Graphic courtesy of National Eye Institute
Like the glass on a watch, the cornea is the clear protective coating on the front of the eye that allows light to pass through it without distortion. It covers the colored iris. The lens of the eye focuses the images transmitted through the cornea to the retina. Therefore, it must be clear and regularly shaped to give good vision. These images are then transferred via the optic nerve to the brain, where sight is interpreted. A "scratched" cornea, when the surface layer is torn, is extremely painful.
The conjunctiva is the clear covering (like cellophane) of the white part of the eye, the sclera. When it gets irritated or infected it becomes red, which is called conjunctivitis. Conjunctivitis can be allergic or infectious; viral conjunctivitis is often called "pink eye."
The lids of each eye are vital to the preservation of sight. They maintain vision by keeping moisture inside, and foreign particles outside of the eye. Without eyelids, our eyes would quickly dry out from exposure to air, or could be damaged severely by a cinder, pebble, or some other foreign object. Think of how fast you blink when something approaches your eye.
There are many ways the lids may be damaged. They may be cut or burned in an accident, be involved in infections, or be invaded by many different types of tumors. These tumors can be benign (not cancerous) or malignant (cancerous). Two other common lid problems are droopy eye lids (ptosis) and baggy eyelids (dermatochalasis). Droopy eyelids can be acquired or congenital. Droopy eyelids are seen most often in children and baggy eyelids are more common in older people. However, both can usually be corrected quite successfully.
The lacrimal (tear) system is a miniature drainage network, which prevents tears from continuously rolling down the cheeks. From the surface of the eye, the tears flow along the edge of the lids toward the nose. Just before reaching the corner of the eye, the tears slide into two tiny drain tubes, and then into a larger tube which carries the tears into the nose. This explains why crying often causes one to blow one's nose.
Many problems may cause the tear system to drain poorly. For instance, an accident may cut one of the two tiny drainage tubes, a long-standing infection may scar these tubes, or tumors may invade parts of the tear system. All of these may lead to poor drainage causing the tears to well up in the eye and roll down the cheek. Fortunately, surgery or other kinds of procedures can usually relieve this uncomfortable situation and restore a functional system.
Every camera must have a lens to properly focus the picture. Your eye has a lens, too, which lies directly behind the pupil in the sac-like capsule. Your lens, which is about the size of an "M&M" piece of candy, is normally clear and transparent. It focuses images onto the retina, which acts as the film that records the picture. The picture is then transmitted by the optic nerve to the brain, where the image is interpreted. It’s the brain that does the actual seeing.
The macula is a part of the eye that contains special light-sensitive cells which allows us to see fine details clearly. It is located in the retina. The macula is an important part of the eye because even small changes can cause severe vision loss.
The optic nerve is the pathway that connects the eye to the brain. It is the means through which images captured by the retina reach the brain, where those images are interpreted.
The orbit is the bony housing in which the eyeball sits. If you place your finger on your brow and press down, you will feel the edge of the orbit. By continuing to move your finger around in a circle, you can feel the orbit protection provided the eyeball on all sides, except in the front where the lids protect the eye. Between the bony housing and the eyeball are other structures such as fat, muscle, blood vessels and glands. These are known as the orbital contents. The orbital contents may develop a tumor, causing the eye to protrude. Often, these tumors have to be removed to help maintain normal eye function. The orbit can also suffer fractures during trauma to the eye. Therefore, it is always wise to use certified, shatterproof eyewear whenever engaging in sports.
The dark center of the iris (the colored part of the eye) is the pupil. The pupil decides how much light is need for the eye to see properly. It changes sizes to adjust for changes in light.
The retina is the light-sensitive part of the eye. It has one major artery and one major vein, which is called the central retinal vein. Sometimes, branches of this vein can be blocked.
The sclera is the "white" part of the eye.
The uvea is the middle section of the eye. It has three parts: the iris (the colored part of the eye), the ciliary body and the choroid. Inflammation (or swelling) of any of these parts or their adjacent tissues is called "uveitis."
The vitreous is the clear jelly-like substance that fills the middle part of the eye.