The Ophthalmic Center - How the Eye Works

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How the Eye Works

The EYE is a remarkable organ, intricate in design and unparalleled in its ability to anchor us in the world around us. This section of our website will give you a brief overview of the some of the better-known parts of the eye, what their function is and how they interrelate with each other. Simply click on the highlighted part of the eye you would like to learn more about.

The EYELIDS

The eyelids and lashes are important protective elements. Your eyebrows: are actually there to help prevent perspiration from dripping into your eyes from your forehead. Your eyelashes: or cilia as they re known serve as a sensor and screen against foreign particles that may come into contact with the eye. Eyelashes fall out after 3-5 months and new ones grow and take their place. If an eyelash is pulled out, a new one will be full-grown in about 2 months. Your eyelids: have glands that secrete oils that protect the delicate surface of the eye. This special oil prevents evaporation of your tearfilm and helps keep the tears in your eye so they do not spill out constantly. The skin that comprises the eyelid is thinner than anywhere else on the body so it folds easily and opens and closes rapidly. Eyes normally blink spontaneously about 15 times per minute and blink duration is about 0.3-0.4/seconds. Reflex blinking results from many different stimuli such as objects coming towards the eye or loud noises.

The SCLERA

"Don't one of you fire until you see the whites of their eyes!" Israel Putnam was referring to the sclera when he directed his men to look for the whites of the enemy's eyes at the Battle of Bunker Hill. The sclera extends from the cornea to the optic nerve and is made of a tough, fibrous tissue. It is part of the protective apparatus of the eye and shields the rest of the eye from germs, dust and other harmful matter. As with the rest of the eye, the sclera is actually avascular, and the blood vessels that are visible when you are tired or your eyes are irritated are largely "in transit". The sclera attaches to the orbit of the eye via 6 tiny muscles that also control the eye's movements.

The AQUEOUS

You may well ask how the eye gets its nutrients if there are so few blood vessels in it. The answer is that nutrients to and from the eye are carried by the aqueous humor. The aqueous has a viscosity not unlike water, but it is made up of proteins and other nutrients necessary for the health of the eye. Think of aqueous as a slow moving stream that carries nutrients to the eye and then carries cellular waste products from the eye. The steady formation and drainage of the eye's aqueous fluid maintains the intraocular pressure of the eye. It is this that is affected by diseases like glaucoma.

The VITREOUS

The vitreous is a jelly-like substance that is approximately 98.5% water and contains collagen and hyaluronic acid (a substance used in facial aesthetics.) The vitreous fills the back of the eye giving the eye its shape. It supports the retina and allows nutrients to pass through to it. The vitreous is also transparent and light travels through it to reach the retina. Because of this, there is a refractive element to the substance. The viscosity or thickness of the vitreous begins to thin with age and can be affected by certain diseases.

The CORNEA

The cornea is the clear covering over the eye that you see when you look in the mirror and touch when you poke yourself in the eye. Looks are deceiving in this case, for the cornea is actually a highly organized and complex group of cells and proteins The cornea is extremely sensitive because of the rich supply of sensory nerves it contains. If the surface of the cornea is scratched, the injury heals quickly and new cells generate before infection or changes in vision can set in. However, if the injury goes into the deeper layers of the cornea, the healing will take longer and there may be considerable pain, light sensitivity, tearing and blurred vision. The cornea provides a protective covering over the eye filtering out some of the most damaging ultraviolet (UV) wavelengths in sunlight. If not for the cornea, these dangerous rays have the potential to damage the lens and the retina. The cornea is the most powerful lens in the optical system of the eye. When light enters the cornea, it is bent or refracted onto the lens, which further refracts the incoming light onto the retina. In order to see clearly, light rays must be focused by the cornea to fall precisely on the retina.

The LENS

The lens has the shape of a flattened globe and sits behind the iris inside the eye. It is a transparent organ and focuses light onto the retina after it passes through the cornea and the pupil. The lens is made up of special fibers, completely surrounded by a capsule that helps maintain its shape and is suspended in position by tiny filaments. The lens of the eye expands and contracts to change the focusing power and refractive capabilities of the eye: this is known as accommodation and allows the eye to focus on objects far away and close up. Thirty-three percent of the total weight of the lens is made up of protein. This is more per size than any other organ in the body. (The brain is 10% protein.) Chemical changes begin to occur in the lens over time and it begins to harden, losing its elasticity and turning white. We call this process a cataract.

The IRIS and the PUPIL

The iris is a sponge-like fiber that is connected over the lens, doming out slightly. As the lens beneath it
expands, the iris fibers contract like the accordions on a fan. The iris varies in color from light blue to a dark brown depending on the number of pigments. In a blue eye, the light is absorbed over the body of the iris much like across a body of water where reflected rays make the water look blue. In a brown eye, there are many more pigments present reflecting light, hence the darker color. The iris contains two muscles, one that contracts the iris and one that expands it. The pupil is actually an opening in the center of the iris that allows light to enter the eye. Light rays bounce off the object you are looking at and comes back to your eye. When light is dim, the iris constricts to expand the opening of the pupil, allowing more light to enter the eye. When the light is bright, the iris decreases the size of the opening of the pupil, regulating just the right amount of light that enters the eye. All of this is done involuntarily and happens automatically.

The RETINA and OPTIC NERVE

If the eye is like a camera, then the retina is the film. Light enters the eye through the pupil and is projected onto the retina that comprises the back of the eye. The retina has seven layers and includes 130 million light sensitive cells. The light signals are converted into neural signals that the brain understands through a process called signal transduction. The photoreceptor cells of the retina are called rods and cones. Information leaves the eye and travels to the brain through the ganglion cells. The axons of the ganglion cells make up the optic nerve and are part of the brain. The brain interprets the impulses to determine how we actually see the world. For instance, the curvature of the cornea and lens of the eye flip images so that they are upside down when they reach the retina. The brain flips them again so that we see them right side up. Much of what we see is based on the relation between images. We use three dimensions to determine spatial relationships between objects. When images are flat, or only two dimensional, we lose a point of reference. In order to give you a better understanding of the relation between the light impulses that enter our eyes and what the brain does to interpret them, take a look at the following optical illusions: Optical means vision. An illusion is something that is not what it seems to be. An optical illusion is something that plays tricks on your vision. Optical illusions teach us about how the eye and brain work together to create vision. In our everyday three-dimensional (3-D) world, our brain gets clues about depth, shading, lighting, and position to help us interpret what our eyes see. But when we look at two-dimensional (2-D) images that lack some of these clues, the brain can be fooled.