Converts light into nerve impulses.
• Contains photoreceptors:
  • Cones → detect color (red, green, blue).
  • Rods → enable night and peripheral vision.
• Disorders:
  • Myopia (nearsightedness) → light focuses in front of the retina.
  • Hyperopia (farsightedness) → light focuses behind the retina.
  • Color blindness → cone malfunction.

🔗 Learn more: National Eye Institute – Retina

9. Macula and Fovea

• Macula → yellow, oval region in the retina responsible for central vision.
• Fovea → tiny depression within the macula, richest in cones, crucial for sharp vision (e.g., reading, recognizing faces).
• Degeneration leads to macular degeneration, a leading cause of vision loss in older adults.

10. Aqueous Humor

• Clear fluid filling the anterior and posterior chambers.
• Functions:
  • Nourishes the cornea and lens.
  • Maintains intraocular pressure.
• Poor drainage can cause glaucoma (optic nerve damage).

11. Vitreous Humor

• Gel-like substance filling two-thirds of the eyeball behind the lens.
• Helps maintain eye shape and keeps the retina in place.
• Imperfections or clumps in the vitreous create “floaters”, seen as small specks in vision.

Common Eye Diseases (Summary Table)

Disease/Condition	Affected Part	Main Effect
Astigmatism	Cornea	Blurred or distorted vision
Cataracts	Lens	Cloudy vision, glare sensitivity
Presbyopia	Lens	Difficulty focusing on near objects
Glaucoma	Aqueous humor/Optic nerve	Progressive vision loss
Macular Degeneration	Macula/Fovea	Loss of central vision
Color Blindness	Retina (cones)	Difficulty distinguishing colors
Floaters	Vitreous humor	Shadows or spots in vision

Final Thoughts

The human eye is a highly specialized sensory organ that allows us to interact with the world. From the cornea that first bends light to the retina that sends signals to the brain, each part works together seamlessly.

Maintaining eye health through regular eye exams is essential for preventing diseases such as glaucoma, cataracts, and macular degeneration.

For further reading:

• National Eye Institute
• American Academy of Ophthalmology
• Mayo Clinic – Eye Diseases