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Chapter Title
Anatomy and
Physiology of the Eye
CONTENTS
Chapter 1. Anatomy of the Eye
3
Overview ...............................................................................................................3
Learning Objectives ..............................................................................................4
External Structures of the Eye...............................................................................4
Other External Structures .....................................................................................4
Anterior Segment ..................................................................................................5
Cornea ..................................................................................................................5
Conjunctiva ...........................................................................................................7
Iris .........................................................................................................................7
Anterior Chamber .................................................................................................8
Posterior Chamber.................................................................................................8
Lens ......................................................................................................................9
Posterior Segment ..............................................................................................10
Vitreous Humor ...................................................................................................10
Retina .................................................................................................................10
Summary ...............................................................................................................12
Knowledge Check ................................................................................................13
Knowledge Check Answers .................................................................................16
Chapter 2. Physiology of Vision
17
Overview ............................................................................................................17
Learning Objectives ............................................................................................17
Focusing the Image ........................................................................................... 18
Accommodation .................................................................................................18
Retinal Function ..................................................................................................19
Higher Processing ...............................................................................................19
Summary ...............................................................................................................20
Knowledge Check ................................................................................................21
Knowledge Check Answers ................................................................................22
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References 23
Glossary
24
1
Ciliary Body
Anatomy and Physiology of the Eye
The eye is a complex organ that captures information from our environment in the form
of light, focuses it, and converts it into neural signals that are transmitted to the brain for
processing and storage. Knowing the basics of ocular structure and function will help you
build your confidence and increase your credibility with customers. You will be better able
to speak your customers’ language, understand their questions and concerns, and
establish a professional relationship built upon mutual respect.
Choroid
Posterior
Chamber
Optic
Nerve
Lens
Anterior
Chamber
Cornea
Pupil
Iris
Retina
Zonules
Sclera
Conjunctiva
FIGURE 1. STRUCTURES OF THE EYE
Chapter 1. Anatomy of the Eye
OVERVIEW
The foundation for learning about the eye begins with its anatomy, as shown in Figure 1.
This small structure, approximately 24 mm (1 in) in diameter, is a highly intricate organ
perfectly suited for vision. It is protected in its bony location in the orbit of the skull (eye
socket), surrounded by a cushion of fat and moved by muscles that attach to the skull.
The visual information collected by the eye is transferred by way of the optic nerve,
which travels from the back of the eye, through an opening in the skull at the back of the
orbit, to the brain. Let’s begin by looking at the parts of the eye, which work in concert to
give us the sense of sight.
As we discuss the structures of the eye, we’ll move from the front of the eye to the back,
and from the outside in. We’ll begin with the external structures of the eye, taking a look
at the eyelids, at tear production and drainage, and at the muscles that move the eye.
Next, we’ll look at the anterior segment, including the cornea, the conjunctiva, the iris,
the anterior and posterior chambers, and the lens. Along the way, we’ll visit the sclera,
learn about the aqueous humor that fills the anterior segment, and discuss the
surprisingly complex components of tears.
The posterior segment, including the vitreous humor, the retina, the choroid, and the
optic disc, will be our next stop. We’ll learn about the middle, vascular layer of the eye,
called the uvea, which includes the iris, the ciliary body, and the choroid. Many visual
aids, such as Figure 1, will be presented to enhance your understanding of how all of
these structures fit together in such a small space.
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Anatomy and Physiology of the Eye
3
Chapter 1. Anatomy of the Eye
Chapter 1. Anatomy of the Eye
LEARNING OBJECTIVES
ANTERIOR SEGMENT
After completing this chapter, you will be able to
The structures of the eye may be grouped in a number of ways. A common approach is
to divide them into the anterior segment and the posterior segment, as in Figure 4.
Let’s look first at the anterior segment: the cornea, the conjunctiva, the iris, the pupil, the
anterior and posterior chambers, and the lens.
• Describe the individual structures of the human eye.
• Identify the function of each of the 5 basic layers of corneal tissue.
• Identify the 3 layers of the tear film.
• Identify the structures that make up the uvea.
• Differentiate the 2 main segments of the human eye.
• Explain the structure and function of the retina.
Posterior
Segment
Anterior
Segment
Posterior
Chamber
EXTERNAL STRUCTURES OF THE EYE
KEY LEARNING
The ocular structures that are
bathed by the aqueous humor are
grouped into the anterior segment
of the eye. These structures
include the cornea, the iris, and
the lens. Note that the anterior
segment includes both the anterior
and posterior chambers.
The external structures of the eye include
• Eyelids
• Lacrimal system (lacrimal gland, puncta, and nasolacrimal duct)
• Extraocular muscles
The eyelids act as a barrier, protecting the surface of the eye from external objects and
irritants. In addition, they spread the tear film (we will explore the details of the tear film
later in this training) across the ocular surface with each blink. Each eyelid has a row of
eyelashes, acting as a further barrier from debris. Behind the lashes, a row of
meibomian glands produce oil (or “lipid”),
Lacrimal Gland
the top layer of the tear film. The oil prevents
evaporation of the water in the tears, keeping the
Punctum
surface of the eye moist and lubricated.
Upper
Canaliculus
Lacrimal
Sac
Lower Canaliculus
Nasolacrimal Duct
FIGURE 2. LACRIMAL SYSTEM
The lacrimal gland produces the watery, or
aqueous portion, of the tears. After passing
across the ocular surface, tears are pushed
through the upper and lower punctum, tiny holes
in the eyelids. Next, they drain through tubes
called the upper and lower canaliculus, into the
lacrimal sac, and then into the nasolacrimal
duct, or tear duct. Note that the tears eventually
drain into the nose, as seen in Figure 2, which
is the reason the nose runs when we cry. The
production and drainage of tears must be in
balance to avoid both dry eye disorders and
excessive tearing.
OTHER EXTERNAL STRUCTURES
Superior Rectus M.
Each eye has 6 extraocular muscles, as
seen in Figure 3. One end of each muscle
Lateral
is attached to the bone of the orbit, or eye
Rectus M.
socket, while the other end is attached to
the sclera, the white, outer wall of the eye.
These muscles work in combination to move
the eyes in all directions and keep the eyes
Inferior Rectus M.
aligned.
Superior
Oblique M.
Anterior
Chamber
Lens
Cornea
Pupil
Iris
Conjunctiva
FIGURE 4. ANTERIOR SEGMENT OF THE EYE
CORNEA
The cornea is the thin, transparent dome at the front of the eye, measuring about a half
millimeter in thickness. Along with the eyelids and sclera, the cornea protects the inside
of the eye from germs, dust, and other dangers and is the first focusing surface that light
encounters as it travels through the eye. In fact, the cornea, together with the tear film,
account for about 65%–80% of the eye’s refractive (or focusing) power.
The cornea has a high concentration of nerve endings, making it one of the most
sensitive areas of the body. Unlike most tissues, the cornea contains no blood vessels
to nourish or protect it against infection. It must remain transparent to allow light to pass
through it and refract light properly. The presence of even the smallest capillaries would
interfere with this process. Therefore, the cornea receives its oxygen and nourishment
from the tears and from the fluid that fills the chamber behind it.1-4
Medial
Rectus M.
Inferior Oblique M.
FIGURE 3. EXTRAOCULAR MUSCLES
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Anatomy and Physiology of the Eye
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Chapter 1. Anatomy of the Eye
Chapter 1. Anatomy of the Eye
CONJUNCTIVA
The cornea is composed of 5 layers, as seen in Figure 5.
Bulbar
Conjunctiva
Epithelium
Bowman’s Layer
Conjunctival
Fornix
Stroma
Endothelium
Palpebral
Conjunctiva
Descemet’s Membrane
Surface
of the Cornea
FIGURE 5. LAYERS OF THE CORNEA
Epithelium
The epithelium is the cornea’s surface layer. It protects the other layers of the
cornea and provides a smooth surface that absorbs oxygen and cell nutrients
from tears and then distributes those nutrients to the rest of the cornea.
Bowman’s Layer
Lying directly below the epithelium is a transparent sheet of tissue known as
Bowman’s layer. If injured, Bowman’s layer can form a scar, which may lead to
vision loss.
FIGURE 6. CONJUNCTIVA
The conjunctiva is the structure of the eye that is involved in “pink eye” or conjunctivitis.
It contains goblet cells that secrete mucin, the base layer of the tear film. The tear film,
while not a structure of the eye, nevertheless plays an important role in maintaining the
health of the ocular surface. Furthermore, a smooth tear film is important for clear vision.
Stroma
Beneath Bowman’s layer is the stroma, which comprises about 90% of the
cornea’s thickness. It consists primarily of water and collagen, which give the
cornea its strength, elasticity, and shape. The collagen’s unique arrangement
makes it transparent.
escemet’s Membrane
D
Under the stroma is Descemet’s membrane, a thin but strong sheet of tissue
that serves as a protective barrier against infection and injury. Descemet’s
membrane regenerates itself easily after injury.
Endothelium
The endothelium is the innermost layer of the cornea and is only 1 cell thick.
Endothelial cells are essential for keeping the cornea clear by pumping any
excess fluid out of the stroma. In a healthy eye, a perfect balance is maintained
between the fluid moving into and being pumped out of the cornea. If the balance is disrupted because of the loss of endothelial cells, the stroma swells
with water, becoming hazy and ultimately opaque. Once endothelial cells are
destroyed by disease or trauma, they are lost forever.
Recently, a sixth corneal layer, Dua’s layer, was discovered between Descemet’s
membrane and the stroma. The function of this layer, which is only present in the central
cornea and not at the periphery, remains unknown.5
The border between the clear cornea and the opaque sclera is called the limbus. This
is an important landmark for differentiating among many anterior segment abnormalities
as well as for surgery.
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The conjunctiva is a transparent
mucous membrane that begins at the
outer edge of the cornea, covering the
visible part of the sclera, and lining the
inside of the eyelids. The portion of the
conjunctiva lying on the surface of the
eye is called the bulbar conjunctiva,
while the portion lining the eyelids is
called the palpebral conjunctiva. The
dead end where the conjunctiva folds
back on itself between the bulbar and
palpebral portions is referred to as the
conjunctival fornix. This area, seen in
Figure 6, prevents objects, such as
contact lenses, from getting lost behind
the eye.
Lipid Layer
Aqueous Layer
Mucin Layer
FIGURE 7. THE 3 LAYERS OF TEAR FILM
KEY LEARNING
The ocular surface includes the
cornea, the conjunctiva, and the
tear film.
The tear film is composed of
3 layers:
• Lipid layer, composed of oil
secreted by the meibomian
glands of the eyelids
• Aqueous layer, the watery
component of tears produced
by the lacrimal gland
• M
ucin layer, secreted by the
goblet cells of the conjunctiva
The tear film is composed of 3 layers, as
seen in Figure 7. The lipid layer is the top
layer and is composed of oil secreted by
the meibomian glands of the eyelids.
The oil prevents the water in the tears
from evaporating too quickly. The aqueous
layer, the middle layer, is the watery
component of tears produced by the
lacrimal gland. Finally, the mucin layer is
the base layer, secreted by the goblet
cells of the conjunctiva.
IRIS
The colored portion of the eye is called the iris. It controls light levels inside the eye
much like the aperture on a camera would. The round opening in the center of the iris is
called the pupil. This is not a structure, per se, but rather a hole in the iris through which
light can pass. The iris is composed of muscles that dilate (widen) the pupil size in dim
light and constrict (narrow) the pupil size in bright light, allowing the optimal amount of
light to enter the eye in any lighting condition. The color, texture, and pattern of each
person’s iris are unique. The color of the iris is determined by the amount of pigment
present in the iris structure. In the absence of all pigment, as in an albino, the iris will
appear pink. Increasing amounts of pigment result in blue, green, hazel, and brown
irises, respectively.
Anatomy and Physiology of the Eye
7
Chapter 1. Anatomy of the Eye
Chapter 1. Anatomy of the Eye
ANTERIOR CHAMBER
LENS
The anterior chamber is the open space posterior to (behind) the cornea and anterior
to (in front of) the iris. It is filled with a watery fluid called the aqueous humor, which
supplies oxygen and nutrients to the endothelial layer of the cornea, as well as to the
lens (to be discussed later in this chapter). The production and drainage of aqueous
humor must remain in balance to maintain normal intraocular pressure. The majority of
the aqueous humor drains out of the eye through the angle formed between the cornea
and the iris, as shown in Figure 8. Microscopically, this angle is composed of a network
of cells arranged in a meshwork pattern, termed the trabecular meshwork.6 After
passing through the trabecular meshwork, the aqueous humor enters the
Canal of Schlemm and then travels into ocular veins, joining the bloodstream.
The lens is a transparent, fibrous tissue located behind the iris. The lens accounts for
20%–35% of the eye’s refractive power. It is enclosed in a capsule and suspended from
slender filaments, or zonules, which are attached to the ciliary body, as seen in
Figure 9.
Lens
Zonules
Cornea
Trabecular Meshwork
Ciliary
Body
Anterior
Chamber
Canal of Schlemm
Iris
Aqueous Humor
Flowing Into the
Bloodstream
Iridocorneal Angle
Sclera
Ciliary Body
Aqueous Humor
Production
Lens
FIGURE 9. LENS SUSPENDED FROM CILIARY BODY BY SLENDER THREAD-LIKE ZONULES
The tension of these zonules, controlled by the ciliary muscle, adjusts the shape of the
lens to allow it to focus on images at various distances, as demonstrated in Figure 10.
Ciliary Muscle Relaxes
Ciliary Muscle Contracts
Posterior
Chamber
FIGURE 8. FLOW OF AQUEOUS HUMOR THROUGH ANTERIOR CHAMBER
Suspensory
Ligament Taut
POSTERIOR CHAMBER
The open space behind the iris and in front of the lens is called the posterior chamber.
This space also contains aqueous humor, which is produced by the ciliary body. This
structure lies behind the base of the iris and is composed of the ciliary processes, which
secrete aqueous humor, and the ciliary muscle, which adjusts the shape of the lens for
focusing. Figure 8 shows the movement of aqueous humor through the eye, beginning
at the ciliary body, moving past the lens, which absorbs nutrients from it, through the
pupil into the anterior chamber. Here, more nutrients are absorbed by the corneal
endothelium as the aqueous humor moves toward the iridocorneal angle, out of the
eye through the trabecular meshwork, and eventually into the bloodstream.
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Lens Flattens
Suspensory
Ligament Lax
Lens Becomes
Globular
FIGURE 10. EFFECT OF CONTRACTION OF CILIARY MUSCLE ON LENS SHAPE
DURING ACCOMMODATION
Anatomy and Physiology of the Eye
9
Chapter 1. Anatomy of the Eye
Chapter 1. Anatomy of the Eye
POSTERIOR SEGMENT
KEY LEARNING
The posterior segment of the eye
includes the structures behind the
lens. These include the vitreous,
the retina, the choroid, and the
optic nerve.
The posterior segment of the eye includes the structures behind the lens, as you see
in Figure 11. These include the vitreous, the retina, the choroid, and the optic nerve.
Let’s take a look at each of these in more detail.
Posterior
Segment
The choroid is a vascular layer beneath the retina that supplies blood, oxygen, and
nutrients to the outer portion of the retina. The vascular tissues of the eye—the iris, the
ciliary body, and the choroid—are also referred to as the uvea, or uveal tract,
as depicted in Figure 13.
Ciliary Body
Anterior
Segment
Choroid
Iris
Optic
Nerve
KEY LEARNING
The uvea, sometimes referred to
as the middle ocular layer, is
composed of the vascular tissues
of the eye. These include the iris in
the anterior segment, the choroid
in the posterior segment, and the
ciliary body between the anterior
and posterior segments.
Vitreous
Choroid
Retina
FIGURE 13. UVEAL TISSUES—IRIS, CILIARY BODY, AND CHOROID
FIGURE 11. POSTERIOR SEGMENT OF THE EYE
VITREOUS HUMOR
Behind the lens is a large cavity filled with a gelatinous substance called the
vitreous humor. Unlike the aqueous humor, the vitreous is stationary and changes
very little throughout life. Occasionally, small opacities may form because of clumping
or shrinking of the vitreous. These are experienced as “floaters”, which appear to move
as the eye moves.
RETINA
The retina is the highly complex, thin tissue that lines the vitreous cavity. It transforms
light into nerve signals via photoreceptor cells called rods and cones. These signals
then travel through 1.2 million nerve fibers, which converge to form the optic nerve.
The nerve then carries the signals on to the brain, where they are converted to visual
information. The optic nerve head, or optic disc, has no photoreceptors and therefore
forms a “blind spot” in the retina.
The central portion of the retina is called the macula,
an area devoid of blood vessels, seen in Figure 12.
While both rods and cones are found throughout the
retina, the macula, and particularly a depression within
it called the fovea, has a high concentration of cones.
This high concentration of cones allows the sharpest
imaging. The peripheral retina, in contrast, contains
more rods than cones. Rods are very sensitive to light
but do not detect color, whereas cones detect color but
are less sensitive to light.
10
Fovea
Optic Disc
Macula
FIGURE 12. NORMAL RETINA
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Anatomy and Physiology of the Eye
11
Summary
Knowledge Check
Summary
Knowledge Check
• T
he eyelids protect the eye and contain meibomian glands, which secrete oil for the
top layer of the tear film.
1. Label the anatomy of the eye.
a.
b.
c.
d.
e.
f.
g.
h.
i.
j.
k.
l.
m.
• The lacrimal gland produces the watery component of the tears.
• T
he cornea is a 5-layered tissue that accounts for the majority of the eye’s
refractive power. These layers, from the outside in, are the epithelium, Bowman’s
layer, the stroma, Descemet’s membrane, and the endothelium.
• The cornea contains no blood vessels and receives its oxygen and nutrients from the
tears and the aqueous humor.
• T
he conjunctiva covers the visible part of the sclera and lines the inside of the
eyelids. It secretes mucin, the base layer of the tear film.
• T
he tear film is composed of 3 layers—lipid on the top, aqueous in the middle, and
mucin on the bottom.
• The pupil is an opening in the center of the iris. It constricts in bright light and
dilates in dim light to allow the optimal amount of light to enter the eye in any
viewing condition.
• Aqueous humor is produced by the ciliary body, flows through the anterior chamber,
and drains out of the eye through the iridocorneal angle.
e
a
f
b
g
• The trabecular meshwork is a network of cells in the angle, through which the
aqueous humor drains out of the eye.
h
• The anterior segment includes all ocular structures from the lens forward. This
segment includes the spaces known as the anterior and posterior chambers.
i
j
• The lens is suspended from zonules, which are controlled by the ciliary muscle. This muscle
allows the lens to change shape to accommodate, or focus, on near objects.
• The posterior segment includes all structures behind the lens.
k
c
l
• The vitreous humor is the gel-like substance that fills the posterior segment of the eye.
• The retina is a highly complex tissue that transforms light into nerve signals via photoreceptors called rods and cones.
• The macula is the central portion of the retina, responsible for our sharpest
central vision.
• The macula contains a high concentration of cones.
d
m
2. Which of the answers in Question #1 are located in the anterior segment?
• The peripheral retina contains more rods than cones.
• The uveal tract includes the vascular tissues of the eye—the iris, the ciliary body, and
the choroid.
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Anatomy and Physiology of the Eye
13
Knowledge Check
Knowledge Check
3. C
hoose the component of tears paired with the correct ocular structure that
secretes it, and list them in order from outside to inside.
a. Aqueous—lacrimal gland
b. Aqueous—meibomian gland
c. Aqueous—conjunctiva
d. Lipid—lacrimal gland
e. Lipid—meibomian gland
f. Lipid—conjunctiva
g. Mucin—lacrimal gland
h. Mucin—meibomian gland
i. Mucin—conjunctiva
4. Which of the following posterior segment structures is indicated by
the arrow?
a. Optic disc
b. Vitreous humor
c. Macula
d. Choroid
l
5. Describe the function of the structure in Question #4.
6. Which of the following pairs is incorrect?
a. Aqueous humor—anterior chamber
b. Aqueous humor—posterior chamber
c. Vitreous humor—anterior segment
d. Vitreous humor—posterior segment
7. What is the name of the clear mucous membrane that covers the sclera on the
ocular surface?
a. Conjunctiva
b. Cornea
c. Pupil
d. Tear film
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Anatomy and Physiology of the Eye
15
Knowledge Check Answers
Knowledge Check Answers
1. Label the anatomy of the eye.
a. Choroid
b. Optic nerve
c. Retina
d. Sclera
e. Ciliary body
f. Posterior chamber
g. Lens
h. Anterior chamber
i. Cornea
j. Pupil
k.Iris
l. Zonules
m. Conjunctiva
2. Which of the answers in Question #1 are located in the anterior segment?
Conjunctiva, Cornea, Iris, Lens, Anterior and Posterior Chambers, Pupil
3. Choose the component of tears paired with the correct ocular structure that
secretes it, and list them in order from outside to inside.
e. Lipid—meibomian gland
a. Aqueous—lacrimal gland
i. Mucin—conjunctiva
4. Which of the following posterior segment structures is indicated by the arrow?
c. Macula
5. Describe the function of the structure in Question #4.
T
he macula is the central part of the retina, contains mostly cones, and
is responsible for the eye’s sharpest central vision.
6. Which of the following pairs is incorrect?
c. Vitreous humor—anterior segment
7. What is the name of the clear mucous membrane that covers the sclera on
the ocular surface?
a. Conjunctiva
Chapter 2. Physiology of Vision
OVERVIEW
Now that you are familiar with the anatomy of the eye, let’s examine the process of
vision. We begin with focusing an image as it passes through the tear film, the cornea,
and the lens. We will then follow the path of that image through the eye onto the retina,
where it is transformed through a biochemical reaction in the rods and cones into nerve
signals. These signals then travel along the optic nerve to the brain, where higher
processing transforms the information into the perception of what is seen.
LEARNING OBJECTIVES
After completing this chapter, you will be able to
• Describe the process of focusing the image, including the role of the tear film, the
cornea, and the lens.
• Name the common refractive errors of the eye.
• Identify the ocular structures and processes involved in accommodation.
• Describe the function of the rods and cones.
• Describe the brain’s role in visual function.
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Anatomy and Physiology of the Eye
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Chapter 2. Physiology of Vision
Normal vision:
the image is focused on the retina
Farsightedness (hyperopia):
the image is focused behind the retina
FOCUSING THE IMAGE
RETINAL FUNCTION
The first step in the process of vision is
for the eye to focus an image. Light from
the image is refracted, or bent, as it
enters the eye through the tear film and
then the cornea. The light is then refracted again as it passes through the lens of
the eye. Ideally, the image will then be
perfectly focused in the back of the eye
on the retina, specifically the fovea. If
the eye has a refractive error, such as
myopia (nearsightedness), hyperopia
(farsightedness), or astigmatism
(irregular curvature of the cornea), then
the image will not be focused on the
retina, as depicted in Figure 14. This
may be corrected with glasses or contact
lenses, which refract the light to
compensate for the eye’s refractive
error. Another option for correcting this
problem is refractive surgery, which
changes the curvature of the cornea.
Rods and cones, the photoreceptor cells of
the retina, are named for their shapes, as
seen in Figure 16. These specialized cells
react to light through a complex series
of biochemical reactions. While rods far
outnumber cones in the retina, as shown
in the same figure, vision in lighted
conditions is dominated by cones. As
previously mentioned, rods are very
sensitive to small amounts of light,
cannot detect color, and are thus mostly
used in dark conditions. For this reason,
the eye cannot perceive color in the dark.
Cones have red-, green-, or blue-sensitive
FIGURE 16. SCANNING ELECTRON MICROGRAPH OF
RODS AND CONES
pigment, which allows the human eye to
perceive any possible color or shade. The
cones are concentrated in the macula, and especially in the fovea, the area of sharpest
visual acuity. Deficiencies in color vision are due to the absence of one or more of the
genes for red or green pigment or both. Despite common use of the term “colorblindness”, true absence of color vision is extremely rare.7, 8
ACCOMMODATION
Nearsightedness (myopia):
the image is focused in front of the retina
FIGURE 14. COMMON REFRACTIVE ERRORS
Far Object
Chapter 2. Physiology of Vision
HIGHER PROCESSING
Accommodation is the process by
which the eye changes its focal length
for near objects. A near object requires
more refraction than a distant object
to come into clear focus on the retina.
When viewing a near object, the ciliary
muscle increases the curvature of the
lens, thereby increasing its refractive
power, as shown in Figure 15.
Babies are born with a very large
amount of accommodative ability due to
the softness and flexibility of the lens.
In childhood, the near point of vision is
about 3 in (8 cm), increasing to about
7 in (17.5 cm) in young adults, and
about 32 in (83 cm) at 60 years of age.
This diminished ability to focus on close
objects, called presbyopia, results from
gradual stiffening of the lens with age.
The lost ability of the lens to change
shape is the reason that most people
begin to require reading glasses at 40
years of age.2
Visual
Cortex
FIGURE 17. VISUAL CORTEX
Nerve signals are transmitted from the
retina via the optic nerve to the
visual cortex area of the brain, located
at the back of the head as shown in
Figure 17. There, they are processed into
a perceptual image representing the
object viewed by the eyes. A much larger
percentage of the brain is devoted to
processing signals from the macula than
from the periphery of the visual field.
Each eye receives a slightly different view
of the same image because the eyes are
several centimeters apart. The overlap
and differences in the visual fields are
compared by the binocular vision system
of the brain, which uses this information to
calculate the distance of the image. This
process enables us to experience the world
with binocular, 3D vision. People with vision
only in one eye experience monocular
vision and cannot see things in 3D.2, 9-11
Near Object
FIGURE 15. ACCOMMODATION CHANGES
FOCAL LENGTH OF THE EYE
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Anatomy and Physiology of the Eye
19
Summary
Knowledge Check
Summary
Knowledge Check
• Light is refracted by the cornea and the lens, resulting in a focused image on
the retina.
1. Which of the following is NOT involved in focusing the image in the eye?
a. Tear film
b. Cornea
c. Aqueous humor
d. Lens
2. What is accommodation and how does it change with age?
• Refractive errors result in an unfocused image. These may be corrected with glasses,
contact lenses, or refractive surgery.
• Accommodation is the process of changing the focal length of the eye from
distance to near. With accommodation, the lens of the eye becomes more curved,
increasing the refraction of light.
• The retina contains photoreceptors that convert light to nerve signals. Rods
are able to detect low levels of light, while cones require bright light to function.
• Cones contain red-, green-, or blue-sensitive pigment, which are used in
combination to identify all colors.
• The
fovea contains the highest concentration of cones and is the portion of
the retina responsible for the sharpest visual acuity.
• T
he stimulated photoreceptors send visual information to the optic nerve, which
transmits the information to the visual cortex of the brain.
• The brain uses higher processing functions to interpret the nerve signals as vision.
3. At what age does presbyopia generally begin?
a. 30
b. 40
c. 50
d. 60
4. Which of the following statements is correct regarding the retina as
a whole?
a. Cones outnumber rods, and rods are responsible for color vision.
b. Cones outnumber rods, and cones are responsible for color vision.
c. Rods outnumber cones, and rods are responsible for color vision.
d. Rods outnumber cones, and cones are responsible for color vision.
5. Describe the role of the brain in the process of vision.
6. Which of the following is NOT a refractive error?
a. Hypertropia
b. Hyperopia
c. Myopia
d. Astigmatism
7. Which of the following structures transmits visual information from
the eye to the brain?
a. Optic nerve
b. Oculomotor nerve
c. Olfactory nerve
d. Abducens nerve
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Anatomy and Physiology of the Eye
21
Knowledge Check Answers
References
Knowledge Check Answers
References
1. Which of the following is NOT involved in focusing the image in the eye?
c. Aqueous humor
1. Rolando M, Zierhut M. The ocular surface and tear film and their dysfunction in
dry eye disease. Surv of Ophthalmol. 2001;45 (suppl 2): S203–S210.
2. What is accommodation and how does it change with age?
Accommodation is the process of changing the shape of the lens of
the eye to focus on a near object. The lens is flexible in childhood,
but gradually stiffens and loses its ability to change shape, resulting
in presbyopia.
2. Martini FH. Fundamentals of Anatomy and Physiology. 6th ed. San Francisco,
CA: Benjamin Cummings; 2004:565–587.
3. At what age does presbyopia generally begin?
b. 40
4. National Eye Institute. Facts about the cornea and corneal disease. 2013.
http://www.nei.nih.gov/health/cornealdisease/. Accessed August 18, 2013.
4. Which of the following statements is correct regarding the retina as a whole?
d. Rods outnumber cones, and cones are responsible for color vision.
5. Dua HS, Faraj LA, Said DG, Gray T, Lowe J. Human corneal anatomy redefined: A
novel pre-Descemet’s Layer (Dua’s Layer). Ophthalmology. 2013. May 25. [Epub
ahead of print].
5. Describe the role of the brain in the process of vision.
The brain receives the visual nerve signals from the eye and uses higher
processing to convert them to the perception of vision. The brain
receives slightly different input from both eyes, calculates the difference,
and perceives images in 3 dimensions.
6. Which of the following is NOT a refractive error?
e. Hypertropia
7. Which of the following structures transmits visual information from the eye
to the brain?
a. Optic nerve
3. MedicineNet.com. Cornea. 2007. http://www.medterms.com/script/main/art.
asp?articlekey=7248. Accessed August 18, 2013.
6. Llobet A, Gasull X, Gual A. Understanding trabecular meshwork physiology: A key
to the control of intraocular pressure? Physiology. 2003;18(5):205–209.
7. Purves D, Augustine GJ, Fitzpatrick D, et al (eds). Neuroscience. 2nd ed.
Sunderland, MA: Sinauer Associates; 2001. http://www.ncbi.nlm.nih.gov/books/
NBK10848/. Accessed July 28, 2013.
8. Kimball JW. The Human Eye. http://users.rcn.com/jkimball.ma.ultranet/
BiologyPages/V/Vision.html. Accessed July 28, 2013.
9. Waugh A, Grant A. Ross and Wilson Anatomy and Physiology in Health and Illness.
9th ed. Edinburgh, Scotland: Churchill Livingstone; 2003:191–212.
10. Scanlon V, Sanders T. Essentials of Anatomy and Physiology. 4th ed.
Philadelphia, PA: FA David Co; 2003:186–206.
11. Guyton AC, Hall JE. Textbook of Medical Physiology. 11th ed. Philadelphia,
PA: Elsevier Saunders; 2006:640–650.
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Anatomy and Physiology of the Eye
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Glossary
Glossary
Accommodation: process of changing the focal length of the eye from distance to near.
With accommodation, the lens of the eye becomes more curved, increasing the
refraction of light.
Iris: colored portion of the eye, which controls light levels inside the eye by
changing the size of the pupil.
Lacrimal gland: orbital structure that produces the watery, or aqueous, portion
of the tears.
Angle: area through which aqueous humor drains out of the eye. It is located at the
angle formed between the cornea and the iris.
Lacrimal sac: pouch in the tear drainage system that connects the canaliculi to
the nasolacrimal duct.
Anterior chamber: space behind the cornea and in front of the iris, which is filled with
the aqueous humor.
Lens: transparent ocular structure located behind the iris. It refracts light and changes
shape during accommodation to focus on near objects.
Anterior segment: front portion of the eye, which includes the cornea, the
conjunctiva, the iris, and the lens.
Limbus: border between the clear cornea and the opaque sclera.
Aqueous humor: watery fluid that supplies oxygen and nutrients to the corneal
endothelium and to the lens.
Astigmatism: refractive error due to irregular curvature of the cornea.
Bulbar conjunctiva: portion of the conjunctiva that covers the visible part of the sclera
on the ocular surface.
Canal of Schlemm: duct surrounding the outer edge of the iris at the cornea. This canal
drains aqueous humor into the bloodstream.
Canaliculus: small tube that connects the punctum of the eyelid to the lacrimal sac.
Choroid: vascular layer beneath the retina that supplies blood, oxygen, and
nutrients to the outer portion of the retina. The choroid is the posterior uveal tissue.
Ciliary body: structure found behind the base of the iris, composed of the ciliary
processes and the ciliary muscle. The ciliary body secretes aqueous humor and controls
accommodation. It is the intermediate uveal tissue.
Cones: photoreceptor cells that sense color; concentrated in the fovea for sharpest
visual acuity.
Conjunctiva: transparent mucous membrane that begins at the outer edge of the
cornea, covers the visible part of the sclera, and lines the inside of the eyelids. It
secretes mucin, the base layer of the tear film.
Cornea: thin, transparent dome at the front of the eye, which is the first focusing surface
that light encounters as it travels through the eye.
Fovea: portion of the retina responsible for the sharpest visual acuity, located at a
depression in the center of the macula. It contains the highest concentration of cones in
the retina.
Hyperopia: refractive error commonly known as farsightedness.
Iridocorneal angle: area through which aqueous humor drains out of the eye.
It is located at the angle formed between the cornea and the iris.
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Glossary
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Macula: central portion of the retina.
Meibomian glands: oil glands in the eyelids that secrete the top layer of the
tear film.
Myopia: refractive error commonly known as nearsightedness.
Nasolacrimal duct: tear duct via which the tears drain from the eyes into the nose.
Optic disc: portion of the optic nerve that is visible in the retina.
Optic nerve: nerve that carries visual information from the retina to the brain.
Orbit: cavity in the skull in which the eye and its appendages are located. This is
commonly called the “eye socket”.
Palpebral conjunctiva: portion of the conjunctiva that lines the inside of the eyelids.
Photoreceptor: type of specialized sensory cell that responds to light.
Posterior chamber: space behind the iris and in front of the lens that contains
aqueous humor.
Posterior segment: structures behind the lens, including the vitreous, the retina, the
choroid, and the optic disc.
Presbyopia: literally, “old eye”; diminished ability to focus on near objects.
Punctum: tiny hole in the eyelid through which tears drain.
Pupil: round opening in the center of the iris that dilates and constricts in changing light
conditions.
Refractive error: imperfection in the eye’s ability to focus an image on the retina.
Common refractive errors include myopia, hyperopia, and astigmatism.
Retina: highly complex, thin tissue that transforms light into nerve signals via
photoreceptor cells called rods and cones.
Rods: photoreceptor cells that are exceedingly sensitive to low levels of light.
Anatomy and Physiology of the Eye
25
Glossary
Sclera: white, outer wall of the eye.
NOTES
Trabecular meshwork: tissue in the drainage angle of the eye, through which the
aqueous humor leaves the anterior chamber.
Uvea: vascular tissues of the eye, namely, the iris, the ciliary body, and the choroid.
Visual acuity: measure of the sharpness or clarity of vision.
Visual cortex: part of the brain dedicated to visual processing.
Vitreous humor: gelatinous substance filling the space behind the lens of the eye.
Zonules: slender filaments that suspend the lens from the ciliary muscle.
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Anatomy and Physiology of the Eye
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NOTES
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