Ординатура / Офтальмология / Английские материалы / Relearning To See_Quackenbush_2000

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T H E T H R E E L A Y E R S O F T H E E Y E

See Plate 2: The Three Layers of the Eye.

The eye can be classified into three basic groups:

1. The three layers of the eye: The outer layer consists of the sclera and cornea; the middle layer consists of the choroid, ciliary body, lens, and iris; and the inner layer consists of the visual and non-visual portions of the retina.

2. The fluids and chambers of the eye: The anterior and posterior chambers are filled with aqueous humor; and the vitreous chamber is filled with vitreous humor.

3.The external parts of the eye: the optic nerve, eyelids and tear glands, and the six external muscles.

T H E O U T E R L A Y E R : S C L E R A A N D C O R N E A

several dozen layers of epithelial cells, which are like the sheets of glass used to make safety glass in automobiles.

Because blood vessels are excluded from the cornea, Ught can pass through it more perfectly to the retina. The cornea receives nutrients on its inner surface from the aqueous humor, on its outer surface from tears and oxygen from the air, and along its circumference from blood vessels in the sclera.

The cornea is a convex lens and accounts for 80% of the curvature needed to focus light rays onto the retina. By bending light rays inward, the cornea and lens shrink the large image of the world down to the size of a nickel onto the retina.

T H E M I D D L E L A Y E R : C H O R O I D , CILIARY B O D Y , L E N S , A N D I R I S

The Sclera

The sclera (pronounced skleh'-rah; from the Greek skleros, meaning "hard") is the eye's protective, leather-like outer layer. It is strong, thick, and opaque. This "white of the eyes" covers about % of the outer surface of the eyeball.

The Cornea

The clear, crystalline front of the eye is called the cornea (from the Latin corneus, meaning "horn-like"). The hard, tough cornea is the part of the sclera that has become transparent, and it allows light to enter the eye. The cornea bulges forward in a dome-like shape. In adults the cornea is about one-half inch in diameter—a little smaller than the size of a dime—and covers the remaining lA of the eye's outer surface. The cornea consists of

The Choroid

The choroid lies between the sclera and the retina. The choroid consists of many blood vessels and provides nutrients to the entire eye, but especially to the retina.

The choroid is discussed further in Chapter 17, "The Retina."

The ora serrata is the notched junction between the choroid and the ciliary body.

The Ciliary Body

The ciliary body is a highly vascularized, enlarged continuation of the choroid that encircles the lens.

Within the ciliary body is the ciliary process, which produces aqueous humor. Suspensory ligaments extend between the ciliary process and the lens capsule, 3600 around the lens.

The ciliary body contains a circular (parasympathetic) ciliary muscle, and a merid- ional-radial (sympathetic) ciliary muscle. The contraction of the circular muscle decreases the circumference of the ciliary body, like the narrowing of the iris in bright light. The contraction of the radial muscle expands the ciliary body, like the enlarging of the iris in dim light.

Most orthodox books on eyesight state that the contraction and expansion of the internal ciliary muscle changes the shape of the front side of the lens to give it more and less curvature, respectively. More on this theory in later chapters.

The Lens

Behind the iris and in front of the vitreous body lies the double convex, transparent lens. The front side of this "living crystal" touches the back side of the iris and is nourished by the aqueous humor. The back side of the lens contacts the vitreous body.

The lens is enclosed in a transparent membrane called the elastic capsule. The suspensory ligaments between the lens capsule and the ciliary body "suspend" the lens vertically, behind the iris.

The lens is composed of many microscopic, onion-skin-like layers, and accounts for the remaining 20% of the curvature needed to focus light rays onto the retina.

The lens grows slowly each year due to a constant addition of external layers. The older, inner layers, which cannot be absorbed or discarded, are compressed in the middle of the lens. The lens doubles in size between the ages of 20 and 80.

Orthodox textbooks state that the hardening of the lens into a relatively flat shape

Chapter Two: Anatomy

is the reason many people lose their ability to see clearly up close around age 40; this is called presbyopia, or "old-age" sight.

Theories of the role of the lens are discussed more in later chapters.

The Iris

In front of the lens lies the iris. The iris is a colored (pigmented), circular, and variable diaphragm- A pupillary sphincter muscle along the inner circumference of the iris surrounds the pupil. When the pupillary sphincter muscle contracts, the pupil becomes smaller. When the dilator muscle contracts, the pupil becomes larger. The pupil is not a physical structure; it is an opening in the center of the iris, through which light enters the eye.

The iris regulates the amount and distribution of light entering the eyeball. In the brightest light, the diameter of the pupil is about 1.5 mm (with an area of only 2 mm2); in very low levels of light, the diameter expands to about 9 mm (with an area of 64 mm2); the average diameter is about 4 mm (with an area of 13 mm2).

Changes in the pupil size can easily be observed in a mirror while turning a light on and off; the iris constricts and dilates, respectively. The pupil normally appears black because most of the light entering the eyeball is absorbed by the retina and choroid. Very little light is reflected out through the pupil.

See Plate5: Suzie Q's Red Eyes. Red pupils appear in some photographs. In dim light, the pupil is large. When the high-intensity bulb on the camera flashes, a lot of light enters the eye. The retina glows red because lights reflects from the blood vessels in the retina and choroid.

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Modern cameras have been able to reduce "red eyes" by turning on a special "red-eye reduction lamp" before the picture is taken. The pupil has a chance to contract small and thus much less light enters the eye. The result is a picture with a normal, black pupil.

T H E I N N E R L A Y E R : T H E R E T I N A

The retina is the inner third layer, covering about 95% (back, sides, and part of the front) of the interior surface of the eye. The entire eyeball is designed for the retina.

There are two parts of the retina: the visual and non-visual portions.

The Visual Portion of the Retina

The rear 70% of the retina contains light receptors, called cones and rods.

The design of the visual portion of the retina is discussed in great detail in Chapter 17, "The Retina."

side of the iris. The much smaller posterior chamber lies between the back side of the iris and the lens, lens capsule, suspensory ligaments, and ciliary body.

These two chambers contain aqueous humor, which means "watery fluid." Aqueous humor supplies the cornea and the lens with nutrients. Aqueous humor is referred to by an ophthalmologist consultant as "clear blood." Aqueous humor is produced by the ciliary process and secreted into the posterior chamber. From there, it travels slowly around the iris through the pupil into the larger anterior chamber. The entire volume of the aqueous humor is replenished every hour. The aqueous humor's pressure helps maintain the

cornea's convex shape.

Aqueous humor also "percolates" from the posterior chamber into the vitreous chamber.

Excess aqueous humor, along with dead cornea cells, drains away through the Canal of Schlemm, which encircles the cornea. The Canal of Schlemm discharges these fluids and cells into veins.

The Non-Visual Portion of the Retina

The other 30% of the retina, the non-visual portion, extends forward from the visual portion at the ora serrata, along the back part of the ciliary process and the back side of the iris up to the pupil.There are no light receptors in the non-visual portion of the retina.

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O F T H E E Y E

A Q U E O U S H U M O R , A N D T H E A N T E R I O R A N D POSTERIOR C H A M B E R S

V I T R E O U S H U M O R A N D C H A M B E R

The vitreous chamber lies behind the lens and comprises the majority of the volume of the eye. It is almost completely surrounded by the visual portion of the retina. Filling the vitreous chamber is a "jelly-like" clear liquid called the vitreous humor.

Positive intraocular pressure created by the vitreous humor helps hold the rear fourfifths of the eye in its round shape.

I HE EX'l E R N A L P A R rS OF T H E EYE

See Plate4:Aqueous Humor.

The anterior chamber lies between the back (inner) side of the cornea and the front

The external parts of the eye consist of the optic nerve, eyelids and tear glands, and the six external (extrinsic) museks.

The eye socket is lined with fatty tissue which: 1) cushions the eye from blows to the head; 2) lubricates the continually moving eyeball; and 3) provides warmth.

T H E O P T I C N E R V E

The optic nerve is the second cranial nerve and the second-largest nerve in the human body. This nerve transmits the signals from the 137 million light receptors in the retina to the brain. The central nervous system is directly exposed to light stimulation via the retina and optic nerve—the only part of the human body where this occurs.

Chapter Two: Anatomy

rior), outer (lateral), and inner (medial) parts of the eye. When contracting, a rectus muscle shortens and pulls backward on the part of the eye where it is attached. For example, when the superior rectus muscle contracts, the eye rotates upward. When the medial rectus muscle contracts, the eye rotates inward, and so on.

Much of Bates' research was directed toward the role of these muscles in errors of refraction and accommodation.

In this chapter we discuss various types of refractive, or "corrective," lenses that are com­ monly used in prescription glasses and con­ tact lenses. In this book, the term "lense" refers to an artificial lense, while "lens" refers to the natural lens inside the eye.

F O U R T Y P E S O F R E F R A C T I V E L E N S E S

Figure 3-1 shows a piano lense and four types of refractive, or "corrective," lenses com­ monly used in glasses or contact lenses—con­ cave, convex, cylindrical, and prismatic.

Of course, the term "corrective" does not mean that the lense corrects the cause of the vision problem; only the angle of light rays entering the eyes changes. As Bates stated, "corrective" lenses are more correctly referred to as "compensating" lenses.

Figure a shows a piano lense. Since a piano lense has no curvature, parallel light rays con­ tinue in straight paths through the lense; it is not really a "corrective" lense. A piano lense has no focal point. Notice the image seen through the lense on the right is the same as the original image on the left.

Piano lenses are often used in safety glasses

to protect the eyes from injury. They are also used for cosmetic reasons. For example, if one eye has no sight, but the other eye uses a cor­ rective lense, a piano lense can be placed in glasses in front of the sightless eye.

Figure b-i shows a double concave lense, which can compensate for the refractive error in nearsightedness. A double concave lense is a diverging lense because the light rays "spread out" after passing through the lense. A diverging lense has a "virtual" focal point in front of the lense.

A meniscus lense is concave on one side and convex on the other. Notice how the front side of the meniscus lense in Figure b-2 is convex, while the back side has a higher degree of concavity. This type of meniscus lense is a diverging lense. Contact lenses are often meniscus lenses.

In glasses for nearsightedness, a meniscus lense is usually used in place of a single or double concave lense, mainly for cosmetic reasons.

Figure с shows a double convex lense, which can compensate for the refractive error in farsightedness. A convex lense is a

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• • С

b-1. CONCAVE, NEARSIGHTED, "MINUS" LENSE: b-2. MENISCUS LENSE

Since the direction of the light rays emerging from a piano lense does not change, the image does not change.

converging lense because light rays converge to a point after passing through the lense. A converging lense has a focal point beyond the lense.

Figure d shows a cylindrical lense, which can compensate for the refractive error in astigmatism.

Figure e shows a prismatic lense, which can compensate for an eye with strabismus.

Glasses can have more than one "correc­ tion" combined into one lense. For example, a lense can be both diverging and cylindrical, compensating for nearsightedness and astig­ matism.

mathematical definition of a diopter is the reciprocal (or inverse) of the focal length in meters.

When parallel light rays from a distant object travel through a typical (piano) win­ dow in a home, the light rays simply continue straight through—without changing direc­ tion. Diopters do not apply to piano lenses because there is no focal point.

Most corrective lenses are made in multi­ ples of 0.25 D—for example, 0.50 D, 1.25 D, and 3.75 D. Some lenses are made in 0.125 D increments. A total correction of less than 0.25 D in one eye is seldom prescribed.

U N D E R S T A N D I N G L E N S E S : D I O P T E R , AXIS , A N D B A S E

DIOPTERS

A diopter, abbreviated "D," is a unit of mea­ surement of the refractive power of a con­ cave, convex, or cylindrical lense. The number of diopters indicates the light-bending abil­ ity of a lense. The more diopters, the more refractive power of a lense, and, generally, the more curvature in a lense.

When parallel light rays pass from air through a curved piece of glass or other trans­ parent material, they change direction. When parallel light rays pass through a convex lense, the rays converge to a focal point at some dis­ tance beyond the lense. This distance, mea­ sured in meters, is called the focal length. The greater the curvature of the lense, the greater the change in direction of the emerging light rays, and the shorter the focal length.

Since it is convenient to have a system of measurement in which a lense with a higher refractive power corresponds to a higher value, the dioptric system was created. The

DIOPTERS AND DIVERGING LENSES

A diverging lense with a small amount of cur­ vature and a long focal length of -2 meters is a -0.50 D lense; 1 ч- -2 meters - -0.50 D.

The minus sign in front of the 0.50 D indi­ cates there is a virtual focal point located in front of the diverging lense. Since parallel light rays emerging from a concave lense diverge, there is no focal point beyond the lense. How­ ever, there is a virtual focal point located in front of the lense. This focal point is deter­ mined by drawing rays in the opposite direc­ tion of the diverging rays, so that they converge at a point in front of the lense. Since the focal length is in the opposite direction of the direction of original light rays, the num­ ber of meters has a minus sign in front of it.

A diverging lense that has a little more cur­ vature with a shorter focal length of-i meter is a -1.00 D lense; 1 -=—1 meter = -1.00 D. A diverging lense that has much greater curva­ ture with a much shorter focal length of -% meter is a -6.00 D lense; 1 -f -A meter =

D.

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The nearsighted eye is too long from front to back. A "-" diverging lense is used to focus light rays farther back into the eyeball, onto the retina.

Some materials have a higher index of refraction than others. Therefore, a lense with a high index of refraction and low curvature may have the same refractive power as a lense with a low index of refraction and a high curvature. Lenses with a high index of refraction are sometimes used in glasses for people with high errors of refraction, i.e., very blurred vision. The thinner lenses are lighter and cosmetically pleasing. However, some people have difficulty adjusting to them.

DIOPTERS AND CONVERGING LENSES

A converging lense with a small amount of curvature and a long focal length of 2 meters is a +0.50 D lense; 1-7-2 meters = +0.50 D.

The plus sign indicates the focal point is beyond the converging lense.

A converging lense that has a little more curvature with a focal length of 1 meter is a +1.00 D lense; 1 -f 1 meter = +1.00 D. A much stronger convex lense with a focal length of only Vs meter is a +5.00 D lense; 1 -f Vs meter

= +5.00 D.

The farsighted eyeball is too short from front to back. A converging lense is used to focus light rays closer to the front of the eye, onto the retina. Glasses made with converging lenses are often called "magnifiers" or "readers."

DIOPTERS AND CYLINDRICAL LENSES

Diverging and converging lenses have equal curvatures in all planes. These lenses bend light equally in all planes and bring the rays to a focal point.

A cylindrical lense bends light rays in only one plane. Think of a lense in the shape of a can of soup that has been cut in half vertically. When a horizontal plane of parallel light rays passes through this cylindrical lense, the light rays come to a vertical "focal line" at some distance beyond the cylinder.

However, when a vertical plane of light rays passes through the same cylmdrical lense, the light rays continue straight through the lense without converging. The direction of the original vertical plane of light rays is not affected by the lense.

Since a cylindrical lense brings a plane of parallel light rays to a "focal line," there is a dioptric measurement associated with the cylindrical lense. A cylindrical lense can have "+" or "-" diopters. The sign in front of cylindrical diopters is not a measure of nearsightedness or farsightedness, and it is not important for the discussion in this book. We will consider only the magnitude, or absolute value, of the number of diopters for astigmatism correction; the plus or minus sign in front of a cylinder diopter number is ignored here.

In nearsightedness and farsightedness, the eyeball is too long and too short, respectively, but it is still round from the front point of view. In astigmatism, the eyeball is oval, or lopsided, from the front point of view, like a teaspoon or football. The amount, or magnitude, of this "ovalness" is measured in the diopters.

The oval shape in astigmatism can be oriented at any angle. It can be horizontal (like a lemon lying on its side), vertical, or any other angle. The angle of astigmatism is called axis. The axis determines the angle, or orientation, of the cylindrical lense put into glasses that compensate for astigmatism. Axis is not a measure of the amount of the astigma- tism—only its angle.

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Chapter Three: Understanding Lenses and Prescription*

Figure 3-2: Images Distorted by Astigmatism.

In astigmatism, one plane of light can focus in back of the retina, as in farsightedness, while another plane of light can focus in front of the retina, as in nearsightedness. This is because one plane of the cornea can have too much curvature, while

another plane has too little curvature.

A prism uniformly changes the angle of all incoming parallel light rays. The path of the light rays simply changes to a new direction. Since there is no focal point, the power of the prism is not measured in diopters.

Prism correction for strabismus is measured in units of base, and is indicated by the prism symbol, A. For example, iABO, called "one prism base out," is a relatively small correction for an eye that turns slightly inward.

4ABI, "four prism base in," is a larger correc-

In astigmatism, the shapes of objects at all distances or only at specific distances can be distorted. Astigmatism can also create multiple images of, or shadows around, an object.

With astigmatism, vertical lines on a piece of paper may appear to be darker or lighter than horizontal lines.

The discovery of astigmatism is attributed to

ASTIGMATISM WHEEL

While looking at the stripes of the Astigmatism Wheel, move this page closer and farther from you; then move the page in a circular motion; then rotate the page clockwise and counterclockwise. Do the same while looking at the figures on the right.

Do some stripes appear gray while others appear black? Do some of the stripes appear less clear than others? If so, you may have astigmatism.

Figure 3-3: Astigmatism Chart.

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tion for an eye that turns outward. 2ABU,"two prism base up," is for an eye that turns down. 2ABD, "two prism base down," is for an eye that turns up.

Strabismus and prisms are discussed fur­ ther in Chapter 18, "Stereoscopic Vision."

VISUAL ACUITY AND EYE CHARTS

DISTANCE "20/20" VISION

In 1864, a test for visual acuity was devised by the Dutch ophthalmologist Herman Snellen. Using the sight of an anonymous young Dutch man (actually, Snellen's assis­ tant) as his standard for normal vision, Snellen created a chart with letters on it. The "Snellen" chart was used to test the sight of children reading the chalkboard from the back of a classroom.

The Snellen chart has different-size letters on it.

A black letter E, which is 3/s" in height and width, placed twenty feet away, occupies a 50 area in the macula. The macula is the area in the center of the retina with a high concen­ tration of cones. Cones pick up sharp detail (acuity). If the three black horizontal lines and the two horizontal white spaces in the letter E are of equal width, a horizontal stroke or white space occupies a i° area in the center of the macula, called the fovea. The fovea con­ tains the highest concentration of cones within the macula (and the retina).The letter E's three horizontal black strokes plus the two horizontal white spaces, at i° each, equal 50.

The distance of twenty feet is important, because, for all practical purposes, the eye accommodates only within the first twenty feet. If an object at twenty feet is clear, (usu­ ally) objects farther away will also be clear.

Snellen placed several Ун" letters, like the letter E described above, in a row. Snellen's

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Figure 3-4: Snellen Eye Chart.

assistant could read this line, so Snellen called it the "20/20" line. When all of the letters on the 20/20 line can be read with one eye, with­ out correction, you are said to have "normal," "perfect," or "20/20" sight for distance vision in that eye.

Larger letters on the eye chart correspond to vision less than 20/20 sight. For example, if you can read all of the lAn letters, you have at least 20/30 sight; reading the line with %" letters is 20/40 sight. The fine with а 3У2" let­ ter is 20/200. This is usually the largest (top) letter, the letter E on the Snellen chart.

If you can read all of the letters on the 20/30 line, but only most of the letters on the 20/20 line, you may have slightly less than 20/20, or 20/20", vision. If you read all of the letters on the 20/100 line and some of the let­ ters on the 20/80 line, you may have slightly better than 20/100 vision, or 20/100* sight.

MACULAR I

Figure 3-5: The 20/20 "E" for Distance.