Clinical Anatomy and Physiology of the Visual System

297 Pages·2011·24 MB·English

by  Lee Ann Remington OD MS

LEE ANN REMINGTON, OD, MS, FAAO
Professor of Optometry
Pacific University College of Optometry
Forest Grove, Oregon
3251 Riverport Lane
St. Louis, Missouri 63043
CLINICAL ANATOMY AND PHYSIOLOGY OF THE VISUAL SYSTEM, THIRD EDITION ISBN: 978-1-4377-1926-0
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To Dan, Tracy, Ryan, Aaron, Angela, Isaiah, Shalane, Danielle, Taryn, Payton, and Taylor.
You bring joy to my life.
Preface
C
linical Anatomy and Physiology of the Visual System was The branches of the internal and the external carotid
written to provide the optometry and ophthalmology arteries that supply the globe and adnexa are identified
student, as well as the clinician, with a single text that in Chapter 11. The cranial nerve supply to orbital struc-
describes the embryology, anatomy, histology, physi- tures, including both sensory and motor pathways, is
ology, blood supply, and innervation of the globe and clarified in Chapter 12, with an emphasis on the clinical
ocular adnexa. The visual and pupillary pathways are relevance and implications of interruptions along the
covered as well. The text is fully referenced, and infor- pathways. Significant detail on the relationship between
mation gathered from historical and current literature the structures of the visual pathway and neighboring
is well documented. An overview of the visual system as structures and on the orientation of the fibers as they
well as a short review of histology and physiology is pro- course through the cranium en route to the striate cor-
vided in the introductory chapter. Chapters 2 through tex is presented in Chapter 13. Examples are given of
5 include the anatomy, the detailed histology, and the characteristic visual field defects associated with injury
physiology of the structures constituting the globe. Each to various regions of the pathway. The final chapter pres-
of the three coats of the eye—cornea-sclera, uvea, and ents the autonomic pathways to the smooth muscles of
retina—is covered in a separate chapter. Included in each the orbit and to the lacrimal gland. The pupillary path-
is an emphasis on similarities and differences between way is included in this chapter, as is a treatment of the
regions within each coat and notations about layers that more common pupillary abnormalities and the rela-
are continuous between structures and regions. The crys- tion between the pathway and the clinical presentation.
talline lens is covered in Chapter 5 and the globe is com- Some of the common pharmaceutical agents and their
pleted in Chapter 6, with descriptions of the chambers actions and pupillary effects are covered as well.
of the eye and the production and composition of the In the format used in the text, terms and names of
material that occupies those spaces. structures are noted in bold print when they are first
In my experience, students can more easily grasp the described or explained. The name for a structure that
intricacies of ocular development after gaining a com- is more common in usage is presented first, followed
prehensive understanding of the composition of the by other terms by which that structure is also known.
structures; therefore, ocular embryology is covered in Current nomenclature tends to use the more descriptive
Chapter 7. The tissue and structures associated with and name rather than proper nouns when identifying struc-
surrounding the globe are described in the next three tures, but that is not always the case, especially when the
chapters. First is a review of the bones and important proper name of an individual has been linked so closely
foramen of the entire skull and then the detail regarding historically (e.g., Schwalbe’s line and Schlemm’s canal).
the orbital bones and connective tissue. This is followed Experienced clinicians know that the knowledge of
by a chapter detailing eyelid structure and histology, structure and function provides a good foundation for
including the roles that the muscles and glands have recognizing and understanding clinical situations, condi-
in tear film secretion and drainage. The chapter on the tions, diseases, and treatments. For this reason, “Clinical
extraocular muscles describes movements that result Comments” are included throughout the book to empha-
from contraction of the muscles with the eye in various size common clinical problems, disease processes, or
positions of gaze; an explanation of the clinical assess- abnormalities that have a basis in anatomy or physiology.
ment of extraocular muscle function based on the anat-
omy is included. Lee Ann Remington
vii
Acknowledgments
I
have had the pleasure of interacting with many bright, photographs from the second edition were done by
engaging students during the past 25 years while teach- Neil VanderHorst, O.D., and the original line drawings
ing at Pacific University College of Optometry. Their included from the first edition were done by Tracey
questions, corrections, suggestions, and enthusiasm Asmus, O.D.
motivate me to continually improve and update my I am grateful to Dan, my husband and friend, for his
understanding of the process we call vision. I am grate- patience, encouragement, and loving support during the
ful for their kindness; they make my days richer. months spent preparing this manuscript. I am indebted
I am also fortunate to work with an extraordinary to my family for their love and support.
group of colleagues, the faculty at Pacific, who create an Christie Hart, my editor at Elsevier, has guided me
enjoyable environment conducive to academic growth. with kindness and tact throughout the entire process,
I am grateful to Dean Jennifer Smythe for the constant and for that I am grateful. Andrea Campbell, my project
level of support she provides, and to the optometry fac- manager at Elsevier, competently combined the text and
ulty for their warm encouragement and help during this figures into a cohesive whole. I appreciated her thought-
process. ful and gentle suggestions.
I thank Seth Taylor for his diligence in assisting me in
the search for pertinent literature sources. The histology
viii
C H A P T E R
1
Visual System
T shape of the globe, providing resistance to the pressure
he visual system takes in information from the envi- of the fluids inside. The sclera is the opaque white of
ronment in the form of light and analyzes and interprets the eye and is covered by the transparent conjunctiva.
it. This process of sight and visual perception involves a The transparent cornea allows light rays to enter the
complex system of structures, each of which is designed globe and, by refraction, helps bring these light rays
for a specific purpose. The organization of each structure into focus on the retina. The region in which the transi-
enables it to perform its intended function. tion from cornea to sclera and conjunctiva occurs is the
The eye houses the elements that take in light rays and limbus.
changes them to a neural signal; it is protected by its loca- The vascular layer of the eye is the uvea, which is made
tion within the bone and connective tissue framework of up of three structures, each having a separate function
the orbit. The eyelids cover and protect the anterior sur- but all are interconnected. Some of the histologic layers
face of the eye and contain glands that produce the lubri- are continuous throughout all three structures and are
cating tear film. Muscles, attached to the outer coat of derived from the same embryonic germ cell layer. The
the eye, control and direct the globe’s movement, and the iris is the most anterior structure, acting as a diaphragm
muscles of both eyes are coordinated to provide binocu- to regulate the amount of light entering the pupil. The
lar vision. A network of blood vessels supplies nutrients, two iris muscles control the shape and diameter of the
and a complex system of nerves provides sensory and pupil and are supplied by the autonomic nervous sys-
motor innervation to the eye and surrounding tissues and tem. Continuous with the iris at its root is the ciliary
structures. The neural signal that carries visual informa- body, which produces the components of the aqueous
tion passes through a complex and intricately designed humor and contains the muscle that controls the shape
pathway within the central nervous system, enabling of the lens. The posterior part of the uvea, the choroid,
an accurate view of the surrounding environment. This is an anastomosing network of blood vessels with a
information, evaluated by a process called visual percep- dense capillary network; it surrounds the retina and sup-
tion, influences myriad decisions and activities. plies nutrients to the outer retinal layers.
This book examines the macroscopic and micro- The neural tissue of the retina, by complex biochem-
scopic anatomy and physiology of the components in ical processes, changes light energy into a signal that can
this complex system and the structures that support it. be transmitted along a neural pathway. The signal passes
through the retina, exits the eye through the optic
THE E YE nerve, and is transmitted to various parts of the brain
for processing.
The interior of the eye is made up of three cham-
ANATOMIC FEATURES
bers. The anterior chamber is bounded in front by the
The eye is a special sense organ made up of three coats, cornea and posteriorly by the iris and anterior surface
or tunics, as follows: of the lens. The posterior chamber lies behind the
1. The outer fibrous layer of connective tissue forms the iris and surrounds the equator of the lens, separating
cornea and sclera. it from the ciliary body. The anterior and posterior
2. The middle vascular layer is composed of the iris, cili- chambers are continuous with one another through
ary body, and choroid. the pupil, and both contain aqueous humor that is
3. The inner neural layer is the retina. produced by the ciliary body. The aqueous humor pro-
Within this globe are three spaces: the anterior cham- vides nourishment for the surrounding structures, par-
ber, posterior chamber, and vitreous chamber. The ticularly the cornea and lens. The vitreous chamber,
crystalline lens is located in the region of the posterior which is the largest space, lies adjacent to the inner
chamber (Figure 1-1). retinal layer and is bounded in front by the lens. This
The outer dense connective tissue of the eye provides chamber contains a gel-like substance, the vitreous
protection for the structures within and maintains the humor.
1
2 Clinical Anatomy of the Visual System
Iris Cornea
Anterior chamber
External scleral sulcus
Corneoscleral border
Bulbar conjunctiva
Ciliary muscle
Ora
serrata
Pars plicata
Ciliary body
Pars plana
Medial
rectus
Lateral
rectus
Retina
Choroid
Fovea
Sclera
Lamina cribrosa
Dural sheath
Long posterior ciliary
artery
Optic nerve
Short posterior ciliary arteries
FIGURE 1-1
The visual system. (From Kronfeld PC: The human eye, Rochester, NY, 1943, Bausch & Lomb
Press.)
The crystalline lens is located in the area of the pos-
ANATOMIC DIRECTIONS AND PLANES
terior chamber and provides additional refractive power
for accurately focusing images onto the retina. The lens Anatomy is an exacting science, and specific terminol-
must change shape to view an object that is close to the ogy is basic to its discussion. The following anatomic
eye, through the mechanism of accommodation. directions should be familiar (Figure 1-2):
CHAPTER 1 t Visual System 3
Sagittal (median)
plane
Coronal (frontal)
plane
Posterior
Superior
Inferior
Anterior
Transverse
(horizontal)
plane
Lateral
Medial
Proximal
Distal
FIGURE 1-3
Anatomic planes. (From Palastanga N, Field D, Soames
R: Anatomy and human movement, Oxford, UK, 1989,
Butterworth-Heinemann.)
FIGURE 1-2
Anatomic directions. (From Palastanga N, Field D, Soames • Sagittal: vertical plane running from anterior to pos-
R: Anatomy and human movement, Oxford, UK, 1989, terior locations, dividing the structure into right and
Butterworth-Heinemann.) left sides.
• Midsagittal: sagittal plane through the midline, divid-
ing the structure into right and left halves.
• Anterior, or ventral: toward the front • Coronal or frontal: vertical plane running from side
• Posterior, or dorsal: toward the back to side, dividing the structure into anterior and poste-
• Superior, or cranial: toward the head rior parts.
• Inferior, or caudal: away from the head • Transverse: horizontal plane dividing the structure
• Medial: toward the midline into superior and inferior parts.
• Lateral: away from the midline Because the globe is a spherical structure, references
• Proximal: near the point of origin to locations can sometimes be confusing. In references
• Distal: away from the point of origin to anterior and posterior locations of the globe, the
The following planes are used in describing anatomic anterior pole (i.e., center of the cornea) is the reference
structures (Figure 1-3): point. For example, the pupil is anterior to the ciliary
4 Clinical Anatomy of the Visual System
body (see Figure 1-1). When layers or structures are
referred to as inner or outer, the reference is to the entire
globe unless specified otherwise. The point of reference
is the center of the globe, which would lie within the
vitreous. For example, the retina is inner to the sclera
(see Figure 1-1). In addition, the term sclerad is used to
mean “toward the sclera,” and vitread is used to mean
“toward the vitreous.” A
REFRACTIVE CONDITIONS
If the refractive power of the optical components of the
eye, primarily the cornea and lens, correlate with the
distances between the cornea, lens, and retina so that
incoming parallel light rays come into focus on the ret-
ina, a clear image will be seen. This condition is called
B
emmetropia (Figure 1-4, A). No correction is necessary
for clear distance vision. In hyperopia (farsightedness),
the distance from the cornea to the retina is too short
for the refractive power of the cornea and lens, thereby
causing images that would come into focus behind
the retina (Figure 1-4, B). Hyperopia can be corrected
by placing a convex lens in front of the eye to increase
the convergence of the incoming light rays. In myopia
(nearsightedness), because the lens and cornea are too C
strong or, more likely, the eyeball is too long, parallel
light rays are brought into focus in front of the retina FIGURE 1-4
(Figure 1-4, C). Myopia can be corrected by placing a Refractive conditions. A, Emmetropia, in which parallel light
concave lens in front of the eye, causing the incoming comes to a focus on the retina. B, Hyperopia, in which parallel
light rays to diverge. light comes to a focus behind the retina (dotted lines). A
convex lens is used to correct the condition and bring the light
rays into focus on the retina. C, Myopia, in which parallel light
OP HTH ALM IC comes to a focus in front of retina (dotted lines). A concave
I NST RU MENTATI ON lens is used to correct the condition and bring the light rays
into focus on the retina. (Courtesy Dr Karl Citek, OD, Pacific
University College of Optometry, Forest Grove, Ore.)
Various instruments are used to assess the health and
function of elements of the visual pathway and the sup-
porting structures. This section briefly describes some of The inside of the eye, called the fundus, is examined
these instruments and the structures examined. using an ophthalmoscope, which illuminates the interior
The curvature of the cornea is one of the factors that with a bright light. The retina, optic nerve head, and
determine the corneal refractive power. A keratometer blood vessels can be assessed and information about
measures the curvature of the central 3 to 4 mm of the ocular and systemic health obtained. This is the only
anterior corneal surface and provides information about place in the body in which blood vessels can be viewed
the power and the difference in curvature between the directly and noninvasively. Various systemic diseases,
principle meridians at that location. The smoothness of such as diabetes, hypertension, and arteriosclerosis, can
the corneal surface can also be assessed by the pattern alter ocular vessels. To obtain a more complete view of
reflected from the cornea during the measuring process. the inside of the eye, topical drugs are administered to
The automated corneal topographer maps the corneal sur- influence the iris muscles, causing the pupil to become
face and gives an indication of curvatures at selected enlarged, or mydriatic. The binocular indirect ophthalmo-
points. This instrument is an important adjunct in the scope allows stereoscopic viewing of the fundus.
fitting of contact lenses in difficult cases. The outside of the globe and the eyelids can be
The optometric physician can objectively determine assessed with a biomicroscope. This combination of an
the optical power of the eye with a set of lenses and a illumination system and a binocular microscope allows
retinoscope. This instrument is beneficial also for assess- stereoscopic views of various parts of the eye. Particularly
ing the accommodative function of the lens. beneficial is the view of the transparent structures, such
CHAPTER 1 t Visual System 5
as the cornea and lens. A number of auxiliary instru- the cells in the surface layer. Only the basal or deepest
ments can be used with the biomicroscope to measure layer of cells is in contact with the basement membrane,
intraocular pressure and to view the interior of the eye. and this layer usually consists of columnar cells.
Technologic advances have produced instrumentation Keratinized, stratified squamous epithelium has
that can provide three-dimensional mapping of retinal a surface layer of squamous cells with cytoplasm that
and optic nerve head surfaces and measure the thickness has been transformed into a substance called keratin, a
of specific retinal layers. Additional instrumentation can tough protective material relatively resistant to mechan-
allow visualization of corneal layers, cells, and nerves ical injury, bacterial invasion, and water loss. These kera-
and can aid in the differentiation of bacterial, viral, para- tinized surface cells constantly are sloughed off and are
sitic, and fungal infection in corneal tissue. replaced from the layers below, where cell division takes
The visual field is the area that a person sees when place.
looking straight ahead, including those areas seen “out
of the corner of the eye.” A perimeter is used to test the
Glandular Epithelium
extent, sensitivity, and completeness of this visual field.
Computerized perimeters provide extremely detailed Many epithelial cells are adapted for secretion and, when
maps of the visual field, as well as statistical informa- gathered into groups, are referred to as glands. Glands
tion on the reliability of the test and the probabilities can be classified according to the manner of secretion—
of any defects. exocrine glands secrete into a duct, whereas endocrine
glands secrete directly into the bloodstream. Glands
can also be classified according to the process of secre-
BAS IC HIS TOLOGIC
tion production—holocrine glands secrete complete cells
FEAT URES laden with the secretory material; apocrine glands secrete
part of the cell cytoplasm in the secretion; and the secre-
Because many of the anatomic structures are discussed tion of merocrine glands is a product of the cell without
in this book at the histologic level, this section briefly loss of any cellular components. Glands can also be
reviews basic human histology. Other details of tissues named according to the composition of their secretion:
are addressed in the pertinent chapters. mucous, serous, or sebaceous.
All body structures are made up of one or more of
the four basic tissues: epithelial, connective, muscle,
CONNECTIVE TISSUE
and nervous tissue. A tissue is defined as a collection of
similar cells that are specialized to perform a common Connective tissue provides structure and support and is
function. a “space filler” for areas not occupied by other tissue.
Connective tissue consists of cells, fibers, and ground
substance. The ground substance consisting of glyco-
EPITHELIAL TISSUE
proteins and water, and the insoluble protein fibers
Epithelial tissue often takes the form of sheets of epi- collectively are called matrix. Connective tissue can be
thelial cells that either cover the external surface of a classified as loose or dense. Loose connective tissue has
structure or that line a cavity. Epithelial cells lie on a relatively fewer cells and fibers per area than dense con-
basement membrane that attaches them to underlying nective tissue, in which the cells and fibers are tightly
connective tissue. The basement membrane can be packed. Dense connective tissue can be characterized as
divided into two parts: the basal lamina, secreted by the regular or irregular on the basis of fiber arrangement.
epithelial cell, and the reticular lamina, a product of Among the cells that may be found in connective
the underlying connective tissue layer.1 The free surface tissue are fibroblasts (flattened cells that produce and
of the epithelial cell is the apical surface, whereas the sur- maintain the fibers and ground substance), macrophages
face that faces underlying tissue or rests on the basement (phagocytic cells), mast cells (which contain heparin and
membrane is the basal surface. histamine), and fat cells. Connective tissue composed
Epithelial cells are classified according to shape. Squa- primarily of fat cells is called adipose tissue.
mous cells are flat and platelike, cuboidal cells are of The fibers found in connective tissue include flexible
equal height and width, and columnar cells are higher collagen fibers with high tensile strength, delicate reticu-
than wide. Epithelium consisting of a single layer of lar fibers, and elastic fibers, which can undergo exten-
cells is referred to as simple: simple squamous, simple sive stretching. Collagen fibers are a major component
cuboidal, or simple columnar. Endothelium is the spe- of much of the eye’s connective tissue. These fibers are
cial name given to the simple squamous layer that lines composed of protein macromolecules of tropocollagen
certain cavities. Epithelium consisting of several layers is that have a coiled helix of three polypeptide chains. The
referred to as stratified and is described by the shape of individual polypeptide chains can differ in their amino