How The Eye Works - A Fantastic Voyage Through The Eye

Vision is the product of sight. It is our interpretation of the world around us as seen through the eyes. Sight occurs when light reflected off an object travels through the various refractive elements of our eye and reaches cells that make up the retina that lines the back of the eye. The retina is similar to the film in a camera. When light reaches the film, the image changes the molecular makeup of this film, creating a picture. The optics of the eye have shifted the original image upside down, so the picture of the image is actually inverted by the time it rests on the retina. The change of the molecules within the retinal "film" causes the nerve fibers attached to send the information through various channels deep within the brain. These channel signals interact with one another and interact with the retina in the other direction (channel to nerve fibers to retina). The criss-crossing of signals back and forth act to orient the eyes toward what you are looking at so the retina can receive the proper information and send it back to the brain. The signals finally reach an area of the brain called the cortex. The cortex is responsible for our interpretation of what we see. The cortex helps us to convert the light-image into a meaningful experience. As newborns, we start with only sight and no experience. It is not until we are able to interpret what we see that true vision takes place and it takes place in the cortex.

Prepare for a "fantastic voyage" through the eye. The journey is the same journey light rays that enter your eye from the computer screen take. Picture yourself as you read this hitching a ride on a light ray from a light source such as the sun or a light bulb as we go along. The light beam leaves the source as the journey begins. The light beam is reflected off your computer screen. The light reflected from the screen first encounters the tear layers that cover the front surface of the eye. The tears are composed of three layers. The first layer our light ray encounters is the exterior oily layer of tears. Small glands that line the eyelid margins are responsible for secretion of the oil layer. They are stimulated to secrete oil by blinking and when the two lid margins touch during a blink, they spread the oil over the surface of the eyes. The next layer our ray travels through is the water layer of tears directly beneath the oil layer. The purpose of the oil layer is to prevent the water layer from evaporation. (See DRY EYE section) Traveling on, our light ray passes through the third and most posterior layer of the tears, the mucous layer. The mucous layer is a sticky layer that help the water layer adhere to the eye surface. Traveling beyond the mucous layer we encounter the front of the Cornea. The Cornea is the dome-shaped clear covering over the colored part, or Iris of the eye. The Cornea resembles a contact lens in appearance. The Cornea is responsible for most of the light bending (refractive power) of the eye. The light ray is bent more by the Cornea than any other structure in the eye. The Cornea is made of several layers. The first layer of the Cornea we travel through is the Corneal Epithelium, analogous to a thin transparent layer of skin on the surface of the Cornea. The Corneal Epithelium is a very thin layer, only about 5 cell layers thick. Next we encounter a very thin membrane called Bowman's Membrane. The function of Bowman's Membrane is unknown but is believed to be related to adhesion of the Corneal Epithelium. Much of Bowman's membrane is destroyed in certain laser refractive surgeries without much consequence. Bowman's membrane delineates the surface of the center of the Cornea, the Corneal Stroma. After squeezing by Bowman's Membrane we enter the Stroma. The Stroma is made up of a matrix of collagen protein organized in structures called Beta-pleated sheets. The sheets are stacked so close and tight on top of one another, that light passes through the stroma unimpeded. The structure of the collagen sheets is why the Cornea is transparent to light. If the packing of cells is disrupted by fluid, trauma or infection, the Stroma may lose some of its transparency and this may cause our light ray to scatter. Exiting the Stroma our ray passes through another more posterior membrane similar to Bowman's. This is Descemet's membrane. Descemet's membrane delineates a boundary between the Corneal Stroma and the next structure, the Corneal Endothelium. We pass through Descemet's, into the Endothelium, which is composed of 1 thin layer of specialized cells responsible for pumping fluid out of the Cornea in order to maintain clarity. Osmosis is the tendency of fluid to move from areas of greater concentration to areas of lesser concentration. Since the back of the Cornea is bathed in fluid from inside of the eye (the Aqueous fluid, which we haven't traveled through yet) there is a tendency of fluid to move into the Cornea. The Corneal Endothelium maintains the integrity of the Corneal Stroma by controlling osmotic influx of fluid into the Stroma from the Aqueous, keeping the Corneal Stroma clear and free of fluid. We pass through the Endothelium in to the Anterior Chamber of the Eye. The Chamber is filled with the Aqueous fluid that bathes the Endothelium, or back surface, of the cornea. The Aqueous fluid helps nourish the Corneal Stroma. Our light ray passes through the Aqueous humor. Directly in front of us it the Iris, or colored part, of the eye. The Iris is donut-shaped, with the hole being the Pupil of the eye. A lot of people are surprised to find out that the pupil is a hole and not a black spot. Our light ray will not touch the Iris, but go past it, through the pupil. The color of the Iris is a function of the amount of pigment deposited on its surface. The natural color of all Irises is blue. Green eyes have just enough brown pigment mixed in with the blue to give the appearance of green. The more brown the eye appears, the more pigment has been laid down on the iris. The Iris acts as a light regulator by manipulating the pupil size. The fibrous matrix that makes up the Iris is muscular and can expand, which enlarges the pupil to let more of our light ray into the eye or contract to let less light into the eye. In dim illumination such as nighttime, the Iris will expand so that more light can enter and we can see better. In bright illumination, the Iris contracts, making the pupil smaller so less light can get in and things won't appear too bright. The Iris is analogous to the sphincter of a camera. We pass through the pupil on our light ray and the next structure we encounter head-on is the lens of the eye. The lens is a clear, convex structure located directly behind the iris. As our light ray passes through the lens, the shape of the lens changes by becoming more convex or less convex, adjusting to aim our light ray for focus on the back of the eye (Retina). The change in convexity is what is known as "focus". The cornea provides the majority of focus for the eye, but corneal focus is fixed, that is, the cornea does not alter in curvature or convexity. Lens focusing is like a fine tuning mechanism and is what helps us bring blurry objects into focus. As our light ray leaves the lens of the eye, it enters the posterior chamber of the eye. The posterior chamber is filled with a gel-like substance called Vitreous Humor, or Vitreous. The Vitreous maintains the shape of the eyeball and holds the thin, sensitive nervous tissue in the back of the eye, the Retina, in place against the back wall of the eyeball. Assuming proper focus adjustment of the lens, our light ray travels through the Vitreous and heads directly for a part of the Retina called the Macula. The Macula comprises 10% of the Retina and is the area of the Retina light focuses on where we achieve our sharpest, most central vision. The Fovea is the center of the Macula. The fovea is analogous to the cross hairs of a tracking system. It helps the eye to move to lock object images onto the center of the macula. If the Fovea or Macula is damaged, anything we look directly at will appear blurry or rubbed-out. The Fovea is where the specialized cells that allow us to see in color are located. These cells are the Cone cells. The Fovea is tightly packed with cone cells. Most of the rest of the Retina is made up of Rod Cells. Rod cells help us differentiate shades of white and black. Images of objects in our side vision (peripheral vision) or objects viewed under poor lighting conditions are also transmitted through the Rod cells. Rod cells are responsible for our night vision. Our light ray has emanated from something we are looking directly at (the computer screen) the ray is focused on the Cone cells. Rod cells and cone cells represent a class of cells known as Photoreceptor cells. Photoreceptor means "receiver of light". The Photoreceptors of the retina house molecules of visual pigment. When light is focused on a Photoreceptor, the energy of the light breaks down the visual pigment molecules within the Photoreceptor into simpler molecules. The breakdown of the molecules triggers an impulse. The Rod and Cone cells are attached to nerve cells. The light of the ray has triggered a chemical change that initiates an electrical signal within a nerve cell attached to the photoreceptor. The nerve cell transmits information from the Retina throughout the visual system. All the nerve cells of the retina converge to form a single nerve, the Optic Nerve. All visual impulses travel through the optic nerve in a highly organized fashion towards the brain. At some point along the voyage, the nerve from the right eye joins the nerve from the left eye. The point where the nerve fibers from the 2 eyes mesh together is called the Optic Chiasm. The nerve fibers traveling posterior to the Chiasm carry images from the retina of both eyes. The visual signals collated in the Chiasm are sent posteriorly through structures called Optic Radiations. Some visual information from both eyes travels to structures in the brainstem called Nuclei. The nuclei of the brain are responsible for chores such as eye movements to ensure images fall onto the fovea, balance and reflexes related to orientation as your eyes see it and many other things, including breathing! Coordination of vision with balance, reflex or motion is termed Motor Coordination. An example of Motor coordination is picking up a cup of coffee. The nuclei of the brain use visual information received from the cells in the retina about the location of the coffee cup and send out impulses. The impulses travel to the appropriate shoulder/arm/hand and an adjustment is made to coordinate the visual and motor systems together to achieve the desired goal, in this instance picking up the cup. Information on location is used from the Retina throughout the process as the hand travels toward its goal to fine-tune the reaching-for-the-cup process. If a target we need to see is off to the left, the light reflected from that object will land to the right of the Fovea. Images landing on the retina to the right of the fovea travel through nuclei responsive to stimuli for that area of the retina. The nuclei get directions on the location of the coffee cup from the Retina and guide the hand to reach for the cup. Information is exchanged from Retina to Nuclei and back as the hand reaches in order to accurately locate and grab for the cup. Imagine being on a platform slowly moving left to right past the coffee cup. As you reach for the cup, your arm would have to continually adjust to counteract the motion of your body in order to grab the cup. Information about movement of the image of the coffee cup on your Retina is transmitted to the motor system nuclei, which direct impulses to the arm and the hand to help make fine-tuning adjustments to grab the cup. Other visual signals are sent to the very back of the brain, the occipital lobe, where sight information is processed into units that are meaningful to us and are used for perception, or our interpretation of what we see. One image can be made of hundreds and thousands of these units, and the combination of these units provides us with the perceptual experience of what we view with sight. The information processed in the back of the brain allows us to understand that the object we are thinking about grabbing for is a coffee cup and not a sharp or dangerous object or something else.

Our light ray has traveled from the computer screen, through tear layers, through cornea, anterior chamber of aqueous, pupil, lens, vitreous humor and to the photoreceptors where the information reflected from the page was broken down into nerve signals and processed in the brain into meaningful units. Sight has been transformed from a mechanical process into a perceptual experience through the complexities of the brain and visual system.