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CHAPTER 2
The Eye
This chapter includes related videos. Links to individual videos are provided within the text; a page
containing all videos in Section 2 is available at www.aao.org/bcscvideo_section02.
Highlights
Hemidesmosomes anchor the basal corneal epithelial cells to the Bowman layer. Disruption
at this level can lead to scarring and recurrent erosion syndrome.
In addition to housing corneal stem cells, the limbus is the site of passage of the collector
channels that link the Schlemm canal to aqueous veins.
The sclera is an avascular tissue with 2 overlying vascular layers (deep and superficial) in
the episclera. Clinically, episcleritis refers to inflammation in the superficial layer, and
scleritis involves the deep layer.
The classification of uveitis, established by the 2005 SUN (Standardization of Uveitis
Nomenclature) Working Group, is based on the primary site of inflammation within the
zones of the uvea: anterior, intermediate, posterior, and all zones (panuveitis).
The blood–ocular barrier prevents extravasation of intravascular contents into the eye. It
consists of intercellular junctions of adjacent cells at various locations in the eye: the blood–
aqueous barrier and the blood–retina barrier (inner and outer).
Optical coherence tomography (OCT) has greatly enhanced visualization, as well as our
understanding, of ophthalmic structures in the anterior and posterior segments. In addition,
OCT angiography provides details of the microvasculature of the retina not previously seen
on fluorescein angiography.
The retina has a dual circulation, with the inner retina perfused by the retinal vessels seen
on routine examination of the fundus and the outer retina perfused by the choroid.
Introduction
The eye is a fascinating and complex organ, an anatomical window into the nervous and vascular
systems that can reveal systemic disease. More than 80% of our sensory input comes through
sight. This chapter discusses the anatomy of the major parts of the human eye. The reader is
encouraged to consult other volumes in the BCSC series for further discussion of many of the
topics presented in this chapter.
Topographic Features of the Globe
The eyeball, or globe, is not a true sphere. The radius of curvature of the prolate (polar radius
greater than equatorial radius, or “pointy”) cornea is 8 mm, smaller than that of the sclera, which
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Figure 2-1 Sagittal section of the eye with absent vitreous and major structures identified.
Dimensions are approximate and are average for the normal adult eye. (Illustration by Christine
Gralapp.)
is 12 mm. This makes the globe an oblate “squashed” spheroid (equatorial radius greater than
polar radius). The anteroposterior diameter of the adult eye is approximately 23–25 mm. The
average transverse diameter of the adult eye is 24 mm (Fig 2-1).
The eye contains 3 compartments: the anterior chamber, the posterior chamber, and the
vitreous cavity. The anterior chamber, the space between the iris and the cornea, is filled with
aqueous fluid. Anterior chamber depth varies among individuals and in regional populations; the
average depth is 3.11 mm. The average volume of the anterior chamber is 220 μL. The posterior
chamber is the anatomical portion of the eye posterior to the iris and anterior to the lens and
vitreous face. It is also filled with aqueous fluid and has an average volume of 60 μL. The largest
compartment is the vitreous cavity, which makes up more than two-thirds of the volume of the
eye (5–6 mL) and contains the vitreous gel (also called vitreous, vitreous body, or vitreous
humor). The total volume of the average adult eye is approximately 6.5–7.0 mL (Table 2-1).
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Table 2-1
The eyeball is composed of 3 concentric layers: an outer protective layer, a middle vascular
layer, and an inner neural layer. The outermost layer consists of the clear cornea anteriorly and
the opaque white sclera posteriorly. This corneoscleral layer is composed of collagen and
protects the internal ocular tissues.
The cornea occupies the center of the anterior pole of the globe. Because the sclera and
conjunctiva overlap the cornea anteriorly, slightly more above and below than medially and
laterally, the cornea appears elliptical when viewed from the front. The limbus, which borders
the cornea and the sclera, is blue-gray and translucent.
The middle layer of the globe, the uvea, consists of the choroid, ciliary body, and iris. Highly
vascular, it serves nutritive and supportive functions, supplying oxygen to the outer retina and
producing aqueous humor.
The innermost layer is the retina. This photosensitive layer contains the photoreceptors and
neural elements that initiate the processing of visual information.
Other important surface features of the globe, such as the vortex veins, the posterior ciliary
artery and nerves, and extraocular muscle insertions are discussed in Chapter 1; the optic nerve
and its surrounding meningeal sheaths are discussed in Chapter 3.
Precorneal Tear Film
The exposed surfaces of the cornea and bulbar conjunctiva are covered by the precorneal tear
film, which was formerly described as having 3 layers: lipid (from meibomian glands), aqueous
(from the lacrimal gland), and mucin (primarily from goblet cells). It is now thought of as a lipid
layer with underlying uniform gel consisting of soluble mucus (secreted by conjunctival goblet
cells), mixed with fluids and proteins (secreted by the lacrimal glands). A glycocalyx mediates the
interaction of the mucoaqueous layer with surface epithelial cells of the cornea.
Maintenance of the precorneal tear film is vital for normal corneal function. The tear film
does the following:
provides lubrication for the cornea and conjunctiva
facilitates the exchange of solutes, including oxygen
contributes to the antimicrobial defense of the ocular surface
serves as a medium to remove debris
Further, the air–tear film interface at the surface of the cornea constitutes a major refractive
element of the eye, because of the difference in the refractive index of air and that of the tear
film. Aberrations in the tear film result from a variety of diseases (eg, dry eye, blepharitis) that
can profoundly affect the integrity of the ocular surface and consequently the patient’s vision.
See Chapter 7 for in-depth discussion of the tear film.
Cornea
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Figure 2-2 A, Histologic section showing the 5 layers of the cornea (thickness given within
parentheses): epithelium (40–50 μm), Bowman layer (8–15 μm), stroma (470–500 μm), Descemet
membrane (10–12 μm), and endothelium (4–6 μm). B, Anterior segment optical coherence
tomography (AS-OCT) of the cornea. B = Bowman layer; D = Descemet membrane; En =
endothelium; Ep = epithelium; S = stroma. (Part A courtesy of George J. Harocopos, MD; part B courtesy of
The cornea is a clear avascular tissue consisting of 5 layers (Fig 2-2):
epithelium
Bowman layer
stroma
Descemet membrane
endothelium
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Vikram S. Brar, MD.)
The cornea covers one-sixth of the surface of the globe. It has a refractive index of 1.376 and an
average radius of curvature of 7.8 mm. With a power of 43.25 diopters (D), the cornea produces
most of the eye’s refractive power of 58.60 D. Oxygen from the air and from the eyelid
vasculature dissolves in tears and is transmitted to the cornea via the tear film. The cornea
derives its macromolecules and nutrients from the aqueous humor.
Characteristics of the Central and Peripheral Cornea
In adults, the cornea measures about 12 mm in the horizontal meridian and about 11 mm in the
vertical meridian. The central third of the cornea is nearly spherical and measures approximately
4 mm in diameter. Because the posterior surface of the cornea is more curved than the anterior
surface, the central cornea is thinner (0.5 mm) than the peripheral cornea (1.0 mm). The cornea
flattens in the periphery, with more extensive flattening nasally and superiorly than temporally
and inferiorly. This topography is important in contact lens fitting. For additional discussion,see
Chapter 2 in BCSC Section 8, External Disease and Cornea, and Chapter 4 in Section 3,
Clinical Optics.
Epithelium and Basal Lamina
The anterior surface of the cornea is covered by a lipophilic, nonkeratinized, stratified squamous
epithelium that is composed of 4–6 cell layers and is typically 40–50 μm thick (Fig 2-3). The
basal cells have a width of 12 μm and a density of approximately 6000 cells/mm2. They are
attached to the underlying basal lamina by hemidesmosomes. Trauma to the epithelium
disrupting this layer can lead to recurrent corneal erosion due to improper re-formation of these
hemidesmosomes.
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Figure 2-3 A, The corneal epithelium is composed of 4–6 cell layers that make up a stratified
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squamous epithelium, which is derived from the surface ectoderm. B, Schematic of the corneal
epithelium demonstrating adhesion between cells and to the underlying basal lamina (purple) and
Bowman layer via hemidesmosomes. B = basal cells; S = surface cells; W = wing cells. (Reproduced
with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of the Eye. 11th ed.
Philadelphia: Elsevier/Saunders; 2011:94.)
Overlying the basal cell layer are 2 or 3 layers of polygonal “wing” cells. Superficial to these
layers are 1–2 layers of corneal epithelial “surface” cells that are extremely thin (30 μm) and are
attached to one another by tight junctions. The tight junctions allow the surface epithelial cells to
act as a barrier to diffusion. Microvilli make the apical membranes of the surface cells highly
irregular; however, the precorneal tear film renders the surfaces optically smooth.
Although the deeper epithelial cells are firmly attached to one another by desmosomes, they
migrate continuously from the basal region toward the tear film, into which they are shed. They
also migrate centripetally from their stem cell source at the limbus. Division of the slow-cycling
stem cells gives rise to a progeny of daughter cells (transient amplifying cells), whose division
serves to maintain the corneal epithelium (see also Chapter 8). Diffuse damage to the limbal stem
cells (eg, by chemical burns or trachoma) leads to chronic epithelial surface defects.
Del Monte DW, Kim T. Anatomy and physiology of the cornea. J Cataract Refract Surg. 2011;37(3):588–598.
Bowman Layer
Beneath the basal lamina of the epithelium is the Bowman layer, or Bowman membrane, a tough
layer consisting of randomly dispersed collagen fibrils. It is a modified region of the anterior
stroma that is 8–15 μm thick (see Fig 2-2). Unlike the Descemet membrane, it is not restored
after injury but is replaced by scar tissue.
Stroma
The stroma constitutes approximately 90% of the total corneal thickness in humans (see Fig 2-
2). It is composed of collagen-producing keratocytes, ground substance, and collagen lamellae.
The collagen fibrils form obliquely oriented lamellae in the anterior third of the stroma (with
some interlacing) and perpendicular lamellae in the less compact posterior two-thirds (see
Chapter 8, Fig 8-3).
The corneal collagen fibrils extend across the entire diameter of the cornea, finally winding
circumferentially around the limbus. The fibrils are remarkably uniform in size and separation,
and this regularity helps determine the transparency of the cornea (see also Chapter 8).
Separation of the collagen fibrils by edema leads to stromal clouding. The stroma’s collagen
types are I (predominant), III, IV, V, VI, XII, and XIV. Type VII forms the anchoring fibril of the
epithelium. Natural crosslinking occurs with aging.
The ground substance of the cornea consists of proteoglycans that run along and between the
collagen fibrils. Their glycosaminoglycan components (eg, keratan sulfate) are negatively charged
and tend to repel each other—as well as draw in sodium and, secondarily, water—producing the
swelling pressure of the stroma. The keratocytes lie between the corneal lamellae and synthesize
both collagen and proteoglycans. Ultrastructurally, they resemble fibrocytes.
The cornea has approximately 2.4 million keratocytes, which occupy about 5% of the stromal
volume; the density is higher anteriorly (1058 cells/mm2) than posteriorly (771 cells/mm2).
Keratocytes are highly active cells rich in mitochondria, rough endoplasmic reticula, and Golgi
apparatus. They have attachment structures, communicate through gap junctions, and have
unusual fenestrations in their plasma membranes. Their flat profile and even distribution in the
coronal plane ensure a minimum disturbance of light transmission.
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Figure 2-4 Histologic section of the posterior cornea. Higher magnification depicts the Descemet
membrane (D) and endothelium (En). A keratocyte nucleus (arrow) is visible in the posterior stroma.
(Courtesy of George J. Harocopos, MD.)
Müller LJ, Pels L, Vrensen GF. Novel aspects of the ultrastructural organization of human corneal keratocytes.
Invest Ophthalmol Vis Sci. 1995;36(13):2557–2567.
Mustonen RK, McDonald MB, Srivannaboon S, Tan AL, Doubrava MW, Kim CK. Normal human corneal cell
populations evaluated by in vivo scanning slit confocal microscopy. Cornea. 1998;17(5):485–492.
Descemet Membrane
The basal lamina of the corneal endothelium, the Descemet membrane, is periodic acid–Schiff
(PAS) positive (Fig 2-4). It is a true basement membrane, and its thickness increases with age. At
birth, the Descemet membrane is 3–4 μm thick, increasing to 10–12 μm at adulthood. It is
composed of an anterior banded zone that develops in utero (4.6 ± 0.4 μm thick) and a posterior
nonbanded zone that is laid down by the corneal endothelium throughout life (average in adults is
11.8 ± 0.4 μm, increasing about 0.1 μm/year) (Fig 2-5). These zones provide a historical record
of the synthetic function of the endothelium. Like other basal laminae, the Descemet membrane
is rich in type IV collagen.
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Figure 2-5 Corneal endothelium and the Descemet membrane. (Illustration by Thomas A. Weingeist,
PhD, MD.)
Peripheral excrescences of the Descemet membrane, known as Hassall-Henle warts, are
common, especially among elderly people. Central excrescences (corneal guttae) also appear
with increasing age.
Endothelium
The corneal endothelium is composed of a single layer of hexagonal cells derived from the neural
crest (Fig 2-6). Therefore, the corneal endothelium is of neuroectodermal origin. In young adult
eyes, approximately 500,000 cells are present, at a density of about 3000/mm2 centrally and up
to 8000/mm2 peripherally. Mitosis of the endothelium is limited in humans, and the overall
number of endothelial cells decreases with age.
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Figure 2-6 Specular microscopy of living corneal endothelium. Normal endothelium is shown on the
left. Note the hexagonal shape of the endothelial cells. The corneal endothelium of a patient with
Fuchs endothelial corneal dystrophy is shown on the right. Demonstrated are polymegathism (larger
cells), pleomorphism (variability in size and shape of cells), and dark areas of endothelial cell loss
(guttae). (Courtesy of Preston H. Blomquist, MD.)
The size, shape, and distribution of the endothelial cells can be observed by specular
microscopy at the slit lamp. The apical surfaces of these cells face the anterior chamber; their
basal surfaces secrete the Descemet membrane. Typically, young endothelial cells have large
nuclei and abundant mitochondria. The active transport of ions by these cells leads to the
transfer of water from the corneal stroma and the maintenance of stromal deturgescence and
transparency.
Endothelial cell dysfunction and loss—through surgical injury, inflammation, or disease (eg,
Fuchs endothelial corneal dystrophy)—may cause endothelial decompensation, stromal edema,
and vision loss. Because endothelial mitosis is limited in humans, destruction of cells causes cell
density to decrease and residual cells to spread and enlarge.
Zheng T, Le Q, Hong J, Xu J. Comparison of human corneal cell density by age and corneal location: an in vivo
confocal microscopy study.BMC Ophthalmol. 2016;16:109.
Limbus
The transition zone between the peripheral cornea and the anterior sclera, known as the limbus
(also called corneoscleral junction or corneal limbus), is defined differently by anatomists,
pathologists, and clinicians. Though not a distinct anatomical structure, the limbus is important
for 3 reasons: its relationship to the anterior chamber angle, its use as a surgical landmark, and its
supply of corneal stem cells. The limbus is also the site of passage of the collector channel that
links the Schlemm canal to aqueous veins.
The following structures are found at the limbus:
conjunctiva and limbal palisades of Vogt, which house the corneal stem cells
episclera (discussed later, under Sclera)
junction of corneoscleral stroma
aqueous outflow apparatus (collector channel)
The corneoscleral junction begins centrally in a plane connecting the end of the Bowman
layer and the Schwalbe line, which is the termination of the Descemet membrane. Internally, its
posterior limit is the anterior tip of the scleral spur (Fig 2-7). Pathologists consider the posterior
limit of the limbus to be formed by another plane perpendicular to the surface of the eye,
approximately 1.5 mm posterior to the termination of the Bowman layer in the horizontal
meridian and 2.0 mm posterior in the vertical meridian, where there is greater scleral overlap (Fig
2-8).
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Figure 2-7 Anterior chamber angle and limbus, depicting the concept of the limbus. Solid lines
represent the limbus as viewed by pathologists; the green dotted line represents the limbus as
viewed by anatomists. (Illustration by Thomas A. Weingeist, PhD, MD.)
Figure 2-8 Limbus. A, Slit-lamp photograph showing the blue-gray corneoscleral limbus. The
striations orthogonal to the cornea are the limbal palisades of Vogt, where the corneal stem cells
reside. B, Photograph of limbus-based trabeculectomy. Note the blue-gray surgical limbus with
corresponding sclerostomy. (Part A courtesy of Cornea Service, Paulista School of Medicine, Federal University
of São Paulo; part B courtesy of Keith Barton, MD, and reproduced with permission from Moorfields Eye Hospital.)
The surgical limbus, an external landmark for incisions in cataract and glaucoma surgery, is
sometimes referred to as the gray or blue zone. Its blue-gray appearance is due to the scattering
of light through the oblique interface between cornea and sclera, which occurs gradually over 1–
2 mm (see Fig 2-8B). The posterior border of the blue-gray zone is a consistent external
landmark that corresponds to the internal junction of cornea and sclera overlying the trabecular
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meshwork in all meridians.
Sclera
The sclera covers the posterior five-sixths of the surface of the globe, with an anterior opening
for the cornea and a posterior opening for the optic nerve. The tendons of the rectus muscles
insert into the superficial scleral collagen. The Tenon capsule covers the sclera and rectus
muscles anteriorly, and both are overlain by the bulbar conjunctiva. The capsule and conjunctiva
fuse near the limbus.
The sclera is thinnest (0.3 mm) just behind the insertions of the rectus muscles and thickest
(1.0 mm) at the posterior pole around the optic nerve head. It is 0.4–0.5 mm thick at the equator
and 0.6 mm thick anterior to the muscle insertions. Because of the thinness of the sclera,
strabismus and retinal detachment surgery require careful placement of sutures.
CLINICAL PEARL
The most common sites of scleral rupture following blunt trauma are
in the superonasal quadrant, near the limbus
in a circumferential arc parallel to the corneal limbus opposite the site of impact
behind the insertion of the rectus muscles
The sclera, like the cornea, is essentially avascular except for the vessels of the intrascleral
vascular plexus, located just posterior to the limbus, and the episcleral vessels. The episcleral
vessels have superficial and deep plexuses (Fig 2-9). The superficial plexus runs beneath the
Tenon capsule in a radial pattern; in episcleritis, it is this vascular plexus that is involved. The
deep episcleral plexus rests on the surface of the sclera and is the layer involved in scleritis.
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Figure 2-9 Episcleral vessels. The sclera is avascular but has overlying episcleral vessels, which
are divided into superficial and deep plexuses. The organization of the conjunctival vasculature,
which is also depicted, is similar to that of the episcleral vessels, with the addition of lymphatics,
shown in green. (Modified with permission from Levin LA, Nilsson SFE, Ver Hoeve J, Wu SM. Adler’s Physiology of
the Eye. 11th ed. Philadelphia: Elsevier/Saunders; 2011:118–119.)
Numerous channels, or emissaria, penetrate the sclera (see Chapter 1, Figs 1-19, 1-20),
allowing the passage of arteries, veins, and nerves:
anterior emissaria: penetration of the anterior ciliary arteries anterior to the rectus muscle
insertions
middle emissaria: exit of vortex veins
posterior emissaria: lamina cribrosa, penetration of the short and long posterior ciliary
vessels and ciliary nerves
Extraocular extension of malignant melanoma of the choroid occurs by way of the middle
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Figure 2-10 External photograph of Axenfeld nerve loops in an arc pattern roughly equidistant from
the limbus. (Reproduced with permission from Jesse Vislisel, MD; EyeRounds.org, University of Iowa. Photograph
by Cindy Montague, CRA.)
emissaria.
Branches of the ciliary nerves that supply the cornea sometimes leave the sclera to form loops
posterior to the nasal and temporal limbus. These nerve loops, called Axenfeld loops, are
sometimes pigmented and, consequently, have been mistaken for uveal tissue or malignant
melanoma (Fig 2-10).
Anterior to the rectus muscle insertions, the episclera consists of a dense vascular connective
tissue that merges deeply with the superficial sclera and superficially with the Tenon capsule and
the conjunctiva. The scleral stroma is composed of bundles of collagen, fibroblasts, and a
moderate amount of ground substance.
Collagen fibers of the sclera vary in size and shape and taper at their ends, indicating that they
are not continuous fibers as in the cornea. The inner layer of the sclera (lamina fusca) blends
imperceptibly with the suprachoroidal and supraciliary lamellae of the uvea. The collagen fibers
in this portion of the sclera branch and intermingle with the outer ciliary body and choroid. The
opaque, porcelain-white appearance of the sclera contrasts markedly with the transparency of
the cornea and is primarily due to 2 factors: the greater variation in collagen fibril separation and
diameter, and the greater degree of fibril interweaving in the sclera (see also Chapter 8). In
addition, the lack of vascular elements contributes to corneal clarity.
Anterior Chamber
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Figure 2-11 Structures of the anterior chamber angle. 1, Peripheral iris: a, insertion; b, curvature; c,
angular approach. 2, Ciliary body band. 3, Scleral spur. 4, Trabecular meshwork: a, posterior; b,
mid; c, anterior. 5, Schwalbe line. (*), Corneal optical wedge.
The anterior chamber is bordered anteriorly by the cornea and posteriorly by the iris diaphragm
and the pupil. The anterior chamber angle, which lies at the junction of the cornea and the iris,
includes the following 5 structures (Figs 2-11 through 2-14):
Schwalbe line
Schlemm canal and trabecular meshwork (also see the section Trabecular Meshwork)
scleral spur
anterior border of the ciliary body (where its longitudinal fibers insert into the scleral spur)
peripheral iris
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