From Light to Vision: Anatomy of the Eye
The Cornea is the clear, round, dome-shaped membrane on the surface of your eye that normally is about 0.5 mm thick (0.002 inches; a dime is about 1.0 thick). Contact lenses rest on the Cornea, and since the Cornea has no blood vessels, it receives most of its oxygen from the air. So, wearing a contact lens is a bit like trying to breathe through a wet towel. Rigid, gas-permeable contact lenses allow the most oxygen to pass through, so they are the best type of lenses, if you are concerned about the health of your eyes. The cornea has more pain fibers per unit area than any other place on the body, so a small scratch here hurts, a lot.
The Cornea, together with the Lens, focuses light on the Retina, much like the lenses in a camera focus light on the film. Most of the focusing power (about two thirds) comes from the Cornea. A cross-section of the cornea should give a circle, but with Astigmatism, a corneal cross-section looks slightly foot-ball shaped. Near-sightedness (Myopia), and Far-sightedness (Hyperopia) are also sometimes due to an “abnormal” shape to the cornea, although they are more often caused by an abnormally large (myopia), or abnormally small (hyperopia) shape to the entire eyeball. Most current Refractive Surgeries irreversibly alter the shape of the Cornea.
The Anterior Chamber is simply a space behind the Cornea and in front of the iris that contains a clear, nutrient fluid aqueous humor. The so called “drainage angle” trabecular meshwork is also located here. The trabecular meshwork, or TM, is a circle of specialized tissue where aqueous humor drains out of the eye. It is located at the intersection of the cornea, the sclera (the white part), and the iris, and when it is damaged, glaucoma is often the result. About 95% of patients with glaucoma have the “open angle” type; anatomically the angle is open and fluid flows up to the TM, but microscopically the TM is clogged. Unfortunately, fluid inside the eye is continuously produced by the cilliary body, and when the TM is clogged, the pressure in the eye rises, just like over inflating a ball. High pressure literally crushes the optic nerve, which becomes progressively damaged, and vision is slowly lost. In the vast majority of cases, glaucoma is completely painless and vision loss occurs so slowly (over many, many years) that people don’t recognize that they have a problem until they are nearly blind in both eyes, or completely blind in one eye.
A small group of people with glaucoma have “narrow angle” or “angle closure” glaucoma. In these people, the iris mechanically blocks the trabecular meshwork, and their eye pressure can rise suddenly, with abrupt onset of pain and vision loss. Most people with glaucoma have a very slow and completely painless vision loss.
The Pupil is literally a hole in the paper thin Iris. The Iris is one part of a layer which wraps itself around the inside of the eye called the Uvea. The parts of the Uvea include the Iris , Cilliary body (a tire like structure just behind the root of the Iris), and Choroid (a thin layer located in between the sclera and retina). The Iris contains a circular muscle that constricts the pupil in bright light, and radial muscles (like spokes on a wheel) that dilate the pupil in low light. Inflammation of the Iris is called Iritis, a painful condition characterized by sensitivity to light.
The Lens is shaped just like a round, magnifying lens. It is located behind the Pupil and Iris, and is attached to the Cilliary Body by hundreds of thread-like Zonules. The Lens works together with the Cornea to focus light on the Retina. When the lens is cloudy or opaque, it is called a Cataract. Cataracts are present, at least to a small degree, in virtually everyone over 60 years of age, but can occur in people of any age, even in newborns. Remarkably, the lens also has the ability to Accommodate. Accommodation is where the lens changes its shape (thickens), thereby changing its power, allowing us to focus on near objects. Between ages 42 and 65, the lens slowly hardens and loses its ability to accommodate. This process is called Presbyopia. People with Presbyopia need glasses to read. Everyone becomes presbyopic.
Anatomically the eye is similar to a basketball, with Vitreous filling the eye much the same way that air fills a ball. Vitreous is also known as Vitreous Humor, and, contrary to what you might have thought, it is not a liquid. Vitreous is really a sticky, gooey gel with several firm attachments to the Retina. By and large the Vitreous is clear, but you may actually have seen your vitreous floating around! There is one area of the vitreous in particular that is thickened and opaque; normally this little “attachment” rests on the head of the Optic Nerve. Since the Optic Nerve Head is our natural blind spot, you can’t see this bit of Vitreous. Eventually, however, this little attachment separates from the optic nerve head, and you can see it floating about on the inside of your eye. Often this spot looks like a circle, or spider web, or fly… there seem to be many insect-like configurations, but other times you will only notice a dark spot floating in your vision. Usually floaters are harmless, but occasionally they signal the beginning of a retinal detachment, bleeding, or infection.
The Retina is a thin film of tissue that coats the inside of the eye. If you were to stick a sharp stick into the white part of your eye, it would first pass through the conjunctiva, a clear protective layer that turns red if you rub your eye (infections of the conjunctiva are called conjunctivitis, or pink eye), then sclera, the white part, then choroid, a pigmented layer (the iris is part of the choroid), then retina, then finally the vitreous cavity. The retina has blood vessels running through it that provide nutrients, like oxygen, but they only bring in enough goods for the inner two-thirds. The outer third of the retina gets its oxygen and nutrients from blood vessels in the choroid. A retinal detachment occurs when the retina separates from the choroid. For example, if a break forms in the retina, fluid from the vitreous goes underneath the retina, and peels it off of the choroid (the layer just beneath the retina). Now, since one third of the retina gets nutrients from the choroid, if you separate the retina away from the choroid… You guessed it… if you separate the retina from the choroid, then it dies. Yes, two thirds of the retinal layer is still alive, but that is not enough to see with, and the retina isn’t ver good at regenerating.
But having a retinal detachment doesn’t necessarily mean that you will go completely blind, if only an area of peripheral retina was detached. As you read this print, light rays originating from the words are striking your retina in a highly specialized area called the Macula. Still looking at the screen, you can also see, in your peripheral vision, your mouse, and a bunch of junk cluttering up your desk. The macula has the highest acuity (and the best color vision), but it only covers a very narrow range, and it only works in relatively bright light. Using your peripheral vision, you can’t read the writing on your mouse, but using your macula, you can. On the other hand, if you are trying to navigate around the house, your peripheral vision is what keeps you from bashing into door frames and furniture. If you only had your maculas, walking in the house would be like trying to map out the inside of a dark room using only a narrow-beamed flashlight. Another nice feature about the peripheral retina is that it is much more sensitive to light than the macula. In fact, in very dim light, your maculas are hardly working at all. Pilots are trained to use their peripheral vision when flying at night for this reason.
Macular degeneration is, unfortunately, a relatively common disease in the elderly where the maculas (a specialized part of the retina) degenerate and can no longer function. In the worst cases central vision is completely lost in both eyes. Imagine holding a basketball in front of your face, hanging off the tip of your nose and you’ve got a pretty good idea of what can happen. Patients with macular degeneration maintain their peripheral vision, so they can function independently, but the lack of central vision makes reading, driving, and most other visually demanding activities difficult, at best. Fortunately the majority of people diagnosed with macular degeneration will maintain reasonable central vision throughout their lives, and some people will have no visual loss whatsoever. Everyone who has been diagnosed with macular degeneration should test their vision daily with an Amsler Grid.
If you think of the eye as a ball, the inside layer is coated with retina. Each cell in the retina is connected to a nerve fiber (imagine a very, very thin thread) and all the nerve fibers come together in a bundle called the Optic Nerve. Ok, the eye is a ball, and the retina and nerve fibers are on the inside, so they have to get out so they can carry information to the brain, and so there has to be a hole in the back of the eye. This hole (anatomically it’s really more like a window with a screen over it) in the sclera is our natural blind spot (one per eye) and it’s filled with the head of the optic nerve, which is just like a bundle of threads (1.5 mm in diameter). When the pressure inside the eye rises (glaucoma), this spot also happens to be the weakest spot in the eye, so the optic nerve literally is crushed, and it dies. For some reason, the nerve fibers that go to our peripheral vision die first, so glaucoma is characterized by peripheral vision loss.
Even though glaucoma is due to an abnormality in the trabecular meshwork, visual loss from glaucoma occurs due to damage to the optic nerve. There are many other diseases of the optic nerve, but fortunately they are quite uncommon.