Ординатура / Офтальмология / Английские материалы / The Eye Book A Complete Guide to Eye Disorders and Health_Cassel, Billig, Randall_2001

10 / INTRODUCTION

and its derivatives, but it is not usually related to an older person’s blood cholesterol level. In some people the ring is very obvious, but the condition is so common as to be virtually normal, and it is of no significance.

The cornea is more than a window; it is a converging lens. As in a camera, it takes light rays and bends and focuses them to the back of the inside of the eye, the retina. The power of the cornea changes during growth but stays relatively constant in adults, with minor fluctuations in curvature causing slight shifts in our eyeglass prescription.

The Iris and Pupil

The iris, visible through the cornea, is composed of connective tissue and muscle with a hole in the middle. The color of the iris is actually due to the amount of pigment in the iris connective tissue layer. Brown eyes have a lot of pigment, blue eyes very little. (In the colloquial sense, if you have brown eyes, your iris is brown, and so on. No one knows why irises come in such a fascinating variety of colors and patterns.) The pupil is the hole in the iris, which allows light to reach the retina. The iris uses one muscle to constrict the pupil and another to actively dilate, or enlarge, it. Like an f-stop on a camera, the pupil is wider in a darkened room and narrower in full sunshine. The iris characteristically loses pigment and thins with aging, resulting in the surprisingly bright “China blue” color of the eyes of some older people. The pupil tends to become smaller with age.

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Around the iris, hidden by the sclera, is the ciliary body, the ring or tether that holds the lens in place, connected in back with the choroid, a bolstering, nourishing layer of blood vessels between the sclera and the retina. Normal aging changes the choroid, but usually not in any remarkable way. The ciliary body produces the aqueous, a watery solution that bathes the lens; manufactured behind the iris, it travels through the posterior chamber and the pupil and leaves the eye through a drain in the anterior chamber angle, where the iris meets the cornea. (This fluid must get out of the eye, and trouble can arise when its exit is blocked. An important job of the eye’s plumbing is to keep the pressure in the eye from becoming too high.) The aqueous is secreted by one group of cells and eventually leaves the eye through a drain or meshwork created by other cells. This balance maintains what’s called the intraocular pressure—and this, like the pressure in a balloon, is essential for maintaining the shape of the eye (and particularly for preserving the curvature of the cornea). (See chapter 8, on glaucoma, which is a problem that develops when eye pressure isn’t normal.)

The Lens

Behind the iris is the lens—about the size and shape of an M&M candy—fastened to the ciliary body by thin fibers called zonules. The lens is elastic: to focus, it stretches and snaps back into place. In the job of focusing light on the retina, the cornea does about three-quar-

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ters of the work and the lens about one-quarter; but the cornea has fixed focus power and the lens variable focus power—at least for a while. This brings us to a universally recognized signpost of aging, which arises when our eyes develop trouble mastering a task called accommodation. Accommodation takes place when a muscle in the ciliary body contracts, relaxing the tension on the zonules, thus allowing the lens to become less flattened and more spherical. Accommodation begins to diminish at about age ten, but the change is usually not noticeable until about age forty, when reading the small type in the phone book suddenly isn’t as easy as it used to be. What happens? The muscle of the ciliary body continues to work as well as ever, but the lens grows harder and less elastic; it no longer changes shape when the tension is relaxed. The result? The dawn of a sobering new phase in adulthood: dependence on reading glasses.

The Retina

Inside the sclera is the vascular choroid; inside the choroid is the retina, the reason for being of everything else in the eye. The exquisitely complicated retina does far more than merely register an image like a bit of photographic film. Indeed, cells in the retina break down an image into countless elements—brightness, position, color, movement—then encode all these elements as electrical signals and transmit them to the brain. Remarkably, all of this is done not only faster—literally— than the blink of an eye, but faster than we can even

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comprehend without the help of highly specialized scientific instruments.

The intricate structure and function of the retina could be the subject of an entire book in themselves, but briefly: First, layers of rods and cones receive the basic units of light, triggering a photochemical reaction in these cells. Then bipolar cells apparently receive, organize, and transmit this information to ganglion cells, which send these signals to the brain through a collection of nerve fibers called the optic nerve. Though many diseases affect the retina, normal aging does not affect the retina in a predictable manner. This is especially true in the macula, the area of the retina responsible for central vision, which is needed for such functions as reading and fine visual acuity. The macula, the most significant part of the retina, is located next to the optic nerve and is responsible for our central vision (as opposed to our peripheral vision). The macula is composed mostly of cones and is also an important part of color vision.

Most of the interior of the eye is filled with vitreous, a nearly transparent substance that resembles Jell-O in texture. Vitreous is supported by scaffolding, a meshwork of collagen fibers, and a gel of hyaluronic acid. In normal aging, this gel slowly liquefies, allowing the meshwork to collapse and clump. The meshwork is thicker against the retina, at a site called the posterior vitreous membrane, attached at the optic nerve, the ciliary body, and various other places in the retina. Sometimes, as the gel liquefies and the meshwork collapses, the posterior vitreous membrane can slide or suddenly pull off from its at-

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tachment with the retina or optic nerve, resulting in visual floaters or flashes—a condition known as a posterior vitreous detachment (see chapter 15).

The Muscles

Part of the process of aiming the eyes to see an object is turning the body and moving the head. But the eyes can also move independently; in other words, you can move your eyes without turning your head. To keep the eyes correctly focused on a moving object, like a baseball, while the body is running and turning, is truly an impressive bit of engineering. There are six extraocular muscles fastened to the eye (in addition to muscles such as the ciliary muscles, which are inside the eye). The extraocular muscles are a medial and lateral rectus, mostly for horizontal movement, and superior and inferior rectus and oblique muscles, for vertical and torsional (twisting) movements. Understanding the actions of individual muscles in different eye positions gets complicated; there is a sophisticated feedback system in the brain to keep the eyes focused together, to maintain binocular vision (that is, two eyes working together), and avoid double vision (diplopia). Unless you have a specific muscle disease or some damage to the brain areas that control eye movements, the coordinated movement of your eyes shouldn’t be affected by aging (see chapter 2).

As this brief overview indicates, the eye is one of the most complicated structures in the body, and its func-

Image not available.

Fig. 1.7. How we see an object

What Happens When We See

Figure 1.7 illustrates what happens when we see something. Light, bounced off an object, reaches the eye and is refracted by the cornea and lens and focused into an image on the retina. But the eye’s workings are like a camera’s, which means that the image made on the retina is upside down and reversed. It’s up to the brain to make sense out of it.

You may be aware that the left side of the brain controls the right hand and vice versa. Well, many functions of vision operate under this same confusing crossover system. With your eyes, in other words, the right half of the visual field in each eye goes to the left half of the brain. As figure 1.8 indicates, the right half of the visual field corresponds to the left half of each retina. The fibers from the left retina of the left eye go to the left half of the brain. The fibers from the left retina of the right eye cross over to the left brain via the optic chiasm (from chi, the Greek letter X).

Image not available.

Fig. 1.8. The crossover system between the eyes and the brain