Detailed Structures of the Eyeball

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  • 0:25 Humor
  • 1:09 Tunics
  • 3:08 Rods & Cones
  • 3:39 Optic Disk
  • 4:25 Fovea Centralis
  • 4:50 Lesson Summary
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Lesson Transcript
Instructor: Rebecca Gillaspy

Dr. Gillaspy has taught health science at University of Phoenix and Ashford University and has a degree from Palmer College of Chiropractic.

Your eyeballs are a lot like fluid-filled balloons wearing three coats or tunics. Structures within these tunics allow you to see colors, focus on objects, and see in dim light. Learn about the structures of the eyeballs and how they help you see.


Your eyes allow you to see what's going on around you, but did you ever wonder how your eyes allow you to see different colors, bring objects into focus, or see even when the lighting is dim? These functions are thanks to the tiny structures found inside your eyeballs. In this lesson, we'll learn about the internal anatomy of the eye to gain a better understanding of how your eyes help you navigate through life.


If we look at the basic structure of the eyeball, we see that it's somewhat like a fluid-filled balloon that's covered by three coats. The fluid inside the eyeball is referred to as the humor. It helps the eyeball maintain its shape. Fluid in the small anterior chamber of the eye is called the aqueous humor, and fluid in the large posterior chamber of the eye is called the vitreous humor.

Aqueous humor is constantly being secreted into the anterior chamber. To avoid pressure build up, aqueous humor is drained through the canal of Schlemm. If this drainage canal gets blocked, pressure within the eye builds, leading to a condition known as glaucoma.


The three coats of the eyeball are collectively referred to as the tunics. The outermost covering of the eye is referred to as the fibrous tunic. It's made up of the sclera and the cornea.

Did you ever hear the military expression, 'Don't fire until you see the whites of their eyes?' Well, the whites of the eyes are actually the sclera. The cornea isn't white; in fact, it's the clear, transparent window at the front of the eyeball. If you ever wear contact lenses, you actually rest the lens on top of your cornea.

The middle covering of the eye is called the vascular tunic. It's made up of the choroid, ciliary body, and iris. The term 'vascular' refers to 'blood supply,' and we see that the choroid is a dark-brown covering that has a rich blood supply.

In the front of the eye, the choroid meets the ciliary body. The ciliary body contains smooth muscle that controls the shape of the lens. Since the lens is the structure that focuses incoming light rays, we can say that the ciliary body helps bring our world into focus.

The iris is the pigmented, muscular ring surrounding the pupil of your eye. Its pigment is what gives you your eye color, and its muscular ring controls the size of the pupil. As the tiny, involuntary muscles of the iris contract and relax, the opening of the pupil changes. This change in size regulates how much light is allowed to enter your eyes. You can see this reflex in action when you shine a low-intensity flashlight into a friend's eyes. The sudden increase in brightness causes the pupils to get smaller.

The innermost covering of the eye is sometimes called the nervous tunic but commonly referred to as the retina. The retina lines the back of the eyeball, senses light, and sends signals to the brain about what is being seen.

Rods & Cones

It's within the retina that we find two types of photoreceptors known as the rods and cones. The prefix 'photo' means 'light,' and the suffix 'receptors' can be thought of as 'responders.' So, your rods and cones are literally the light responders of your eyes. Yet, they don't pick up the same things. The cones allow you to see colors. You can remember that by remembering that cones and color both start with the letter 'C.' The rods allow you to see in gray tones when the light is dim.

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