The External Eye
When you look at someone’s eye, you see the white eyeball which is a tough leathery shell called the Sclera. That’s covered by a clear membrane called the Conjunctiva, which sometimes turns red if it’s inflamed (conjunctivitis, etc.), or turns yellow (jaundiced) in certain liver diseases. Light enters the eye through the Pupil, which is just an opening. The size of the opening is controlled by the Iris, which determines how much light can enter the eye. The Iris makes the Pupil smaller (constricted) in bright light, and opens it wider (dilated) in darkness.
If we cut the eye vertically down the middle & look at it from the side, it’s easier to understand the different structures.
Inside the Eyeball
The thick Sclera is the outside “eyeball” or container. Ocular Muscles attach to it to move the eye. The thin Conjunctiva, which covers the part of the Sclera lying outside the skull, folds back to also line the Eyelids. The Lens, sitting right behind the Pupil, focuses light as it enters the eye. The Ciliary Body stretches or relaxes the Iris to control the size of the Pupil, and also manipulates the Lens to focus near or far. A dome of transparent skin called the Cornea covers the Iris and Pupil; the cornea also helps focus light, but without adjusting, and is where we’d place a contact lens. It’s thinner than in the diagram. The space between the Cornea and the Iris is called the Anterior Chamber; it’s filled with a clear gel called Aqueous Humor to maintain the Cornea’s shape.
After light travels through the Lens, it passes through a clear gel called Vitreous Humor, which maintains the shape of the eyeball. The light then lands on the Retina in the back of the eyeball. The Retina consists of many nerve cells that detect light — cells called Rods identify shapes, cells called Cones identify colors. Nerve connections from the Rods and Cones come together in a single bundle, the Optic Nerve, to carry the nerve impulses to the Brain (not shown). The Brain is what puts together and interprets the images, allowing us to “see”.
Diseases of the Eye
Different diseases can affect the different parts of the eye. Conjunctivitis, caused by a variety of germs & allergies that irritate the conjunctiva, is the most common cause of a red eye (or “pink eye”). The cornea can become scratched from trauma (Corneal Abrasion); infection of the cornea is called Keratitis. Iritis (also called Anterior Uveitis) involves the iris, anterior chamber, and possibly ciliary body; it can be due to infection by a variety of germs, to general inflammatory conditions (like certain types of arthritis, etc.), or even due to some medications. Glaucoma involves build-up of excessive pressure in the anterior chamber. The main condition affecting the lens is a Cataract.
Further back in the eye, infections and some inflammatory conditions of the retina are called Retinitis. The term Retinopathy refers to other types of retinal diseases, usually involving fragile blood vessels that can rupture. The most common is caused by long-standing diabetes; it also happens to premature babies born before 32 weeks, especially before 28 weeks. A Retinal Detachment occurs when part of the retina peels off, which needs to be repaired surgically. Macular Degeneration is a wearing-away of the macular, which is the most sensitive part of the retina & which has the most nerves we need for vision.
Scleritis, inflammation of the sclera, is serious but very rare. Episcleritis, inflammation of the covering of the sclera, is almost always harmless. Endophthalmitis, infection of the entire eyeball as a result of trauma or surgery, requires immediate IV antibiotic treatment to avoid losing the eye.
Any disease occurring in the line of sight, from the cornea to the retina, can cause visual loss and blindness. But note that the most common eye disease, Conjunctivitis, involves the conjunctiva which is located outside this line. The only common sight-threatening germ infecting the conjunctiva is Gonorrhea, which can spread to the entire eyeball.
There are other structures related to the eye, but not part of the eyeball itself. Bones form the eye socket (called the Orbit). Extraocular Muscles attach from the Orbit to the Eyeball, so the eye can move around. The Eyelids protect the eye and constantly blink, keeping it moist. The Lacrimal Gland, which produces tears, is located under the Upper Eyelid by the temple. Tears flow across the eye & drain down the Lacrimal Duct, in the corner of the eye by the nose.
Then there’s Refraction, meaning measuring vision to see if someone needs glasses. Medically, we don’t really consider that part of the eye [!!!!]. Being nearsighted or farsighted has nothing to do with eye diseases. Optometrists measure vision & prescribe glasses or contact lenses. Medical Clinicians deal with everything else.