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

xx / PREFACE

ucts of any close-up task that requires prolonged use of the eye muscles. What can you do to relieve the strain? Take frequent breaks. Rest your eye muscles by periodically looking up or across the room. Keep the monitor twenty-four inches away. You may find it helpful to use a pair of “computer glasses,” so that your eyes don’t have to work so hard. (See your eye doctor.)

If you look cross-eyed long enough, your eyes will stay that way.

They won’t.

P A R T I

Introduction

1

A Guide to the Adult Eye

No matter how hard we fight it, certain unavoidable things happen to the body over time. Our skin starts to sag, for example; our bones begin to thin; so does our hair.

And inevitably, just like the rest of the body, our eyes age, too. But because these changes are often much more subtle and incremental—in other words, because the eyes staring back at us in the mirror still look about the same as always—it’s difficult for most people to detect them immediately.

This chapter is designed to help acquaint you with the parts of the eyes and how they work together so that we can see. In the next chapter we’ll take a look at what’s been happening to your eyes over the years, and later in the book we’ll cover specific problems—and what you can do about them—in much more detail. Now, let’s begin with a quick review of the eye’s basic anatomy and

4 / INTRODUCTION

the changes that most of us can expect to encounter as time goes by.

The Orbit

First, imagine a ping-pong ball. That’s about how big your eye is. But unlike a simple ping-pong ball, your eye is wonderfully complex and intricately layered. Now, think of your eyelids as movable curtains on a stage. What we see of the eye is just the front surface that’s visible between the opened lids; backstage in the eye is just as important and interesting. As you read the following description of the anatomy of the eye, you may find it helpful to refer to figure 1.1, panels A, B, and C.

Nature provided the soft, vulnerable eye with excellent protection—a layered cradle, or socket, of bone called the orbit. The orbit has heavy bone on its outer edges, thinner bone on the inner (nasal) surfaces, and also a pillow of fat, which cushions the eye. This cavity also contains the muscles and nerves that allow your eyes and eyelids to move; blood vessels, which nourish and sustain eye tissue; and the lacrimal gland, which produces the tears that lubricate the eye. (Imagine how difficult and painful it would be to move your eyes back and forth, or open and close your eyelids, without this moisture!) The lacrimal gland is also part of the eye’s defense system: in response to the sting of chemicals or onions, or to such irritants as dust or pollen, it turns on the faucet to dilute and wash away anything that might harm the eye. (For other reasons not entirely understood, this

A GUIDE TO THE ADULT EYE / 5

tear gland also responds to grief, great joy, and other strong emotions.)

Fortunately, unlike other bones, orbital bone—which is comparably thin to begin with—is not susceptible to osteoporosis: it does not thin or weaken significantly with aging. In very elderly persons, the cushion of orbital fat sometimes atrophies, or shrinks, causing the eyes to sink noticeably back into the skull—which may pose an aesthetic problem, but not a functional one. In other people the orbital fat may herniate forward into the eyelids, causing abnormal puffiness or even “bags” under the eyes (see chapter 10).

The Eyelids

The eyelids are covered on their inner aspect with a sheet of thin, slippery membrane called conjunctiva, which folds back and connects to the front surface of the eye. Over this lining, and giving the eyelid some rigid support structure and strength—like a tab insert in a collared shirt—is a tough, fibrous plate of connective tissue called the tarsus; then come layers of muscle and skin. A thinner layer of fibrous tissue, called the orbital septum, connects the tarsus to the periosteum, the outermost layer of bone covering the orbit. The orbital septum is a thin fence that keeps the previously mentioned layer of orbital fat confined inside the bony orbit. As the orbital septum weakens over time, a few chunks of orbital fat can poke through this fence, making unsightly lumps in the skin under the eyes. (Note: Such lumps are “bad” only

Image not available.

Image not available.

8 / INTRODUCTION

in that they’re not terribly attractive. For many people these lumps present only a temporary problem, because they can be, and often are, removed surgically.)

As the skin of the eyelids ages—just like the skin on the face or arms—it tends to lose its suppleness and begins to droop. Sometimes the skin of the upper lid can sag enough to interfere with vision. (In the lower lid, this drooping skin—“bags under the eyes”—does not interfere with seeing, although again, some people consider it a cosmetic problem.) Sagging, excess skin on either or both eyelids can be taken care of with minor surgery, a procedure called blepharoplasty (see chapter 10).

At the edges of the eyelids are ducts for glands that secrete an oily substance, which helps keep tears from oozing out of the eyes and onto the skin. Here also are the bases of the eyelashes: delicate, efficient filters that protect our eyes from dust and myriad other foreign objects. Our eyelashes tend to become more sparse as we get older. Interestingly, they become lighter, but rarely do they turn white with age.

The Sclera and Cornea

The “white” of the eye visible between the lids is the front portion of the sclera, a thick, protective sheath that encircles the eye, with a porthole at its very front. At this porthole, sitting like a watch glass on its casing, is the cornea. The cornea is normally transparent, like a camera lens; through it you can see the iris and pupil. The cornea has no blood vessels, while the conjunctiva and

A GUIDE TO THE ADULT EYE / 9

episclera (two tissue layers that cover the sclera) do—a major difference and important for understanding the discussion of corneal neovascularization later in this book (see chapter 5).

The back surface of the sclera is connected to the tough outer covering of the optic nerve, the cable that links the eye with the brain. (If you think of the eye as a kind of TV camera, the brain is where the electrical signals are sorted out and transformed into an image that makes sense.) Sometimes the white sclera of the eye develops dark areas (a condition called focal senile translucency of the sclera). Don’t worry—these dark areas are of no significance. They’re caused by calcium deposits, which cause the sclera to lose its normal white color, allowing the dark pigment inside the eye to show through. (Oddly, calcium in the cornea causes the same problem in reverse: the cornea loses its transparency and turns white.)

The domed cornea is like a cake with five layers: the epithelial, or outer-lining, cells (the icing, if you will, on this cake); Bowman’s layer; the tough but transparent stroma, the bulk of the cornea (the cake itself ); Descemet’s membrane; and a single layer of endothelial, or inner-lining, cells. The inner endothelial cells tend to decrease in numbers and become thin with age. If too many endothelial cells are lost, through old age or injury, the cornea becomes cloudy, but fortunately the endothelial cells usually last a lifetime. As people get older, the outer cornea tends to develop a white ring called arcus senilis. This corneal ring is composed of cholesterol