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					ANATOMY OF THE EYE
This chapter gives only a brief account of the anatomy
of eyeball and its related structures. The detailed
anatomy of different structures is described in the
relevant chapters.
THE EYEBALL
Each eyeball (Fig. 1.1) is a cystic structure kept
distended by the pressure inside it. Although,
generally referred to as a globe, the eyeball is not a
sphere but an ablate spheroid. The central point on
the maximal convexities of the anterior and posterior
curvatures of the eyeball is called the anterior and
posterior pole, respectively. The equator of the
eyeball lies at the mid plane between the two poles
(Fig.1.2).
Dimensions of an adult eyeball
Anteroposterior diameter 24 mm
Horizontal diameter 23.5 mm
Vertical diameter 23 mm
Circumference 75 mm
Volume 6.5 ml
Weight 7 gm
Coats of the eyeball
The eyeball comprises three coats: outer (fibrous
coat), middle (vascular coat) and inner (nervous coat).
1. Fibrous coat. It is a dense strong wall which
protects the intraocular contents. Anterior 1/6th of
this fibrous coat is transparent and is called cornea.
Posterior 5/6th opaque part is called sclera. Cornea is
set into sclera like a watch glass. Junction of the
cornea and sclera is called limbus. Conjunctiva is
firmly attached at the limbus.
2. Vascular coat (uveal tissue). It supplies nutrition
to the various structures of the eyeball. It consists of
three parts which from anterior to posterior are : iris,
ciliary body and choroid.
3. Nervous coat (retina). It is concerned with visual
functions.
Segments and chambers of the eyeball
The eyeball can be divided into two segments:
anterior and posterior.
1. Anterior segment. It includes crystalline lens
(which is suspended from the ciliary body by zonules),
and structures anterior to it, viz., iris, cornea and two
aqueous humour-filled spaces : anterior and posterior
chambers.
ANATOMY OF THE EYE
.. The eyeball
.. Visual pathway
.. Orbit, extraocular muscles and
appendages of the eye
DEVELOPMENT OF THE EYE
.. Formation of optic vesicle and
optic stalk
.. Formation of lens vesicle
.. Formation of optic cup
.. Changes in the associated mesoderm
.. Development of various ocular
structures
.. Structures derived from the embryonic
layers
.. Important milestones in the development
of the eye
Anatomy and
Development
of the Eye
CHAPTER11
4 Comprehensive OPHTHALMOLOGY
Fig. 1.1. Gross anatomy of the eyeball.
Fig. 1.2. Poles and equators of the eyeball.
.. Anterior chamber. It is bounded anteriorly by
the back of cornea, and posteriorly by the iris
and part of ciliary body. The anterior chamber is
about 2.5 mm deep in the centre in normal adults.
It is shallower in hypermetropes and deeper in
myopes, but is almost equal in the two eyes of
the same individual. It contains about 0.25 ml of
the aqueous humour.
.. Posterior chamber. It is a triangular space
containing 0.06 ml of aqueous humour. It is
bounded anteriorly by the posterior surface of
iris and part of ciliary body, posteriorly by the
crystalline lens and its zonules, and laterally by
the ciliary body.
2. Posterior segment. It includes the structures
posterior to lens, viz., vitreous humour (a gel like
material which fills the space behind the lens), retina,
choroid and optic disc.
VISUAL PATHWAY
Each eyeball acts as a camera; it perceives the images
and relays the sensations to the brain (occipital
cortex) via visual pathway which comprises optic
nerves, optic chiasma, optic tracts, geniculate bodies
and optic radiations (Fig. 1.3).
ORBIT, EXTRAOCULAR MUSCLES AND
APPENDAGES OF THE EYE (FIG. 1.4)
Each eyeball is suspended by extraocular muscles
and fascial sheaths in a quadrilateral pyramid-shaped
ANATOMY AND DEVELOPMENT OF THE EYE 5
Fig. 1.3. Gross anatomy of the visual pathway.
bony cavity called orbit (Fig. 1.4). Each eyeball is
located in the anterior orbit, nearer to the roof and
lateral wall than to the floor and medial wall. Each eye
is protected anteriorly by two shutters called the
eyelids. The anterior part of the sclera and posterior
surface of lids are lined by a thin membrane called
conjunctiva. For smooth functioning, the cornea and
conjunctiva are to be kept moist by tears which are
produced by lacrimal gland and drained by the lacrimal
passages. These structures (eyelids, eyebrows,
conjunctiva and lacrimal apparatus) are collectively
called ‘the appendages of the eye’.
DEVELOPMENT OF THE EYE
The development of eyeball can be considered to
commence around day 22 when the embryo has eight
pairs of somites and is around 2 mm in length. The
eyeball and its related structures are derived from the
following primordia:
.. Optic vesicle,an outgrowth from prosencephalon
(a neuroectodermal structure),
.. Lens placode, a specialised area of surface
ectoderm, and the surrounding surface ectoderm,
.. Mesenchyme surrounding the optic vesicle, and
Fig. 1.4. Section of the orbital cavity to demonstrate eyeball and its
accessory structures.
.. Visceral mesoderm of maxillary process.
Before going into the development of individual
structures, it will be helpful to understand the
formation of optic vesicle, lens placode, optic cup
and changes in the surrounding mesenchyme, which
play a major role in the development of the eye and
its related structures.
6 Comprehensive OPHTHALMOLOGY
FORMATION OF OPTIC VESICLE
AND OPTIC STALK
The area of neural plate (Fig. 1.5A) which forms the
prosencepholon develops a linear thickened area on
either side (Fig. 1.5B), which soon becomes depressed
to form the optic sulcus (Fig. 1.5C). Meanwhile the
neural plate gets converted into prosencephalic
vesicle. As the optic sulcus deepens, the walls of the
prosencepholon overlying the sulcus bulge out to
form the optic vesicle (Figs. 1.5D, E&F). The proximal
part of the optic vesicle becomes constricted and
elongated to form the optic stalk (Figs. 1.5G&H).
FORMATION OF LENS VESICLE
The optic vesicle grows laterally and comes in contact
with the surface ectoderm. The surface ectoderm,
overlying the optic vesicle becomes thickened to form
the lens placode (Fig. 1.6A) which sinks below the
surface and is converted into the lens vesicle (Figs.
1.6 B&C). It is soon separated from the surface
ectoderm at 33rd day of gestation (Fig. 1.6D).
FORMATION OF OPTIC CUP
The optic vesicle is converted into a double-layered
optic cup. It appears from Fig. 1.6 that this has
happened because the developing lens has
invaginated itself into the optic vesicle. In fact
conversion of the optic vesicle to the optic cup is
due to differential growth of the walls of the vesicle.
The margins of optic cup grow over the upper and
lateral sides of the lens to enclose it. However, such a
growth does not take place over the inferior part of
the lens, and therefore, the walls of the cup show
deficiency in this part. This deficiency extends to Fig. 1.5. Formation
of the optic vesicle and optic stalk.
Fig. 1.6. Formation of lens vesicle and optic cup.
ANATOMY AND DEVELOPMENT OF THE EYE 7
Fig. 1.8. Developing optic cup surrounded by mesenchyme.
In the posterior part of optic cup the surrounding
fibrous mesenchyme forms sclera and extraocular
muscles, while the vascular layer forms the choroid
and ciliary body.
DEVELOPMENT OF VARIOUS
OCULAR STRUCTURES
Retina
Retina is developed from the two walls of the optic
cup, namely: (a) nervous retina from the inner wall,
and (b) pigment epithelium from the outer wall
(Fig. 1.10).
(a) Nervous retina. The inner wall of the optic cup is
a single-layered epithelium. It divides into several
layers of cells which differentiate into the following
three layers (as also occurs in neural tube):
some distance along the inferior surface of the optic
stalk and is called the choroidal or fetal fissure
(Fig. 1.7).
Fig. 1.7. Optic cup and stalk seen from below to show
CHANGES IN THE ASSOCIATED MESENCHYME
The developing neural tube (from which central
nervous system develops) is surrounded by
mesenchyme, which subsequently condenses to form
meninges. An extension of this mesenchyme also
covers the optic vesicle. Later, this mesenchyme
differentiates to form a superficial fibrous layer
(corresponding to dura) and a deeper vascular layer
(corresponding to pia-arachnoid) (Fig. 1.8).
With the formation of optic cup, part of the inner
vascular layer of mesenchyme is carried into the cup
through the choroidal fissure. With the closure of
this fissure, the portion of mesenchyme which has
made its way into the eye is cut off from the
surrounding mesenchyme and gives rise to the hyaloid
system of the vessels (Fig. 1.9).
The fibrous layer of mesenchyme surrounding the
anterior part of optic cup forms the cornea. The
corresponding vascular layer of mesenchyme
becomes the iridopupillary membrane, which in the
peripheral region attaches to the anterior part of the
optic cup to form the iris. The central part of this
lamina is pupillary membrane which also forms the
tunica vasculosa lentis (Fig. 1.9). Fig. 1.9. Derivation of various
structures of the eyeball.
8 Comprehensive OPHTHALMOLOGY
Crystalline lens
The crystalline lens is developed from the surface
ectoderm as below :
Lens placode and lens vesicle formation (see page
5, 6 and Fig. 1.6 .
Primary lens fibres. The cells of posterior wall of
lens vesicle elongate rapidly to form the primary lens
fibres which obliterate the cavity of lens vesicle. The
primary lens fibres are formed upto 3rd month of
gestation and are preserved as the compact core of
lens, known as embryonic nucleus (Fig. 1.11).
Secondary lens fibres are formed from equatorial cells
of anterior epithelium which remain active through
out life. Since the secondary lens fibres are laid down
concentrically, the lens on section has a laminated
appearance. Depending upon the period of
development, the secondary lens fibres are named as
below :
.. Fetal nucleus (3rd to 8th month),
.. Infantile nucleus (last weeks of fetal life to
puberty),
.. Adult nucleus (after puberty), and
.. Cortex (superficial lens fibres of adult lens)
Lens capsule is a true basement membrane produced
by the lens epithelium on its external aspect.
Cornea (Fig. 1.9)
1. Epithelium is formed from the surface ectoderm.
2. Other layers viz. endothelium, Descemet's
membrane, stroma and Bowman's layer are derived
from the fibrous layer of mesenchyme lying anterior
to the optic cup (Fig. 1.9).
Sclera
Sclera is developed from the fibrous layer of
mesenchyme surrounding the optic cup (corresponding
to dura of CNS) (Fig. 1.9).
Choroid
It is derived from the inner vascular layer of
mesenchyme that surrounds the optic cup (Fig. 1.9).
Ciliary body
.. The two layers of epithelium of ciliary body
develop from the anterior part of the two layers
of optic cup (neuroectodermal).
.. Stroma of ciliary body, ciliary muscle and blood
vessels are developed from the vascular layer of
mesenchyme surrounding the optic cup (Fig. 1.9).
.. Matrix cell layer. Cells of this layer form the rods
and cones.
.. Mantle layer. Cells of this layer form the
bipolar cells, ganglion cells, other neurons of
retina and the supporting tissue.
.. Marginal layer. This layer forms the ganglion
cells, axons of which form the nerve fibre
layer.
(b) Outer pigment epithelial layer. Cells of the outer
wall of the optic cup become pigmented. Its posterior
part forms the pigmented epithelium of retina and the
anterior part continues forward in ciliary body and
iris as their anterior pigmented epithelium.
Optic nerve
It develops in the framework of optic stalk as
below:
.. Fibres from the nerve fibre layer of retina grow
into optic stalk by passing through the choroidal
fissure and form the optic nerve fibres.
.. The neuroectodermal cells forming the walls of
optic stalk develop into glial system of the nerve.
.. The fibrous septa of the optic nerve are
developed from the vascular layer of mesenchyme
which invades the nerve at 3rd fetal month.
.. Sheaths of optic nerve are formed from the layers
of mesenchyme like meninges of other parts of
central nervous system.
.. Myelination of nerve fibres takes place from
brain distally and reaches the lamina cribrosa just
before birth and stops there. In some cases, this
extends up to around the optic disc and presents
as congenital opaque nerve fibres. These develop
after birth.
Fig. 1.10. Development of the retina.
ANATOMY AND DEVELOPMENT OF THE EYE 9
Iris
.. Both layers of epithelium are derived from
the marginal region of optic cup (neuroectodermal)
(Fig. 1.9).
.. Sphincter and dilator pupillae muscles are
derived from the anterior epithelium (neuroectodermal).
.. Stroma and blood vessels of the iris develop
from the vascular mesenchyme present anterior
to the optic cup.
Fig. 1.11. Development of the crystalline lens.
Fig. 1.12. Development of the eyelids, conjunctiva and
lacrimal gland.
Vitreous
1. Primary or primitive vitreous is mesenchymal in
origin and is a vascular structure having the
hyaloid system of vessels.
2. Secondary or definitive or vitreous proper is
secreted by neuroectoderm of optic cup. This is
an avascular structure. When this vitreous fills
the cavity, primitive vitreous with hyaloid vessels
is pushed anteriorly and ultimately disappears.
3. Tertiary vitreous is developed from neuroectoderm
in the ciliary region and is represented
by the ciliary zonules.
Eyelids
Eyelids are formed by reduplication of surface
ectoderm above and below the cornea (Fig. 1.12). The
folds enlarge and their margins meet and fuse with
each other. The lids cut off a space called the
conjunctival sac. The folds thus formed contain some
mesoderm which would form the muscles of the lid
and the tarsal plate. The lids separate after the seventh
month of intra-uterine life.
10 Comprehensive OPHTHALMOLOGY
Tarsal glands are formed by ingrowth of a regular
row of solid columns of ectodermal cells from the lid
margins.
Cilia develop as epithelial buds from lid margins.
Conjunctiva
Conjunctiva develops from the ectoderm lining the
lids and covering the globe (Fig.1.12).
Conjunctival glands develop as growth of the basal
cells of upper conjunctival fornix. Fewer glands
develop from the lower fornix.
The lacrimal apparatus
Lacrimal gland is formed from about 8 cuneiform
epithelial buds which grow by the end of 2nd month
of fetal life from the superolateral side of the
conjunctival sac (Fig. 1.12).
Lacrimal sac, nasolacrimal duct and canaliculi.
These structures develop from the ectoderm of
nasolacrimal furrow. It extends from the medial angle
of eye to the region of developing mouth. The
ectoderm gets buried to form a solid cord. The cord is
later canalised. The upper part forms the lacrimal sac.
The nasolacrimal duct is derived from the lower part
as it forms a secondary connection with the nasal
cavity. Some ectodermal buds arise from the medial
margins of eyelids. These buds later canalise to form
the canaliculi.
Extraocular muscles
All the extraocular muscles develop in a closely
associated manner by mesodermally derived
mesenchymal condensation. This probably
corresponds to preotic myotomes, hence the triple
nerve supply (III, IV and VI cranial nerves).
STRUCTURES DERIVED FROM
THE EMBRYONIC LAYERS
Based on the above description, the various
structures derived from the embryonic layers are given
below :
1. Surface ectoderm
.. The crystalline lens
.. Epithelium of the cornea
.. Epithelium of the conjunctiva
.. Lacrimal gland
.. Epithelium of eyelids and its derivatives viz., cilia,
tarsal glands and conjunctival glands.
.. Epithelium lining the lacrimal apparatus.
2. Neural ectoderm
.. Retina with its pigment epithelium
.. Epithelial layers of ciliary body
.. Epithelial layers of iris
.. Sphincter and dilator pupillae muscles
.. Optic nerve (neuroglia and nervous elements
only)
.. Melanocytes
.. Secondary vitreous
.. Ciliary zonules (tertiary vitreous)
3. Associated paraxial mesenchyme
.. Blood vessels of choroid, iris, ciliary vessels,
central retinal artery, other vessels.
.. Primary vitreous
.. Substantia propria, Descemet's membrane and
endothelium of cornea
.. The sclera
.. Stroma of iris
.. Ciliary muscle
.. Sheaths of optic nerve
.. Extraocular muscles
.. Fat, ligaments and other connective tissue
structures of the orbit
.. Upper and medial walls of the orbit
.. Connective tissue of the upper eyelid
4. Visceral mesoderm of maxillary process
below the eye
.. Lower and lateral walls of orbit
.. Connective tissue of the lower eyelid
IMPORTANT MILESTONES IN THE
DEVELOPMENT OF THE EYE
Embryonic and fetal period
Stage of growth Development
2.6 mm (3 weeks) Optic pits appear on either
side of cephalic end of
forebrain.
3.5 mm (4 weeks) Primary optic vesicleinvaginates.
5.5 to 6 mm Development of embryonic
fissure
10 mm (6 weeks) Retinal layers differentiate,
lens vesicle formed.
20 mm (9 weeks) Sclera, cornea and extraocular
muscles differen-tiate.
ANATOMY AND DEVELOPMENT OF THE EYE 11
.. Corneal diameter is about 10 mm. Adult size
(11.7 mm) is attained by 2 years of age.
.. Anterior chamber is shallow and angle is narrow.
.. Lens is spherical at birth. Infantile nucleus is
present.
.. Retina. Apart from macular area the retina is fully
differentiated. Macula differentiates 4-6 months
after birth.
.. Myelination of optic nerve fibres has reached
the lamina cribrosa.
.. Newborn is usually hypermetropic by +2 to +3 D.
.. Orbit is more divergent (50°) as compared to
adult (45°).
.. Lacrimal gland is still underdeveloped and tears
are not secreted.
Postnatal period
.. Fixation starts developing in first month and is
completed in 6 months.
.. Macula is fully developed by 4-6 months.
.. Fusional reflexes, stereopsis and accommodation
is well developed by 4-6 months.
.. Cornea attains normal adult diameter by 2 years
of age.
.. Lens grows throughout life.
25 mm (10 weeks) Lumen of optic nerve obliterated.
50 mm (3 months) Optic tracts completed, pars
ciliaris retina grows
forwards, pars iridica retina
grows forward.
60 mm (4 months) Hyaloid vessels atrophy, iris
sphincter is formed.
230-265 mm Fetal nucleus of lens is
complete,
(8th month) all layers of retina nearly
developed, macula starts
differentiation.
265-300mm Except macula, retina is fully
(9th month) developed, infantile nucleus
of lens begins to appear,
pupillary membr-ane and
hyaloid vessels disappear.
Eye at birth
.. Anteroposterior diameter of the eyeball is about
16.5 mm (70% of adult size which is attained by
7-8 years).
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MAINTENANCE OF CLEAR
INTRODUCTION OCULAR MEDIA
  Physiology of tears
  Physiology of cornea
  Physiology of crystalline lens
  Physiology of aqueous humour and
maintenance of intraocular pressure
PHYSIOLOGY OF VISION
  Phototransduction
  Processing and transmission of visual impulse
  Visual perceptions
PHYSIOLOGY OF OCULAR MOTILITY AND
BINOCULAR VISION
  Ocular motility
  Binocular single vision

				
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Description: Common Eye Disease