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Mayo Clinic Health Library

Slide show: A look inside your eyes

Updated: 03-06-2020


Illustration of the eyelid

Your eyes are your windows to the world — but they need to be shielded from the elements to keep you seeing clearly.

Upper and lower eyelids protect the front of your eyeballs by blocking foreign objects and bright light. Your eyelids also involuntarily open and close (blink) every few seconds when you're awake.

Eyelid muscles

The eyelid muscles

One of the muscles involved in blinking is called the orbicularis oculi. During each blink, fluid produced by tear (lacrimal) glands passes over the protective dome of clear tissue at the front of your eyes (cornea) and lubricates the surface of the eyes.

Tear drainage pathway

The lacrimal gland and ducts

Fluid drains out of the eye through the lacrimal ducts. This process helps keep your eyes moist and washes away germs, dust and stray eyelashes.

The front of your eye

Illustration of the pupil, iris and sclera

The front of your eye has major parts:

  • Sclera. This is the white part of your eye. It is a tough outer wall that helps protect the eye's delicate internal structures. A thin transparent tissue, called the conjunctiva, covers the sclera. Some of the blood vessels visible in the white part of your eye are located within the conjunctiva.
  • Pupil. This dark spot is an opening at the center of the iris. It regulates the amount of light that enters your eye.
  • Iris. This is the colored part of your eye. It contains a ring of muscle fibers that expand or contract the size of your pupil to control the amount of light entering your eye. This muscle also helps the eye's lens focus on the object of interest.

Inside the front of your eye

Illustration of cornea, lens and vitreous cavity

Behind the scenes, other parts of your eye work to help you see:

  • Cornea. This is a protective dome of clear tissue at the front of your eye. It functions as a convex surface that helps focus light rays before they're fine-tuned by the lens.
  • Lens. The lens is a clear, elliptical structure that sits behind the iris. The curvature of your lens changes to sharpen your focus.
  • Vitreous cavity. The vitreous cavity extends from the back of the lens to the back of your eyeball, helping to maintain its shape. This area is filled with a clear, jellylike substance called the vitreous.

The back of your eye

Illustration of optic nerve and retina

Structures at the back of your eye:

  • Retina. The retina is a thin layer of tissue that lines the back inner wall of your eyeball. It consists of millions of cells that capture the images focused onto them by your cornea and lens. When light hits these cells, electrical impulses are generated and carried to your optic nerve.
  • Macula. The macula is a specialized part of the retina located in the center of the back of the eye. This patch of densely packed light-sensitive cells is essential to your central vision and allows you to see fine detail.
  • Optic nerve. The optic nerve carries information gathered by your retina to your brain.

How your eyes move

Illustration of eye muscles

Each eyeball has six muscles attached to the sclera — the white part of your eye. These muscles, five of which are shown above, allow you to move your eye and track an object without turning your head. The eye muscles also allow you to shift your field of gaze left, right, up, down and diagonally. Your brain coordinates these eye movements so that both eyes can move together when tracking an object.

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