The Eyes: How They Work

Your visual system consists of your eyes, nerve pathways, and your brain.  Your eyes are where the visual process begins.  Nerve pathways extend from your eyes and carry messages throughout your brain for processing.  Your brain interprets what you see—colors, shapes, movement, and more, to let you know what is going on in your environment.

Your eyes are sensory organs that are used for the sense of vision. Your eyeballs are round and almost an inch in diameter.  They are located in the orbits (eye sockets) at the front of your skull.  Six ocular muscles suspend and move each eye.  The orbit is filled with fat which acts as a shock absorber to protect the eyeball and fill the orbital space.
 
Your eyelids cover and protect your eyes.  Your eyelids keep your eye moist and clean each time you blink.  Your eyelashes keep dirt and particles from touching your eye.  The lacrimal glands produce fluid (tears) to keep your eyes moist and comfortable.  The conjunctiva is a thin film that covers the inside of your eyelids and the outer layer of the sclera.  The sclera is the white part of your eye.  The sclera is a tough protective coat that covers most of your eye.  

Your eyes and brain work together to produce images about your environment.  Light rays first enter your eye through the cornea, the “window” of your eye.  The cornea is a clear dome that helps your eyes focus.
 
The anterior chamber is located behind the cornea and in front of the iris.  The anterior chamber is filled with a fluid that maintains eye pressure, nourishes the eye, and keeps it healthy.  The iris is the colored part of your eye.  Eye color varies from person to person and includes shades of blue, green, brown, and hazel.  The iris contains two sets of muscles.  The muscles work to make the pupil of your eye larger or smaller.  The pupil is the black circle in the center of your iris.  It changes size to allow more or less light to enter your eye.
 
After light comes through the pupil, it enters the lens.  The lens is a clear curved disc.  Muscles adjust the curve in the lens to focus clear images on the retina.  The retina is the inner coating of the eye running from the edge of the iris back to the optic nerve.  It is itself an extension of the optic nerve and contains the photoreceptors that allow you to see.
 
Your inner eye or the space between the posterior chamber behind the lens and the retina is the vitreous body.  It is filled with a clear gel substance that gives the eye its shape.  Light rays pass through the gel on their way from the lens to the retina.

A main purpose of the eye is to focus light on the retina.  The retina is the inner layer of the eye.  Think of the retina as a movie screen on which everything that you see is displayed and interpreted by your brain as 3D images. 
 
The retina is a thin tissue layer that contains millions of nerve cells.  The choroid is the lining underneath the retina.  The choroid contains blood vessels that supply your retina with blood and oxygen to keep it healthy.

 
The nerve cells in the retina are sensitive to light.  Cones and rods are specialized receptor cells.  Cones are specialized  for color vision and detailed vision, such as for reading or identifying distant objects.  Cones work best with bright light.  The greatest concentration of cones is found in the macula and fovea at the center of the retina. 

 

The macula is the center of visual attention.  The fovea is the site of visual acuity or best visual sharpness.  Rods are located throughout the rest of the retina and allow you to function at night and pick up motion.
 
Your eyes contain more rods than cones.  Rods work best in low light.  Rods perceive blacks, whites, and grays, but not colors.  They detect general shapes.  Rods are used for night vision and peripheral vision.  High concentrations of rods at the outer portions of your retina act as motion detectors in your peripheral or side vision.  
 
The receptor cells in the retina send nerve messages about what you see to the optic nerve.  The optic nerves extend from the back of each eye and join together in the brain at the optic chiasm.  The optic chiasm is the place where the optic nerves from the right and left eye meet and cross one another.  From the optic chiasm, the nerve signals travel along two optic tracts in the brain, and eventually to the occipital cortex, where vision is perceived.