Textbook of Visual Science and Clinical Optometry Bikas Bhattacharyya
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Section 1
1Anatomy2

Anatomy of the EyeballCHAPTER 1

 
INTRODUCTION
Eyeball is the peripheral organ of vision. Image of external world is reflected here and is transferred into visual impulses. It consists of segments of two spheres, where the smaller transparent one called cornea is placed in front of the other. Due to this anatomical shape anteroposterior diameter is more than vertical and horizontal diameter.
Eyeball consists of three coats or tunics (Fig. 1-1):
  1. External fibrous—It is formed by transparent cornea in front, opaque sclera behind and their junction called limbus.
  2. Intermediate vascular—It is formed by uveal tract consisting of iris, ciliary body and choroid.
  3. Internal neural—It is formed by retina, which along with optic nerve is considered as the anterior prolongation of the brain.
 
CONTENTS
  1. Aqueous humour
  2. Crystalline lens held by zonule of Zinn
  3. Vitreous humour.
 
DIMENSIONS (IN EMMETROPIC EYE)
Anteroposterior
—
24 mm
Horizontal
—
23.5 mm
Vertical
—
23 mm
Anteroposterior length of the eyeball at birth is 17.3 mm and it reaches adult size at 7-8 years.4
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Fig. 1-1: Horizontal section of the eyeball
 
LOCATION
It is situated in the anterior part of the orbit, closer to the roof than the floor.
 
CHAMBERS
There are three chambers inside the eyeball:
  1. Anterior chamber
  2. Posterior chamber
  3. Vitreous chamber.
    Movements of the eyeball are governed by six extrinsic muscles.
 
CORNEA
  • Cornea forms the transparent and anterior 1/6th of the external fibrous coat of the globe of the eyeball.
  • It is just like glass cover of a watch which is set on the sclera.
  • It is oval from front and circular from behind.
  • It is the main refracting medium of the eye.
    5
 
DIMENSIONS
Front/anterior
Horizontal diameter
Vertical diameter
— 12 mm
— 11 mm
Back/posterior
Horizontal diameter
Vertical diameter
— 11.5 mm
— 11.5 mm
Thickness
At the center
At the periphery
— 0.50 to 0.58 mm
— 1 mm
Radius of curvature
Anterior surface
Posterior surface
— 7.8 mm
— 6.5 to 7 mm
Refractive index
— 1.376
Dioptric strength
— +42.5 Diopter
At birth the size of the cornea is 80% of it's adult size and it reaches it's adult size at 3 years of age.
 
HISTOLOGY
Cornea consists of five layers (Fig. 1-2).
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Fig. 1-2: Layers of the cornea
From the external surface the layers are as follows.
 
Stratified Squamous Epithelium
It is composed of 5–6 layers of nonkeratinised cells (Fig. 1-3), mounted on a basement membrane and continuous with the bulbar conjunctiva at the limbus.6
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Fig. 1-3: Layers of the corneal epithelium
Life span of epithelial cells is roughly a week.
  1. Superficial cells (or Squamous cells): It is 2–3 cell layered and the cells are polyhedral. Outer surfaces of these cells have projections called microvilli and microplicae. These projections extend into the mucin layer of precorneal tearfilm and help in retaining the tearfilm.
  2. Wing cells (or Umbrella cells): It consists of also 2–3 layered polyhedral cells with their concave base fitting over the apex of the basal cells. These cells send wings or process between the basal cells.
  3. Basal cells: It is a single layer of columnar cells with flat bases and rounded apices (or head). These are germinal cells and show mitosis. New cells are gradually pushed superficially.
  4. Basement membrane: It is formed by secretions from the basal cells.
Despite lack of vascularity corneal epithelium is capable of active regeneration after injury or abrasion.
 
Bowman's Membrane
It is the modified condensed anterior layers of the corneal stroma. It is acellular and composed of randomly oriented fine collagen 7fibrils. Once damaged, the Bowma n's membrane cannot regenerate. However, it exhibits strong resistance against infection and injury. It is well-demarcated from the corneal epithelium. So it is easy to strip the epithelium from the Bowman's membrane.
 
Stroma (Substantia propria)
It is the forward continuation of sclera. It accounts for 90% of corneal thickness. It consists of approximately 200 layers of parallel collagen fibrils, parallel to the surface of the cornea, surrounded by a ground substance of mucopolysaccharide. Two types of cells are found in the stroma.
  1. Keratocytes (or Corneal Corpuscles): They secrete collagen and the ground substance and lie within the collagen lamellae.
  2. Wandering leucocytes: They are derived from the limbal vessels.
Cornea is the most sensitive structure in the body due to presence of plenty nonmedullated nerve fibres in the stroma.
 
Descemet's Membrane
It is formed by secretion of corneal endothelium. Hence, it is a modified basement membrane of the endothelium. It is well-defined from the corneal stroma. It has wart-like elevations at the periphery termed as Hassall-Henle bodies. It terminates peripherally at the Schwalbe's line. It is strong and capable of regeneration after injury.
 
Endothelium
It is the deepest layer of cornea consisting of a mosaic of single layer of hexagonal cells, bound together and continuous with the endothelium of the anterior surface of the iris. Endothelial cells are responsible for maintaining relative dehydration (deturgence) of corneal stroma and transparency. Endothelial cells of the cornea can be seen by specular reflection with the slit-lamp biomicroscope. Once damaged, the endothelial cells do not regenerate. At birth the endothelial cell count is 4500 cells/mm2. In the first year of life 25% reduction in cell count occurs. Thereafter, a progressive reduction in endothelial cell count occurs with increasing age. Average endothelial cell count in adult is 2 800 cells/mm2. Great variation 8in size of the endothelial cells is termed polymegathism. It is often observed after ocular disease, trauma and prolonged contact lens wear.
 
BLOOD SUPPLY
The cornea is avascular. However, small plexuses from the anterior ciliary vessels penetrate the periphery of the cornea for roughly 1 mm and are actually within the subconjunctival tissue which overlaps the corneal periphery.
 
NERVE SUPPLY
It is supplied by anterior and posterior ciliary nerves, branches of the ophthalmic division of the trigeminal (Vth cranial) nerve. They form a pericorneal plexus and enter the cornea via the limbus, as 60 – 80 myelinated trunks. They shed their myelin sheaths after reaching few mm inside the cornea and divide into two groups. The superficial group forms plexuses under the Bowman's membrane and the epithelium. The deeper group forms plexuses within the peripheral area of the stroma. The Descemet's membrane, endothelium and the central part of the stroma are devoid of any nerves.
 
SCLERA
  • Sclera forms the tough, white, opaque posterior 5/6th of the external fibrous coat of the eyeball.
  • Sclera is thickest posteriorly (1 mm) and thinnest just behind the insertion of the extraocular muscles (0.3 mm). The thickness around the limbus is 0.6 mm.
  • Scleral spur is a concentric band of sclera, triangular in section, lying posterior to the Schlemm's canal and trabecular meshwork (TM).
  • Sclera is pierced by 3 sets of apertures:
    1. Posteriorly around the optic nerve through which pass the long and short posterior ciliary vessels and nerves.
    2. In the middle, vortex veins (four in number) exit 4 mm behind the equator of the globe.
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    3. Anteriorly pierced by anterior ciliary vessels and perivascular lymphatics.
  • Sclera has three layers from outside inward:
    1. Episclera
    2. Sclera proper
    3. Lamina fusca (or suprachoroid).
 
FUNCTION OF THE SCLERA
It protects the eyeball by rendering mechanical strength and support.
 
BLOOD SUPPLY
It is relatively avascular. However, a rich vascular plexus is formed by episcleral and choroidal vessels, anterior to the insertion of the rectus muscles of the eye. The congestion of these vessels is the basis of the clinical sign “ciliary congestion”.
 
NERVE SUPPLY
It is richly supplied by short ciliary nerves posteriorly and long ciliary nerves anteriorly.
 
LIMBUS
  • It is the transitional area between the cornea on one side and sclera along with conjunctiva on the other.
  • Dimensions – Width
    Superiorly – 2 mm
    Inferiorly – 1.8 mm
    Nasally and Temporally – 1.4 mm
    Thickness – 0.7 mm
  • Knowledge of surgical anatomy of the limbus (Fig. 1-4) is essential due to the fact that virtually all surgery for glaucoma is performed at the limbus since it contains trabecular meshwork (TM) internally.
  • Midlimbal line is useful landmark because it overlies Schwalbe's line. It represents the junction between the bluish zone and the white sclera.
    10
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Fig. 1-4: Surgical anatomy of the limbus, dimensions and landmarks
 
ANTERIOR CHAMBER
  • It is a space filled with aqueous humour.
  • It is bounded in front by the cornea and behind by the iris and pupil, i.e. part of the anterior surface of the lens. It is bounded laterally by the angle of the anterior chamber which is bounded by cornea and sclera anteriorly and root of the iris, the anterior part of the ciliary body and the scleral spur posteriorly (Fig. 1-5).
  • Volume of anterior chamber — 0.25 cc
  • Depth of the anterior chamber — 2.5 mm
The filtration structures present at the angle of the anterior chamber consists of (Fig. 1-5) from inside outwards the following:
 
TRABECULAR MESHWORK (TM)
It is triangular in shape with the apex arising from the termination of the Descemet's membrane (Schwalbe's line) and adjacent corneal stromal fibres. The base of the trabecular meshwork (TM) merges into the ciliary body and root of the iris. Trabecular meshwork is composed of circumferentially disposed flattened bands. Each of the bands have multiple perforations. The openings become progressively smaller as Schlemm's canal is approached.11
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Fig. 1-5: Structures of the angle of the anterior chamber and the anterior chamber
So, through these openings tortuous canals (spaces of Fontana) exist between the anterior chamber and the Schlemm's canal.
 
JUXTACANALICULAR TISSUE
It lies between the deeper part of the trabecular meshwork and the Schlemm's canal.
 
SCHLEMM'S CANAL
It is a circular venous sinus and plays significant role in the drainage of the aqueous humour. It is lined by a continuous layer of endothelial cells joined by junctions which are not truly tight. Only 1% of aqueous drains through these tight junctions. Villi from the cytoplasm of the endothelial cells projects into both juxtacanalicular tissue and the Schlemm's canal. Macropinocytic vesicles and micropinocytic vesicles presents in the cytoplasm act as the major outflow pathway.12
 
COLLECTOR CHANNELS
Twenty-five to thirty-five collector channels drain aqueous from the outer wall of the Schlemm's canal to the anterior ciliary veins via the intrascleral and episcleral plexuses. Aqueous vein also drains aqueous directly into the anterior ciliary veins.
 
POSTERIOR CHAMBER
  • It is also a space filled with aqueous humour
  • Aqueous humour is secreted here by the ciliary processes
  • Volume of posterior chamber — 0.06 cc.
  • It is bounded in front by the posterior surface of the iris and anterior surface of the lens and zonules of Zinn from behind. It is bounded laterally by the ciliary processes of the ciliary body.
 
UVEAL TRACT
This is the intermediate vascular coat of the eyeball consisting of the three following parts; Iris, Ciliary Body and Choroid.
 
IRIS
It is the most anterior part lying in front of the crystalline lens and behind the cornea. It is circular in shape with a central opening called pupil (like a diaphragm of a camera). It is peripherally attached to the middle the anterior surface of the ciliary body. Anterior surface of the iris is divided by a ridge called collarette (thickest part) into smaller pupillary zone and larger ciliary zone (Fig. 1-6). The collarette is formed by roughly circular series of ridges and minor arterial circle of iris. Major arterial circle which supplies blood to the iris is located in the ciliary body adjacent to the root of the iris (Fig. 1-7). The peculiarity of the iris vessels is that, they usually do not bleed when the iris is cut. This is due to the fact that they are enclosed by thick collagen bundles.
 
Histology
It consist of 4 layers:
  1. Anterior endothelium —It is continuous with the corneal endothelium. Iris crypts of Fuchs are pit-like depressions (Fig. 1-6) caused by the focal absence of anterior endothelium.13
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    Fig. 1-6: Anterior surface of the iris
    zoom view
    Fig. 1-7: Location of arterial circles and unstriated muscles of the iris
    These crypts are present near collarette and the root of the iris. Iris crypts located peripherally are difficult to visualise without gonioscopy.
  2. Stroma —The sphincter pupillae muscle, vessels and nerves of the iris and pigment cells are embedded in the connective tissue of the stroma. Sphincter pupillae muscle is, unstriated and 14involuntary, 1 mm wide, present near the posterior surface in a circular fashion (Figs 1-5 and 1-7).
  3. Posterior membrane —It consists of a thin layer of unstriated and involuntary muscle fibres, called dilator pupillae. Dilator pupillae is arranged radially near the root of the iris (Fig. 1-7). Dilator pupillae fibres originate from the ciliary body and fuse with the sphincter pupillae muscle (Fig. 1-5). Contraction of the dilator pupillae draws the pupillary margin towards the ciliary body to cause dilation of the pupil.
  4. Posterior epithelium —It consists of two layers of pigmented epithelium, both of which have originated developmentally from the retina. They are continuous with each other at the pupillary margin. Anterior layer of epithelium contains flat spindle cells whereas, the posterior layer contains large cubical cells.
 
Function of the Iris
The central opening of the iris, i.e. pupil regulates the entry of light into the eye through the actions of the dilator pupillae and the sphincter pupillae muscles.
 
Blood Supply
It is from the minor and major arterial circle of the iris.
 
Nerves Supply
  1. Sphincter pupillae—Oculomotor (IIIrd cranial) nerve
  2. Dilator pupillae—Nerves from the cervical sympathetic
  3. Sensory—Nasociliary nerve [a branch of 1st division of the trigeminal (Vth cranial) nerve].
 
CILIARY BODY
It is the intermediate part of the uveal tract. It extends from ora serrata to the root of the iris, where it is attached to the scleral spur. It is a circular band width of which is 5.9 mm nasally and 6.7 mm temporarily. It is divided into two anatomical parts:
  1. Pars plicata: Anteriorly about 70 ridges (ciliary processes) are arranged in a radiating manner. The region of the ciliary 15processes is th e most vascular area of the eye. The ciliary processes are actively involved in the secretion of the aqueous humour. Width of the pars plicata is 2 mm.
  2. Pars plana: This part is smooth and extends upto ora serrata. During the operations of vitrectomy and lensectomy, ports are made in this area due to relative avascularity and it's location away from the crystalline lens.
It is roughly triangular in sagittal section with the base facing anteriorly. The external side of the triangle is against the sclera and here lies ciliary muscle which is unstriated and involuntary. Ciliary muscles form the chief mass of the ciliary body. It has three parts and a common ring shaped origin mostly from the scleral spur and partly from the trabecular meshwork (TM). The parts of the ciliary muscle are as follows:
  1. Meridional (or Brucke's muscle): It runs anteroposteriorly to be inserted into the suprachoroid and forms the main mass of the ciliary body (Fig. 1-5).
  2. Radial: They are embedded in the meridional fibres and inserted into the root of the iris close to the dilator pupillae muscle.
  3. Circular (or Müller's muscle): They remain in the anterior and inner portion of the ciliary body and run parallel to the limbus to form a concentric ring (Fig. 1-5).
Ciliary muscle is innervated by the parasympathetic fibres derived from the oculomotor (IIIrd cranial) nerve and sympathetic fibres through the short ciliary nerves. Stimulation of the parasympathetic nerve causes contraction of the ciliary muscle resulting in shortening of it's length. Thus, the whole muscle moves forward and inward. Consequently the zonule of Zinn (or suspensory ligament of lens), which suspends the lens, relaxes. This results in release of tension on the capsule of the lens, allowing it to become more convex. This is the basis of accommodation. Contraction of this muscle also opens up trabecular meshwork openings and facilitate aqueous humour outflow.
Inner surface of the ciliary body is covered by layers of epithelium, and are continuous with the similar layers of the iris. The outer layer consisting of flattened cells is continuous with the anterior pigment epithelium of iris and is also pigmented. The inner layer which consists of cubical cells, is continuous with the 16posterior pigment epithelium of iris. However, this layer is non-pigmented.
 
Functions of the Ciliary Body
  1. Production of aqueous humour
  2. It is involved in accommodation:
    zoom view
  3. Increase outflow of aqueous through the trabecular meshwork:
    zoom view
  4. Inner nonpigmented layer of the epithelium secretes hyaluronic acid, the essential component of the vitreous humour.
 
Blood Supply
It is supplied by two long posterior ciliary arteries and seven anterior ciliary arteries via major arterial circle of iris.
 
Nerve Supply
  1. Ciliary muscle— It is supplied by the oculomotor (IIIrd cranial) nerve and the sympathetic nerve.
  2. Sensory— Nasociliary branch of the trigeminal (Vth cranial) nerve.
    17
 
CHOROID
It is a highly vascular thin tunic located between the sclera and the retina and extends from ora serrata to optic nerve. It consists of four layers from outside inwards (Fig. 1-8):
  1. Suprachoroid (Lamina fusca)
  2. Layer of blood vessels
    1. Outer larger vessel layer (Haller's layer)
    2. Inner smaller vessel layer (Sattler's layer).
  3. Choriocapillaries— It is a layer of capillary plexus of fenestrated vessels and it nourishes outer half of the retina.
  4. Membrane of Bruch—It is avascular, separating choriocapillaries from the pigment epithelium of the retina. It consists of outer lamina elastica and inner lamina vitrea, i.e. basement membrane of the pigment epithelium of the retina. It is an important constituent of the blood retinal barrier.
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Fig. 1-8: Layers of choroid
 
Functions of the Choroid
  1. It provides blood supply and nutrition to the retinal pigment epithelium (RPE) and outer half of the sensory retina.
  2. It regulates ocular temperature.
    18
 
Arterial Supply
it is supplied by the following group of arteries:
  1. Short posterior ciliary arteries (20 in number)
  2. Long posterior ciliary arteries (2 in number)
  3. Anterior ciliary arteries (7 in number).
 
Venous Drainage
Venous blood from the iris, ciliary body and choroids is collected by a series of intermediate small veins, which drain into vortex veins (usually 4 in number). The vortex veins are located behind the equator of the eyeball. The vortex veins drain into cavernous sinus through superior and inferior ophthalmic veins.
The walls of the choriocapillaries are fenestrated which allow relatively free movement of fluids and solids between the choroids and the adjacent retinal pigment epithelium (RPE) via the Bruch's membrane. The Bruch's membrane offers no resistance to the fluid traffic.
 
RETINA
  • It is the light receptive inner neural coat of the eyeball consisting of outer retinal pigment epithelium (RPE) and inner sensory retina with a potential space called subretinal space between them.
  • It lies between the choroid and the vitreous.
  • It extends from the optic disc to the ora serrata.
  • Point of importance must be noted that the two pigment epithelium layers of the iris may be traced back upto retina. The anterior pigment epithelium of the iris continues as the outer pigment epithelium of the ciliary body and later forms the retinal pigment epithelium (RPE). The posterior pigment epithelium layer of the iris similarly continues to become the inner nonpigmented epithelium of the ciliary body. This again continues to form the inner nonpigmented sensory layer of the retina (Fig. 1-9).
  • Surface area of the retina— 266 mm2
    19
  • Thickness at the ora serrata – 125 µ
    Thickest at the macula – 350 µ
    Thinnest at the centre of the fovea – 90 µ
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Fig. 1-9: Continuation of the retina to epithelial layers of the ciliary body and iris
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Fig. 1-10: Layers of the retina
20
 
LAYERS OF RETINA
Retina consists of 10 distinct layers (Fig. 1-10) from outside inwards of the following layers:
  1. Retinal pigment epithelium (RPE): It is a single layer of flattened, mostly hexagonal cells which is firmly adherent to lamina vitrea of the choroid. On direct ophthalmoscopy the retina exhibits fine mottled appearance due to the following facts:
    1. The RPE cells are not equally pigmented.
    2. Pigments in each RPE cells are distributed at the periphery of the cells and the central nuclear area remains relatively pigment free.
    The taller and narrower pigment cells at the macula confer darker colour to this region. RPE cells transport substances to the photoreceptor cells which are needed for metabolism. Free exchange of products of metabolism occur between the RPE cells and the photoreceptor cells.
  2. Layer of the rods and cones: Rods and cones with their nucleus and processes form the sensory receptor. They are arranged on the external limiting membrane like a palisade. The rods contain visual purple called rhodopsin which combines vitamin A with protein. Rhodopsin is responsible for vision in dim light (scotopic vision) and peripheral vision. The cones are responsible for acuity of vision, vision in strong light (photopic vision) and colour vision.
    No. of rods— 125 million No. of cones— 7 million
    Each photoreceptor, i.e. rod and cone consist of 3 parts;
    1. Outer and inner segment connected by a tubular connection called cilium in the layer of the rods and cones.
    2. Cell body and nucleus in the outer nuclear layer.
    3. Cell processes, i.e. axons that extend into the outer plexiform layer.
    Rod outer segment, cylindrical in shape, contains dense vertical stack of numerous lamellar discs. The inner segment of the photoreceptors consist of outer ellipsoid (containing large number of mitochondrias) and an inner myoid portion (containing endoplasmic reticulum). The cone outer segment is conical in shape. New rod discs are produced in the inner 21segments and are progressively displaced towards the pigment epithelium via the outer segment. Discarded rod discs are phagocytosed by the RPE cells.
  3. Outer limiting membrane: It is a fenestrated membrane formed by fibres of Müller and pierced by the processes of the rods and cones.
  4. Outer nuclear layer: This layer contains nuclei of the rods and cones.
  5. Outer plexiform/molecular layer: It consists of;
    1. Arborisation of the axons of the rod and cone nuclei with the dendrites of the bipolar cells
    2. Processes of the horizontal cells
    3. The fibres of Müller.
  6. Inner nuclear layer: This layer consists of;
    1. Bipolar cells
    2. Horizontal cells
    3. Amacrine cells
    4. Nuclei of the fibres of Müller
    5. Capillaries of the central retinal artery and vein.
    Bipolar cells form the first order of neurons. Their nuclei are located in the inner nuclear layer and the dendrites arborise with the axons of the rods and cones in the outer plexiform layer. The axons of the bipolar cells synapses with the dendrites of the ganglion cells in the inner plexiform layer. The “midget” bipolar cells synapse with the individual cone feet plates while the rest of the bipolar cells synapse with both the rod spherules and cone feet.
    Horizontal cells are flat cells situated near the outer plexiform layer and send processes horizontally.
    Amacrine cells are pear shaped and send single process inwards to terminate in the inner plexiform layer. They are located near the inner plexiform layer. Both horizontal cells and amacrine cells form horizontal connections between adjacent rods and cones.
  7. Inner plexiform/molecular layer: This layer contains;
    1. Mainly arborisation of the axons of the bipolar cells with the dendrites of the ganglion cells.
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    2. Processes of the amacrine cells
    3. Fibres of Müller
    4. Branches of the retinal arteries and veins.
  8. Ganglion cell layer: Ganglion cells are multipolar nerve cells with clear oval nucleus and well-developed Nissl granules. Ganglion cells form the second order of neurons. Their axons pass into the nerve fibre layer and arborize on cells in the lateral geniculate body and superior colliculus (central nervous system).
  9. Nerve fibre layer: It consists of;
    1. Axons of the ganglion cells which pass through lamina cribrosa and form the optic nerve.
    2. Fibres of Müller
    3. Retinal vessels.
    The bundles of axons of ganglion cells run parallel to the surface of the retina and converge towards the optic disc. They are nonmyelinated. Nasal fibres reach optic disc directly. The temporal fibres pass above and below the macula to reach the optic disc (Fig. 1-11). The macular fibres directly reach the temporal side of the optic disc and form the papillomacular bundle (Fig. 1-11).
  10. Inner limiting membrane: It is a thin hyaloid membrane separating the retina from the vitreous. The feet of the fibres of Müller are attached to it.
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Fig. 1-11: Direction of nerve fibres reaching optic disc
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LANDMARKS ON THE RETINA
  1. Ora serrata: It is the extreme anterior end of the retina. It is located 8 mm from the limbus and 6 mm from the equator of the eyeball. Here the retina is firmly attached to the choroid. Also the vitreous is firmly attached to ora serrata. All the retinal elements tend to cease just posterior to ora serrata.
  2. Optic disc: It is a vertically oval structure and it is called blind spot due to it's failure to evoke visual sensation. Only the nerve fibre layer is present here. It is pink coloured due to the presence of capillaries. There is a funnel shaped depression/excavation at the center and slightly to the temporal side. It is called physiological cup and is lighter in colour.
  3. Macula lutea: It is an oval area situated temporal to the optic disc. It's centre lies 3.5 mm lateral to the optic disc. It is of the same size as the optic disc. At the macula, the ganglion cells are arranged in several layers and the outer plexiform layer is thickest. Rods progressively decrease in number towards the centre. The cones progressively increase towards the centre. It is nourished by the choriocapillaries.
  4. Fovea centralis: It is the central depression of the macula lutea. Only cones are present in the fovea. It is the thinnest area of the retina and characterised by:
    1. Absence of rods, fibres of Müller, ganglion cells and nerve fibre layer.
    2. Extreme thinness of inner plexiform layer, outer plexiform layer and inner nuclear layer.
    3. Tall and slender cones.
Here the layers of the retina are spread aside to allow the light to fall straight on the cone to evoke maximum visual acuity. It is located 2 disc diameter, i.e. 3 mm temporal to the optic disc and just below the horizontal meridian. Here each cone is connected to only one ganglion cell through “midget” bipolar cells and synaptic connections. The above fact also makes fovea centralis responsible for maximum visual acuity. The centre of the fovea is called foveola (500 µm). The foveola doesn't contain any blood vessels and is called foveolar avascular zone (FAZ).24
Retina is topographically divided into central 4.5 mm diameter zone (fovea centralis being the centre) called posterior pole, peripheral retina and intermediate equatorial retina.
 
BLOOD SUPPLY OF RETINA
  1. Retinal pigment epithelium, layers of the rods and cones, outer limiting membrane and outer nuclear layer are avascular layer and are nourished by the choriocapillaries.
  2. Outer plexiform layer is supplied by both the choriocapillaries and the retinal vessels.
  3. Rest of the retinal layers are supplied by the retinal vessels.
  4. Macula lutea and fovea is exclusively nourished by the choriocapillaries. Macula is occasionally supplied by a cilioretinal artery which emerges from the temporal margin of the optic disc. Cilioretinal vein is extremely rare.
The endothelial cells present in the capillaries of the retina are closely adherent to each other by tight junctions and prevent free flow of fluids and other solutes between the capillaries and the retinal tissue and form the blood-retinal barrier.
 
OPTIC NERVE
  • Optic nerve consists of approximately 1 million axons of the ganglion cells of retina, glial cells and the meningeal sheaths.
  • It extends from the optic disc to the optic chiasma.
  • It can be divided into 4 portions depending on it's location;
    1. Intraocular— It extends from the optic disc to just posterior to the lamina cribrosa
    2. Orbital
    3. Intracanalicular— The portion within the optic foramen
    4. Intracranial.
The optic disc is vertically oval, 1.5 mm in diameter. It is strikingly paler in colour than the surrounding retina. It is located 3 mm nasally and slightly at a upper level than the fovea. Physiological cup, the funnel shaped depression within the optic disc is lighter in colour than the peripheral neuroretinal rim. The nerve fibres are transparent within the retina and at the optic disc since, they are nonmyelinated. Just behind the lamina cribrosa they 25become opaque due to addition of myelination. The central retinal artery lies nasal to the central retinal vein at the optic disc.
The nerve fibres, i.e. axons of the ganglion cells pass through a sieve like structure in the sclera called lamina cribrosa and receive myelination resulting in increased diameter of 3.6 mm of the optic nerve.
 
BLOOD SUPPLY
The optic nerve is supplied from following sources;
  • Arterial circle of Zinn
  • Choroidal vessels
  • Branches of retinal arterioles
  • Intraneural branches of central retinal artery
  • Pial branches of choroidal arteries
  • Ophthalmic artery.
 
LENS
  • It is a transparent biconvex crystalline structure situated between the iris and the vitreous humour.
  • The older cells are concentrated towards the centre, whereas the younger cells remain at the periphery of the lens.
  • It is attached to the ciliary body by the zonules of Zinn (or suspensory ligament). Zonuler fibres form 3 groups;
    1. Arising from the pars plana and insert into the lens capsule anterior to the equator.
    2. Arising from the summits and valleys of the ciliary processes and pass backward to be inserted into the lens capsule posterior to the equator.
    3. Arising from the summits of the ciliary processes and insert directly at the equator.
  • It is devoid of any nerve, vessel and connective tissue.
  • Dimensions
    Diameter – 9 mm
    Thickness – 4 mm
    Radius of curvature of anterior surface – 10 mm
    Radius of curvature of posterior surface – 6 mm
    26
 
STRUCTURE
Lens consist of (Fig. 1-12) the following:
 
Lens Capsule
It is acellular and envelops the lens completely. It is a basement membrane of the lens epithelium. It is thinnest at the posterior pole. Only the anterior capsule is lined by the single layer of epithelium.
 
Lens Epithelium
It consists of a single layer of cuboidal cells present in the anterior lens capsule (A – cells) and in the equatorial bow region (E – cells). The A – cells (LEC) present in the anterior capsule are not directly involved in the formation of new lens fibres. The equatorial bow/E – cells show mitotic activity to form new lens fibres. As new cells are formed, these lens fibres elongate and lose their nuclei. Older fibres are continuously pushed centrally.
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Fig. 1-12: Structure of the crystaline lens
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Lens Substance/Material
It consists of the cortex, epinucleus, nucleus and sutures. The most externally located lens fibres which lie beneath the lens capsule form the cortex.
The fibres of central embryonic nucleus meet at the sutures shaped Y. The anterior Y suture is erect, whereas the posterior suture is inverted Y. As development proceeds successive layers of nucleus are formed externally and added to the central embryonic nucleus, viz. foetal, infantile and adult nucleus.
 
VITREOUS HUMOUR
It is a clear, transparent, colourless jelly that fills the posterior 4/5th of the eye, i.e. the space behind the lens and the zonule of Zinn. Volume of vitreous is 4.5 ml (approx.). Anteriorly vitreous has a saucer like depression called fossa patellaris to lodge the lens. Through it's centre runs hyaloid canal, which is remnant of the hyaloid artery. Vitreous humour is loosely adherent to the retina. However, it is firmly attached to the;
  1. Margin of the optic disc (Area Martegiani)
  2. Pars plana of the ciliary body near ora serrata. 1.5 mm broad zone of the ciliary epithelium next to ora serrata (termed vitreous base).
  3. Macula
  4. Central 9 mm diameter zone of the posterior capsule of the lens (ligamentum hyaloideo-capsulare of Weiger,(Fig. 1-13)).
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    Fig. 1-13: Anatomical areas of the vitreous chamber
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From developmental point, vitreous is divided into 3 parts:
  1. Primary vitreous— From the posterior pole of the lens to the optic disc is a space called Cloquet's/Stilling's/hyaloid canal (Fig. 1-13) containing remnants of the primary vitreous.
  2. Secondary vitreous— It fills the vitreous chamber.
  3. Tertiary vitreous— The zonules of Zinn represent tertiary vitreous.
 
BLOOD SUPPLY OF EYEBALL
 
ARTERIAL SUPPLY
The eyeball is supplied by the branches of the ophthalmic artery, which is a branch of the internal carotid artery. Ophthalmic artery enters orbit through the optic foramen and divides into two sets of vascular system. The retinal vasculature, i.e. central retinal artery supplies inner half of the retina, whereas the ciliary system formed by other branches supply the uveal tract, the outer half of the retina and the optic nerve. The branches of ophthalmic artery are:
  1. Central retinal artery: It enters the optic nerve 15–20 mm behind the eyeball. It divides at or just posterior to the surface of the optic disc into two main trunks superior and inferior. They again subdivide into superior and inferior temporal and nasal arterioles which are located within the nerve fibre layer of the retina. These retinal arterioles are end arteries and each supply a quadrant of the retina. The capillaries on the surface of the optic disc are derived from these retinal arterioles.
  2. Short posterior ciliary arteries (20 in number)— They pierce the sclera in the ring around the optic nerve and form the circle of Zinn. They enter the uveal tract and divide into smaller vessels to form the vessels of the choroid and choriocapillaries.
  3. Long posterior ciliary arteries (2 in number)— They pierce the sclera on either side of the optic nerve and pass through the suprachoroidal space in horizontal location to reach ciliary body. Now, each artery divides into two branches and the two branches again anastomose with each other and anterior ciliary arteries, at the base of the iris to form “major arterial circle of iris” (Fig. 1-14).
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  4. Anterior ciliary arteries (7 in number)— They are the terminal branches of muscular branches of the ophthalmic arteries. They enter the eyeball through rectus muscles (two for each recti, except lateral rectus which has only one). They pierce the sclera 5 to 6.5 mm from the limbus and contribute to the formation of “major arterial circle of iris”.
  5. Recurrent meningeal artery
  6. Lacrimal artery
  7. Numerous recurrent arteries.
Twigs from the major arterial circle run radially into the iris and form a circular plexus of anastomosis at the collarette near the pupillary margin, called “minor arterial circle of iris” (Fig. 1-14).
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Fig. 1-14: Vascular supply of the eyeball
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VENOUS DRAINAGE
It occurs through:
  1. Vortex veins— Majority of venous drainage occurs through the tributaries of 4 vortex veins, which enter the sclera just behind the equator and pass obliquely through it. They drain into ophthalmic veins.
  2. Anterior ciliary veins – They drain blood from the outer part of the ciliary muscle and the ciliary body via ciliary venous plexus.
  3. Short posterior ciliary veins – They drain blood away from the sclera.
  4. The retinal veins – The 4 tributaries that correspond roughly to the branches of the central retinal artery unite at or just behind the optic disc to form the central retinal vein. The central retinal vein usually drains into cavernous sinus, after giving a branch to the superior ophthalmic vein. It may drain into the superior ophthalmic vein occasionally.
The major branches of the central retinal artery and tributaries of central retinal vein are located within the nerve fibre layer of the retina. In most of the retinal area two groups of capillary network exist. The superficial one is located within the nerve fibre layer, whereas the deep one is located between the outer plexiform layer and inner nuclear layer.
It is easy to distinguish the retinal arteries from the veins. The arteries are narrower (3/5th of the veins), bright red in colour with a well-defined light streak along their lumen. The veins are wider and less bright in colour. Retinal venous pulsation is physiological and is seen in significant number of people. Retinal arterial pulsation is pathological and is seen in glaucoma, aortic incompetence, etc.