The Eyes

The human eyes are made up of the following important structures:

  • Cornea
  • Conjunctiva
  • Sclera
  • Iris
  • Lens
  • Vitreous
  • Choroid
  • Retina
  • Optic Nerve
  • Extraocular muscles
Simple Eye

Cornea

The front, clear structure of the eye. Both the cornea and lens of the eye bend light to make light focus on the retina (light-sensitive structure) of the eye. Twice as much of the bending of light is done here compared to the lens. The cornea is also densely packed with nerves, so it's very sensitive! Contact lenses are often placed directly on the cornea to help correct blurry vision.

Conjunctiva and Sclera

The conjunctiva is the thin, clear, vascular tissue that covers the sclera - the "white of the eye" - and the back surface of the eyelids. The conjunctiva can become irritated and pink, hence the term "pink eye" - which is more correctly called conjunctivitis. The sclera is very tough - it helps the eye keep its round structure and protects the eye from anything that might otherwise damage the delicate internal structures inside.

Iris

The iris is the colored muscular tissue that dilates and constricts to control and regulate how much light enters the eye through the hole in the middle - the pupil.

Lens

Also called the "crystalline lens", the lens is suspended just behind the iris by a network of fine fibers called zonules. The ciliary muscle relaxes and constricts, which allows the lens to adjust and become thinner or fatter to change how light is bent. Unlike the cornea, the lens flexes to fine-tune the image on the retina during a process called accommodation.

Vitreous

The vitreous body or vitreous humor is a thick, jelly-like fluid that makes up most of the volume of the eye. The vitreous helps disperse nutrients and also acts as a shock absorber for the eye. You may see some small spots that move with your eyes - these floaters are clumps of vitreous that are casting a shadow on the retina of the eye.

Choroid

The choroid is the vascular network of the internal structures of the eye. These are relatively large vessels (relative to the eye) that help transport nutrients to and waste products away from the eye's internal structures.

Retina

The retina is the light-sensitive tissue of the eye. It's only about the thickness of a postage stamp, but this delicate tissue is the first structure to start the long visual pathway of taking light information and transforming it to electrical impulses that will be used to create a visual image. The retina is made up of rods (poor clarity but good motion perception cells) and cones (good clarity but poorer motion perception cells). The majority of cones are packed into a small area of the retina called the macula. This tiny area allows us to see with excellent clarity once an image is focused on it. The macula is also the area affected by the eye disorder macular degeneration. Patients with damage to this area maintain peripheral vision but may lose some or all of their fine central vision.

Optic Nerve

The optic nerve is bundle of nerve tissue called ganglion cells. Ganglion cells are present throughout the retina of the eye and function to collect nerve impulses from rods and cones and transport this information out of the eye and through the early portion of the visual system. The eye condition glaucoma can adversely affect optic nerve function.

Extraocular Muscles

The extraocular muscles work together to coordinate eye movements for vision. There are a total of 6 extraocular muscles that are controlled by 3 different cranial nerves (CN III, IV, and VI).

Take a look at these two neat videos of how the eyes allow us to see!

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