EMBRYOLOGY OF THE EYEBALL
The human eye originates from neuroectoderm, surface ectoderm and the extracellular mesenchyme which consists of both neural crest and mesoderm. Ocular development in the human embryo begins around 3rd week into embryonic life and continues through the 20th week of life (Table 1.1).
- Both the sensory and pigmentary layers of retina are developed from the neuroectoderm. These layers continue anteriorly to give rise to ciliary epithelium and the pigmented layer of the iris. The neuroglial and neural portions of the optic nerve are also originated from neuroectoderm.
- The surface ectoderm is the primordial of the crystalline lens (Table 1.1), the conjunctiva, corneal epithelium and the eyelids with the epithelium of their glands.
- Neural crest cells are themselves derived from the ectoderm and lie close to the neural tube. They are responsible for development of sclera, cornea—Descemet's membrane and endothelium and connective tissue, and bony structure of the orbit.
- The mesoderm is the primordial of the extraocular muscles, endothelial lining of blood vessels of the eye, choroidal blood vessels, sclera and choroid, vitreous, zonular fibers, iris sphincter/dilated muscles, and iris stroma.
- Eye development is initiated by the master control gene Pax6. The Pax6 gene locus is a transcription factor for the various genes and growth factors involved in eye formation.
THE EYE AT BIRTH
- Orbit is more divergent (50°) as compared to adult orbit.
- Eyeball is 70% of adult length, being almost fully developed at the age of 8 years.
- Cornea is 80% of its adult size, being fully developed at the age of 3 years.
- The newborn is hypermetropic by +2.5D.
- Pupil is small and does not dilate fully.
- Anterior chamber is shallow and the angle is narrow.
DEVELOPMENT (FIGS 1.1A TO E)
On either side of the cephalic end of forebrain, a lateral depression appears, known as optic pit (3rd week).
Figs 1.1A to E: The development of the eye
Solid red—neural ectoderm; dotted area—mesoderm; hatched area—surface ectoderm
GROSS ANATOMY OF THE EYEBALL (FIG. 1.2)
- It is not a true sphere, but consists of the segments of two modified spheres, one in front of the other.
- The cornea is more convex than the rest of the globe (7.8 mm radius as opposed to 12 mm).
- The anteroposterior diameter (axial length) is about 24 mm, while the vertical is 23 mm, and the horizontal is 23.5 mm.
- The eyeball is shorter in hypermetropes but longer in myopes.
THE GLOBE
- Three concentric layers or tunics:
- Outer supporting layer: It consists of transparent cornea, opaque sclera and their junction, the limbus.
- Middle vascular layer: It is called uvea, and consists of choroid, ciliary body and the iris.
- Inner neural layer: It is called retina, composed of two parts (1) a sensory portion and (2) a layer of pigment epithelium.
- A crystalline lens: The transparent structure is located immediately behind the iris, and is supported by fine fibers, called zonules.
- Three chambers:
- Anterior chamber: It is located between the iris and the posterior surface of the cornea. It communicates with the posterior chamber through the pupil.
- Posterior chamber: It is minute in size, bounded by the lens and zonules behind, and there is in front.
- Vitreous cavity: It is the largest and located behind the lens and zonules, and adjacent to the retina throughout.
THE CORNEA
Cornea (Fig. 1.3) is elliptical from front, being 12 mm horizontally, and 11 mm vertically. Posteriorly, it is circular with a diameter of 11.5 mm.
- Thickness: 500 micron at the center and 800–1000 micron at the periphery.
- Radius of curvature: Anterior surface 7.8 mm and of posterior surface 6.5 mm.
- Refractive index: 1.37.
- It is the main refracting surface of the eye.
- Dioptric power is about +43.0 to +45.0D.
Structures
It consists of five layers:
- Stratified squamous epithelium
- 5–6 cells deep, continuous with the conjunctival epithelium.
- The basal cells are tall with oval nuclei.
- The intermediate layers (2–3 cells deep) are polyhedral wing cells.
- The surface layers (about 2) are very flat, but not keratinized.
- The corneal epithelium rests on a basement membrane, secreted by the basal cells.
- Bowman's layer (anterior elastic lamina)
- Anterior condensation of substantia propria.
- Once eroded, it does not regenerate and leaves behind a superficial corneal scar.
Fig. 1.3: Transverse section of the cornea
1—Epithelium; 2—Bowman's membrane; 3—Stroma; 4—Descemet's membrane; 5—Endothelium
- Substantia propria (stroma)
- Comprising 90% of corneal thickness.
- Composed of collagen fibrils arranged in sets of lamellae, lying parallel to the surface.
- They form a crystalline lattice, in a ground substance of glycoprotein and mucopolysaccharides.
- Scattered between the lamellae are the keratocytes, and a few wandering leukocytes and macrophages.
- Descemet's membrane (posterior elastic lamina)
- Strong, homogenous and very resistant membrane.
- It readily regenerates after an injury.
- It is secreted by endothelial cells and essentially their basement membrane.
- In old age, it may bear some warty elevations called Hassall-Henle's bodies at its periphery.
- Endothelium
- A single layer of flattened polygonal endothelial cells.
- Continuous with the endothelium over the anterior surface of the iris.
- It does not regenerate in human being.
Nerve Supply
The corneal nerves are derived from the long and short ciliary nerves, branches of the ophthalmic division of trigeminal nerve.
They form the pericorneal plexus just outside the limbus, and then pass onto the cornea as 60–70 trunks. They loose their myelin sheaths after a millimeter or two, and reach the cornea. Cornea does not have proprioceptive sensation.
Blood Supply
Cornea is a vascular, but the corneoscleral limbus is supplied by the anterior conjunctival branches of anterior ciliary arteries and forms a perilimbal plexus of blood vessels.
THE SCLERA
It is a dense tough fibrous envelope that covers posterior five-sixths of the eye.
- Sclera has two large openings, the anterior (the corneal window) and the posterior (for optic nerve).
- Structures piercing the sclera are:
- Four vortex veins (4 mm behind the equator).
- Anterior ciliary nerves and vessels.
- Sclera is thickest posteriorly surrounding the optic nerve (1 mm), and is thinnest just posterior to the insertion of recti muscles (0.3 mm).
Structures
Sclera has three parts:
- Episclera: It is the loose fibrous tissue, containing numerous fine capillaries.
- Sclera proper: It is a dense network of collagen fibers. The sclera is white, because of variable diameter and irregular arrangement of collagen fibers of the stroma.
- Lamina fusca: It is the inner layer, located adjacent to the choroid.
Blood Supply
It is from the episcleral and choroidal vessels. Anterior to the insertion of recti muscles, the anterior ciliary arteries form a dense episcleral plexus. These vessels become congested in ciliary congestion.
Nerve Supply
Short ciliary nerves posteriorly, and long ciliary nerves anteriorly, provide sensory innervation. Because of generous innervation, inflammation of the sclera is extremely painful.
THE LIMBUS
Transitional zone between the cornea and the sclera. It is 1–2 mm wide.
- Its internal boundaries are scleral spur posteriorly, and the Schwalbe's line.
- Its external boundaries are by sclerolimbal junction posteriorly, and corneolimbal junction anteriorly.
- Sclerolimbal junction (Fig. 1.4A) is the only consistent landmark of the limbus, used during cataract and glaucoma surgery.
- Limbus contains trabecular meshwork internally, through which the aqueous humor leaves the anterior chamber.
Fig. 1.4A: Surgical anatomy of the limbus
C+T—Conjunctiva and Tenon's capsule; SL—Schwalbe's line; SS—Scleral spur; CLJ—Corneolimbal junction (at the termination of the Bowman's membrane); SLJ—Sclerolimbal junction (junction of the white sclera and the translucent bluish limbus)
Angle of the Anterior Chamber
- Anterior chamber is bounded in front by the cornea, behind by the anterior surface of iris and part of the anterior surface of the lens which is exposed at the pupil (Figs 1.5A and B).
- The peripheral recess of the anterior chamber is known as angle of the anterior chamber, which is also known as the cockpit of glaucoma.
- It is bounded anteriorly by the corneosclera, and posteriorly by the root of the iris and the ciliary body.
- At this part in the inner layers of sclera, there is a circular venous sinus (sometimes broken up into more than one sinus) called canal of Schlemm. It is of great importance in the drainage of aqueous. The Schlemm's canal is lined by endothelial cells.78Figs 1.5A and B: A. The angle of the anterior chamber; B. The anterior segment of the eyeE—Corneal and conjunctival epithelium; D+E—Descemet's membrane + Endothelium; TM—Trabecular meshwork; SC—Schlemm's canal; SVP—Scleral venous plexus; CM—Ciliary muscle; CP—Ciliary processes; L—Lens; I—iris; S—Sclera; CS—Corneal stroma; AC—Anterior chamber; PC—Posterior chamber; Z—Zonules
- At the periphery of the angle, between Schlemm's canal and the recess of anterior chamber, there lies a loose meshwork of tissues, known as trabecular meshwork.
- Trabecular meshwork is almost triangular in shape. Its apex arises from the termination of Descemet's membrane and the adjacent part of corneal stroma, and its base merges into the tissues of the ciliary body and the root of the iris.
- The trabecular meshwork is made up of circumferentially disposed flattened bands and each is perforated by numerous oval openings. Through these tortuous pas sages, communication exists between the Schlemm's canal and anterior chamber.
- The junctions between these cells are not ‘tight’, and the cells themselves have pores for aqueous drainage.
- Angle of the anterior chamber is best visualized by a gonioscope. Normally, the angle is wide open, and is about 20–45°. If the angle is less than 10°, there will be every chance of developing angle-closure glaucoma.
THE UVEA
The middle coat, or uvea means grape consists of three parts:
- Anterior: Iris, a free circular diaphragm with a central opening called pupil.
- Intermediate: Ciliary body
- Posterior: Choroid.
Functions
- The iris with its central opening, pupil controls the amount of light entering the eye.
- The ciliary body secretes aqueous humor and contains smooth muscles responsible for changing the shape of the lens during accommodation.
- The choroid, a vascular layer, provides the blood supply to the retinal pigment epithelium (RPE) and the outer half of the sensory retina.
THE IRIS AND THE PUPIL
The iris lies in front of the lens and the ciliary body. It separates the anterior chamber from the posterior chamber. Pupil is situated just inferior and slightly nasal side of the center.
- Its periphery or ‘root’ is attached to the anterior end of the ciliary body.
- Anterior surface of the iris is divided into two zones (Fig. 1.6):
- Central papillary zone
- Peripheral ciliary zone.
- Their junction is a circular ridge called collarette (Fig. 1.7) which marks the embryonic site of the minor vascular circle of iris, from which the embryonic pupillary membrane originated.
- The ciliary zone is marked by many ridges and crypts, but the pupillary zone is relatively flat.
Structure
- Anterior endothelium: It is continuous with the corneal endothelium.
- Stroma: It consists of spongy connective tissue with nerves, smooth muscles, and radial blood vessels forming the minor circle of iris.
- Smooth muscles: They are two in number:
- Sphincter pupillae: A circular bundle of smooth muscles running around the pupillary margin causes constriction of the pupil.
- Dilator pupillae: Arranged radially near the root of the iris, causes dilatation of the pupil.
- Posterior two-layered epithelium: Both layers are pigmented and developmentally derived from the retina. The anterior layer consists of flattened cells and the posterior layer consists of cuboidal cells.
Nerve Supply
- Sensory: Nasociliary nerve, branch of 1st division of 5th cranial nerve.
- Sphincter pupillae: Oculomotor nerve (3rd cranial).
- Dilator pupillae: Nerves derived from cervical sympathetic chain.
THE CILIARY BODY
The ciliary body is a ring of tissue about 6 mm wide that extends from the scleral spur to the ora serrata of the retina.
Structure
In anteroposterior section it is roughly an isosceles triangle, with base forwards. Iris is attached at the middle of its base.
- The chief mass of the ciliary body is composed of unstriped muscle fibers, called ciliary muscle. It has three parts, with a common origin circumferentially at the scleral spur:
- Longitudinal: The greater part, running antero-posteriorly.
- Circular: Concentrically with the root of the iris.
- Radial.
- The inner surface of ciliary body is divided into two regions:
- Pars plicata: Anterior part; about 70 plications are visible. Microscopically, they have ciliary processes responsible for the production of aqueous.
- Pars plana: Posterior smooth part; a relatively safe and a vascular zone for pars plana lensectomy and/or vitrectomy operation.
- They are covered by two layers of epithelium, continuous with the iris anteriorly, and retina posteriorly.
Ciliary body extends backward as far as the ora serrata. At this point the retina proper begins abruptly.
Nerve Supply
- Sensory: Via nasociliary branch of 5th cranial nerve.
- Ciliary muscles: Oculomotor (3rd cranial) and sympathetic nerves.
Blood Supply
By branches of major circle of iris which is formed by two long posterior ciliary arteries and seven anterior ciliary arteries (the minor circle lies within the iris stroma).
Functions
- Formation of aqueous humor by the ciliary processes.
- Ciliary muscles also help in opening up the Schlemm's canal, and thus facilitate in aqueous outflow.
THE CHOROID
- This vascular sheet separates the sclera from the retina. It is 0.25 mm thick at the posterior pole and 0.1 mm thick anteriorly.
- It is attached firmly to the sclera around the optic nerve and at the points of exit of the vortex veins.
Structures
It consists of three layers of blood vessels, having supporting structures on either side, i.e. suprachoroid (lamina fusca) on outer side and the basal lamina (Bruch's membrane) on the inner side.
Three vessel layers are (Fig. 1.8):
- The outer vessel layer (of Haller): It is nearest to the sclera and consists of large veins that lead to the vortex veins.
- The middle vessel layer (of Sattler): It consists of medium-size veins and arterioles, with fibrous tissues.
- The inner choriocapillaries: It consists of large fenestrated capillaries.
Bruch's membrane: It is about 7 µ thick, and separates the choriocapillaries from the RPE. Electron microscopically, it consists of five layers (from outside to inside):
- Basement membrane of the choriocapillaries
- Outer collagen layer
- Middle elastic layer
- Inner collagen layer
- Basement membrane of the RPE.
Bruch's membrane is important for bloodretinal barrier function.
Blood Supply (Fig. 1.9)
- Short posterior ciliary arteries: Originate from the ophthalmic artery as 2–3 branches. These branches are subdivided into 10–20 branches which perforate the sclera around the optic nerve, and directly communicate with the choriocapillaries.
- Two long posterior ciliary arteries: Perforate the sclera on either side of the optic nerve → via suprachoroidal space → to the ciliary body. There, each divides into two branches that extend circumferentially to form ‘major arterial circle’ of the iris, located in the ciliary body. Branches extend anteriorly to the iris.
- Anterior ciliary arteries: They are the terminal branches of two muscular arteries of each rectus muscle (except the lateral rectus, which has one muscular artery). The vessels provide supply to the ciliary body, and send branches to the major arterial circle of iris and also to the choriocapillaries.
Venous blood is collected from the iris, ciliary body and the choroid by a series of veins. These lead of four (or more) large vortex veins located behind the equator of the globe. The vortex veins drain into superior and inferior ophthalmic veins → cavernous sinus.
THE CRYSTALLINE LENS
The lens is a transparent biconvex body of crystalline structure.
- It is about 9 mm in diameter and 4 mm in thickness when the suspensory ligament is relaxed.
- Radius of curvature of anterior surface is 10 mm and that of posterior surface is 6 mm.
- The lens is held in its position by the suspensory ligament, called zonules of Zinn. They arise from the sides of the ciliary processes, and the valleys between them.
- The zonular fibers insert into the anterior and the posterior lens capsule near the equator, and extend further over the anterior surface more than the posterior surface.
Structures (Fig. 1.10)
- The lens capsule that envelops the entire lens.
- An anterior lens epithelium.
- A lens substance, consisting of the cortex (newly formed lens fibers) and the nucleus (a dense central area of old lens fibers).
- The capsule: It is a smooth, homogenous, a cellular highly elastic envelope, (but contains no true elastic tissue).The anterior capsule is the basement membrane of the anterior lens epithelium, and the thickest basement membrane of the body. The lens capsule is thickest on the anterior and posterior surface just central to the insertion of zonular fibers (14 µ) (i.e. pre-equator regions). It is thinnest at the posterior pole (3–4 µ).
- The lens epithelium: It consists of a single layer of cuboidal cells just deep to the capsule. There is no corresponding posterior epithelium. Towards the equator the anterior cuboidal cells gradually become columnar and elongated, and eventually converted into lens fibers. At the equator, the division of lens fibers is most active and mitosis is frequently observed.
- Lens substance: It consists of elongated lens cells (fibers). Mature lens fibers are cells which have lost their nuclei, and are no longer in contact with the posterior capsule. They form the greatest portion of the lens substance.
- In the infantile lens, each lens fiber starts and finishes on the anterior and posterior Y sutures respectively in such a way, that the nearer the axis of the lens it commences, the farther away it ends (anterior Y is straight and the posterior Y is inverted).
- Once this fetal nucleus is formed, these outlines become more irregular and more complicated.
- In the beam of slit-lamp, various layers or zones of discontinuity may be seen and these represent the boundaries as follows (Fig. 1.11):
- Embryonic nucleus (first 3 months of fetal life)
- Infantile nucleus (up to puberty)
- Adult nucleus (rest of life).
CHAMBERS OF THE EYE
The eye consists of three chambers—anterior chamber, posterior chamber, and vitreous cavity.
- Anterior chamber: It is bounded anteriorly by the cornea, posteriorly by the front surface of the iris and lens, and peripherally by the anterior chamber angle. It is deepest at its central portion (2.5–3.0 mm), and its volume is 0.25 mL.
- Posterior chamber: It consists of various boundaries are as follows:
- Anteriorly: Iris,
- Laterally: Ciliary processes,
- Medially: Equator of the lens and
- Posteriorly: Anterior surface of the lens.Its volume in adults is 0.06 mL.
- Vitreous cavity: It contains vitreous humor which is a transparent gel-like structure. It is composed of a network of collagen fibers suspended in a viscous liquid containing hyaluronic acid.It has a saucer-like depression anteriorly, for the lens, called patellar fossa. Here, the vitreous humor is condensed, called anterior hyaloids face. The vitreous humor adheres firmly to the:
- Ciliary epithelium in the region of ora serrata (vitreous base),
- Peripheral retina,
- Margin of the optic disc, and
- Posterior capsule of the lens (Weigert's hyaloideocapsular ligament).
Running down the center of the vitreous, from the optic disc to the posterior pole of the lens—there is a canal, called hyaloid canal of Cloquet. Embryologically, the vitreous is divided into three parts:
- Primary (mesenchymal, Cloquet's canal).
- Secondary (most of the adult vitreous).
- Tertiary (the lens zonules).
The vitreous contains a few cells, called hyalocytes, which are believed to be phagocytes (macrophage type).
Volume of the vitreous cavity is 4.5 mL.
THE RETINA
The retina is the membranous light sensitive coat of eyeball. It is transparent in life, and whitish after death.
The retina is derived from the inner and outer layers of the embryological optic cup. These two primary layers are loosely adherent across the potential space (representing the primary optic vesicle) so that they are readily separated by injury or disease. The outer pigmented layer remains as one-cell deep but the inner layer; the sensory retina becomes several layers by various visual relaying cells.
Structures
Retina apparently consists of ten layers (Fig. 1.12). From outside (choroid-side) inwards, they are:13
- Retinal pigment epithelium
- Layer of rods and cones
- External limiting membrane
- Outer nuclear layer
- Outer plexiform layer
- Inner nuclear layer
- Inner plexiform layer
- Ganglion cell layer
- Nerve-fiber layer
- Internal limiting membrane.
- The retinal pigment epithelium: It is a single layer of flattened hexagonal cells with fine cytoplasmic villi, projecting for a short distance between the bases of rods and cones.The cells of RPE contain varying amount of melanin. The cells are taller at the fovea and contain more pigments (hence, the darker color in this region). Around the optic disc they are heaped up as a choroidal ring.
- Layer of rods and cones: They are the outer segments of photoreceptor cells, arranged in a palisade manner. The rods are about 125 million in number, but the cones are about 7 million. Each rod and cone may be divided into three parts (Fig. 1.13):
- An outer segment: It is cylindrical in shape, with its base related to the projecting villi of RPE. It consists of a dense vertical stack of 700 discs that originates from in folding of double layer of cell membrane. The infoldings contain the visual pigments.
- Acilium: It is a tubular connection with the inner segment that contains linear striations.
- An inner segment: It is divided into an outer ellipsoid and an inner myoid portion. The myoid part is densely packed with endoplasmic reticulum and Golgi bodies.
- External limiting membrane: It is a thin lamina, formed by the supporting fibers of Muller on which the rods and cones rest, and pierced by the fibers of these photoreceptors.
- Outer nuclear layer: It contains the rod and cone nuclei. Cone nuclei are larger and more oval than the rod nuclei, and carry a layer of cytoplasm.
- Outer plexiform layer: It is formed by the anastomoses of the photoreceptor cells, with bipolar and horizontal cells. The innermost portion of each rod and cone cell is swollen with several lateral processes, known as rod spherules and cone pedicles. Many photoreceptors converge onto one bipolar cell and interconnect with one another. But at the fovea, each ‘midget’ cone transmits to a single bipolar cell.
- Inner nuclear layer: It consists of the nuclei of bipolar, horizontal and amacrine cells. Amacrine cell processes pass inwards to synapse in the inner plexiform layer. The nuclei of Muller's fibers are also found here. Capillaries from the retinal vessels reach up to this layer, but the outer layers are a vascular, the rods and cones being nourished by the choriocapillaries.
- Inner plexiform layer: It consists of arborizations of the bipolar cells, with the ganglion cells and amacrine cells.
- Ganglion cell layer: It consists of large multipolar nerve cells, with clear round nuclei containing nucleoli, and have Nissl's granules in their cytoplasm. In the retinal periphery, a single ganglion cell may synapse up to a hundred bipolar cells, but in the macular region, there tends to be a single connection with the ‘midget’ bipolar cells.
- Nerve fiber layer: It consists of bundles of ganglion cell axons, running parallel to the retinal surface. The layer increases in depth as it converges towards the optic disc. These nerve fibers are fine and non-medullated. The macular fibers themselves pass directly to the disc as the papillomacular bundle.
- Internal limiting membrane: It is a thin lamina separating retina from the vitreous. It is formed by the union of terminal expansions of Muller's fibers, and essentially a basement membrane.
Regions
- Ora serrata: It is the anterior termination of retina, located about 8 mm from the limbus. Here, only two layers of primitive optic vesicle fuse, and continue forward as the ciliary epithelium.
- Central retina (Fig. 1.14): It is 4.5 mm in diameter. It extends from the fovea centralis—nasally, almost to the optic disc; same distance temporarily, and a similar distance above and below the fovea centralis. In this region, ganglion cell layer is more than one layer of cell bodies. The central part of this region is called macula leutea which contains a yellow pigment, xanthophyll.
- The fovea centralis is a depressed area, located in the central retina, about 3 mm (2 dd) temporal to the optic disc and 0.8 mm below the horizontal meridian. It measures about 1.5 mm (1500 µm). Its central depression is called foveola, measuring about 0.5 mm (500 µm).
- The photoreceptors in the fovea are exclusively cones. All cell layers are displaced peripherally so that light falls directly on the cones’ outer segment. The foveola is nourished solely by the choriocapillaries of the choroid and does not contain any vessels, hence, called foveolar avascular zone (FAZ).
- Peripheral retina: The photoreceptors are mainly rods. The ganglion cells are larger, and their cell bodies are arranged in a single layer.
- Functionally, retina is divided into temporal and nasal portion, by a line drawn vertically through the center of the fovea. Nerve fibers originating from the cells temporal to this line pass to the lateral geniculate body of the same side and from nasal side, cross in the optic chiasma to reach the lateral geniculate body of the opposite side.
- Ophthalmoscopically, the ophthalmologist uses the optic nerve as a hub to divide the retina into superior and inferior temporal portions, superior and inferior nasal portions, and a central retina.
Blood Supply
Retina gets its nourishment from two sources:
- Outer portion: It mainly by the choriocapillaries.
- Inner portion: It mainly by the central retinal artery, the first branch of ophthalmic artery which enters the optic nerve about 10–12 mm posterior to the globe.
THE CONJUNCTIVA
It is a mucous membrane covering the inner surface of the eyelids and reflected to cover the anterior part of the eyeball over the sclera, up to the corneal margin.
Parts (Fig. 1.15)
- Palpebral: It consists of marginal, tarsal and orbital part. It is firmly adherent to the deeper tissue.
- Bulbar: It lies over the sclera and it is freely mobile.
- Fornix: It is the cul-de-sac at the junction of palpebral and bulbar conjunctiva.
- Limbal: It is the conjunctiva at the corneal junction which is adherent.
Structures
- Epithelium: There are two layers of epithelium over the palpebral conjunctiva, and transitional stratified squamous epithelium at the intermarginal strip.
- From fornices to the limbus the epithelium is gradually thicker (4–6 layers).
- Again, it is stratified epithelium at the limbus.
- Goblet cells (mucin secreting cells) are present throughout the epithelium, especially more near the fornices.
- Subepithelial layer: It is an adenoid layer of loose connective tissue containing leukocytes.
- Fibrous layer: It is much dense, and blended with the deeper structures (e.g. episclera or tarsus).
Nerve Supply
Ophthalmic division of trigeminal nerve (5th cranial).
Arterial Supply
- Anterior conjunctival artery from the anterior ciliary artery.
- Posterior conjunctival artery from the lacrimal artery.
- Palpebral branch of nasal artery.
TENON'S CAPSULE
Tenon's capsule is a thin membrane which envelops the eyeball from the optic nerve to the limbus, separating it from the orbital fat and forming a socket in which it moves.
Its inner surface is smooth, and is separated from the outer surface of the sclera. Its anterior part (anterior Tenon's capsule) adheres with the undersurface of the conjunctiva and attaches to sclera at the limbus. Posterior Tenon's capsule is made up of the fibrous sheath of the rectus muscles together with the intermuscular membrane.
The fascia is perforated behind by the ciliary vessels and nerves, and fuses with the sheath of the optic nerve and with the sclera around the entrance of the optic nerve.
It is perforated by the tendons of the extraocular muscles, and is reflected backward on each as a tubular sheath. The expansions from the sheaths of the lateral and medial recti are strong, especially that from the latter muscle, and are attached to the zygomatic bone and lacrimal bone respectively. They also check the actions of these two recti and are called the medial and lateral check ligaments.
There is thickening of the lower part of the Tenon's capsule, which is known as suspensory ligament of the eye (of Lockwood). It is slung like a hammock below the eyeball, being expanded in the center, and narrow at its extremities which are attached to the zygomatic and lacrimal bones respectively.
Sub-Tenon's block for ocular surgery: Local anesthetic may be injected into the space between Tenon's capsule and the sclera to provide anesthesia for eye surgery, principally cataract surgery. After applying topical anesthetic drops to the conjunctiva, a small fold of conjunctiva is lifted off and a small nick made. A blunt, curved cannula is passed through the incision into the sub-Tenon's space and then 1.5 mL 2% lignocaine solution is injected. The advantages are a reduced risk of bleeding and of penetration of the globe, compared to peribulbar and retrobulbar blocks. However, the akinesia of the extraocular muscles may be less complete.
THE EXTRAOCULAR MUSCLES (FIG. 1.16)
They are six in number. Four rectus muscles and two oblique muscles.
- Origin of rectus muscles: Common origin from annular tendon of Zinn around the optic foramen at the apex of the orbit.
- Insertion: They are inserted to the sclera as a spiral line (spiral of Tillaux) after piercing the Tenon's capsule. The distance from limbus are as follows:Superior rectus (S) = 7.7 mmLateral rectus (L) = 6.9 mmInferior rectus (I) = 6.6 mmMedial rectus (M) = 5.5 mm
- Origin of superior oblique: Common origin at the apex of the orbit from annular tendon of Zinn → runs to the trochlea, at upper and inner angle of orbit → becomes tendinous → reflected backwards under the superior rectus muscle.Fig. 1.16: Extraocular muscles of the eyeNote: Both oblique muscles insert behind the equator of the globe. The inferior oblique muscle passes inferior to the body of the inferior rectus muscle but beneath the lateral rectus muscle. The numbers indicate the distance of the insertion in mm from the corneoscleral limbus. Medial rectus (M) = 5.5 mm
- Origin of inferior oblique: Anteriorly from the lower and inner orbital walls near the lacrimal fossa. It is the only muscle, which does not arise from the apex of the orbit.
- Insertion: It is inserted in the sclera at in ferolateral part of the posterior pole of the globe (corresponds to the area near the macula).
- Nerve supply: All the muscles are supplied by 3rd (oculomotor) cranial nerve except, lateral rectus—by 6th (abducens) nerve, and superior oblique—by 4th (trochlear) nerve.
- Actions of extraocular muscles: See Chapter 23.
- Medial rectus: Adduction.
- Lateral rectus: Abduction.
- Superior rectus: Elevation on abduction and intorsion.
- Inferior rectus: Depression on abduction and extorsion.
- Superior oblique: Depression on adduction and intorsion.
- Inferior oblique: Elevation on adduction and extorsion.
THE EYELIDS
The eyelids are thin curtains of skin, muscles, fibrous tissue and mucous membrane. The upper eyelid is limited above by the eyebrow, and the lower eyelid merges with the cheek. Each eyelid is divided by a horizontal furrow (sulcus) into an orbital and a tarsal part. The upper furrow is formed by skin—insertions of the levator palpebrae superior is muscle.
- Palpebral aperture: When the eyes are open, the eyelids form an elliptical opening, the palpebral aperture (fissure) which measures about 12 mm by 30 mm. The lateral canthus is about 2 mm higher than medial canthus (may be up to 5 mm in orientals). At the inner canthus, there is a small bay, the lacus lacrimalis, formed by the eminences (papillae) which bear the lacrimal puncta 6 mm lateral to the angle itself. There is a small separated knob of skin, the caruncle, bearing a few hairs or glands, contained in this bay. Just lateral to the edge of the caruncle, there is a crescentic fold of conjunctiva, the plica semilunaris, which is similar to the nictitating membrane in lower vertebrates.
- Lid margin: Located on the free margin of each eyelid are the openings of the lacrimal canaliculi (the puncta), the eyelashes and the openings of the glands. The medial one-sixth of the lid margin (the lacrimal portion) has no eyelash or gland openings, and is rounded. The lateral five-sixths (the ciliary portion) of the lid margin has square edges. The eyelashes on the upper eyelid margin curve upwards, and are more numerous than those of the lower eyelid margin, which curve downwards.
Structures
- Subcutaneous tissue: Loose areolar tissue devoid of fat.
- Muscular layer
- Orbicular is oculi
- Levator palpebrae superioris (LPS) in the upper lid only
- Muller's muscle.
- Fibrous layer
- Orbital septum in the upper part.
- Tarsal plate in the lower part.
Different glands of the eyelid (i.e. Meibomian glands, glands of Zeis, glands of Moll, glands of Krause and Woulfring) lie in this plane. - Palpebral conjunctival layer: Inner most layer of the eyelid (Fig. 1.17).
Muscles of the Eyelid
- Orbicular is oculi
- Action: Closure of lids, blinking, winking, squeezing and helps in the drainage of tears.
- Nerve supply: Zygomatic branch of facial (7th cranial) nerve.
- In case of its paralysis: There will be lagophthalmos (leading to exposure keratitis due to dryness of the cornea and conjunctiva).
- Levator palpebrae superioris: It is present only in the upper lid.
- Origin: From the apex of the orbit, above the annulus of Zinn.
- Insertion: It can be into five parts:
- The main tendinous slip is inserted into the upper margin and the anterior surface of the tarsal plate.
- Anterior slip to the skin of upper lid.
- Posterior slip to the conjunctiva of the upper fornix along with the sheath of superior rectus muscle.
- Medial and
- Lateral slips are attached to the medial and lateral palpebral ligaments respectively.
- Action: Elevates the upper eyelid including upper fornix, and helps in the formation of upper lid-fold.
- Nerve supply: Upper division of oculomotor nerve (3rd cranial nerve). Paralysis of LPS—causes ptosis.
- Müller's muscle (unstriped)
- Upper Müller's muscle: Arises from the stripped fibers of levator muscle, passes downwards behind it, and is inserted into upper border of the tarsus.Action: It elevates the upper lid.
- Lower Müller's muscle: It arises from inferior rectus muscle, lies below it, and is inserted into the lower tarsus.Action: It elevates the lower lid to some extent.
- Nerve supply: Cervical sympathetic nerve.
- Paralysis of cervical sympathetic nerve will cause Horner's syndrome (ptosis, miosis, enophthalmos and anhydrosis of the face).
Glands
- Meibomian glands: They are modified sebaceous glands (tubular) of larger size and responsible for oily secretion of the tear film. They are situated within the substance of tarsal plate, arranged vertically, and each opens by a single duct on the margin of the lid.
- Number: 30–40 in the upper lid, and 20–30 in the lower lid.
- Glands of Zeis: They are sebaceous glands, lie in the lid margin, and open in the follicle of eyelashes.
- Glands of Moll: Modified sweat glands, situated immediately behind the hair follicles, and their ducts open into the ducts of Zeis’ gland, or into the follicle, (not directly onto the skin surface as elsewhere).
- Glands of Krause and Woulfring: They are accessory lacrimal glands situated on the palpebral conjunctival side (Fig. 1.18).
Intermarginal Strip
It is the margin or free edge of the lid. It is covered with stratified epithelium which forms a transition between the skin and conjunctiva.
- Structures (from anterior to posterior):
- Anterior round border
- Eyelashes
- Gray line
- Orifices of the ducts of Meibomian glands and
- Posterior sharp border.
Fig. 1.18: Section through the upper eyelid1—Orbicularis muscle; 2—Sweat gland; 3—Hair follicle; 4—Gland of Zeis; 5—Cilium; 6—Gland of Moll; 7—Marginal part of orbicularis muscle; 8—Subtarsalis part of orbicularis muscle; 9—Inferior arterial arcade, 10—Meibomian gland; 11—Gland of Wolfring; 12—Conjunctival crypts; 13—Superior arterial arcade; 14—Gland of Krause; 15—Muller's muscle; 16—Levator palpebrae superioris muscle; 17—Gray line; 18—Fat - Gray line is an important landmark for operations in which the lid is splitted, since it indicates the position of the loose fibrous tissue between the orbicularis oculi and the tarsus.
Arterial Supply
- In upper lid: In the form of two arches:
- Superior: Lying between the upper border of tarsus and the orbicularis.
- Inferior: In front of tarsal plate, just above the hair follicle at the free edge.
Venous Drainage
Two plexuses in each lid—(i) post-tarsal, draining into the ophthalmic vein, and (ii) pretarsal, into the subcutaneous veins.
Lymphatic Drainage
- From inner half: Into the submandibular lymph node.
- From outer half: To the preauricular lymph node.
Functions
- Protection of eyeball proper, from external injuries, e.g. dust, fumes, foreign body, etc.
- Maintain the pre-corneal tear film (by sharp posterior borders of the lid margin).
- Interrupt and limit the amount of light entering the eye.
- Drainage of the tears by the lacrimal pump system.
- Emotional expressions.
THE LACRIMAL APPARATUS
The lacrimal apparatus consists of two parts:
- Secretory portion: Lacrimal gland and accessory lacrimal glands of Krause and Woulfring.
- Drainage portion: Via which the tears drain into the inferior meatus of the nose.
Secretory Portion
- Lacrimal gland: It is located in the anterolateral portion of the roof of the orbit in the lacrimal fossa. It has two parts—a large orbital portion and a small palpebral portion separated by lateral part of the aponeuroses of LPS muscle. It is a tubule alveolar type of gland. Their ducts open separately onto the superior temporal fornix (Fig. 1.19).Nerve supply: Via facial nerve, parasympathetic from lacrimal (salivary) nucleus.
- Accessory lacrimal glands of Krause and Woulfring: They are located deep in the conjunctiva particularly in the fornices, mostly on the temporal side.
Fig. 1.19: Parts of the lacrimal apparatus
1—Lacrimal gland; 2—Punctum; 3—Common canaliculus; 4—Lacrimal sac; 5—Nasolacrimal duct
Drainage Portion
It is composed of the puncta, the canaliculi, the lacrimal sac and the nasolacrimal duct.
- Two lacrimal puncta: These are two small openings, situated near the posterior border of the free margins of the lid about 6 mm from the inner canthus. The punctum is situated upon a slight elevation (large in elderly people) called lacrimal papilla.
- Two canaliculi: It pass from the punctum to the lacrimal sac. They are first directed vertically for 1–2 mm, then horizontally for 6–7 mm. The canaliculi usually open separately into the outer wall of lacrimal sac. Sometimes, they join together to form a common canaliculus before opening into the sac.
- Lacrimal sac: It lies in the lacrimal fossa formed by the lacrimal bone. When distended it is about 15 mm long and 5–6 mm wide. The upper portion is called fundus, which lies slightly above the level of medial palpebral ligament. Sac itself is covered by fibers of the orbicularis muscles and loose fibrous tissues.21
- Nasolacrimal duct: It is the continuation of lacrimal sac. 12–24 mm long and 3–6 mm in diameter. The duct has two parts; intraosseous and intrameatal. It passes downwards, slightly outwards and backwards to open into anterior part of the outer wall of the inferior meatus of the nose.
The upper end of the nasolacrimal duct is the narrowest part. The mucous lining forms an imperfect valve at the orifice into the nose (valve of Hasner).
THE ORBIT
The orbits are pear-shaped cavities. Their medial walls are parallel, but lateral walls diverge at an angle of 45°. The orbit is roughly 40 mm in height, width and depth. Its volume is about 30 mL.
Portions of seven bones (Fig. 1.20) form the orbit are—(1) frontal, (2) maxilla, (3) zygoma, (4) sphenoid, (5) palatine, (6) ethmoid and (7) lacrimal.
Fig. 1.20: The bony orbit
FB—Frontal bone; MB—Maxillary bone; ZB—Zygomatic bone; SB—Sphenoid bone-greater wing (gw); lesser wing (lw); PB—Palatine bone; EB—Ethmoid bone; LB—Lacrimal bone; 1—Optic canal; 2—Superior orbital fissure; 3—Inferior orbital fissure; 4—Supraorbital notch; 5—Infraorbital foramen
Contents
- The eyeball and intraorbital part of optic nerve
- Retrobulbar fat
- Extraocular muscles
- Ophthalmic arteries and veins
- 3rd, 4th and 6th nerve and first two divisions of 5th nerve
- Ciliary ganglion
- Sympathetic plexus
- Lymphatic vessels
- Tenon's capsule and orbital fascia
- Lacrimal gland and lacrimal sac.
Optic Foramina
It is located at the posteromedial portion of the orbit in the body of the sphenoid bone. It measures 4–10 mm in diameter.
Through it passes:
- The optic nerve with its sheaths,
- Ophthalmic artery and
- Sympathetic nerve from carotid plexus.
Superior Orbital Fissure
It is just lateral to the optic foramen, and is the gap between the greater and lesser wings of sphenoid. The fissure is divided into lateral and medial portions by the tendinous annulus of Zinn (Fig. 1.21).
- Passing through the annulus
- Two divisions of the oculomotor nerve (CNIII)
- Abducens nerve (CNVI)
- Branches of the ophthalmic division of trigeminal nerve (CNV), except the lacrimal and frontal branches.
- Passing through the lateral portion
- Lacrimal nerve
- Frontal nerve
- Trochlear nerve (CNIV)
- Superior ophthalmic veins
- Recurrent lacrimal artery
Fig. 1.21: Structures passing through the superior orbital fissure and the optic foramen
d—Trochlear nerve; e—Superior ophthalmic vein; f—Oculomotor nerve; g—Abducens nerve; h—Nasociliary nerve; i—Inferior ophthalmic vein; j—Superior rectus; k—Inferior rectus; l—Medial rectus; m—Levator palpebrae superioris; n—Superior oblique; o—Optic nerve; p—Ophthalmic artery; q—Recurrent lacrimal artery
Inferior Orbital Fissure
It lies between the maxilla and the greater wing of sphenoid.
It transmits
- Maxillary division of trigeminal nerve
- Infraorbital artery
- Zygomatic nerve
- Branches of inferior ophthalmic vein draining into pterygoid venous plexus.
Surgical Spaces of the Orbit (Fig. 1.22)
From the surgical point of view, there are four spaces of the orbit. They are relatively self-contained, as the inflammatory processes are contained for a considerable period of time and each of which must, if necessary, be opened separately.
Fig. 1.22: Spaces of the orbit
L—Lateral check ligament; LM—Lacrimal portion of orbicularis muscle; LP—Lateral palpebral ligament; LS—Lacrimal sac; M—Medial check ligament; MP—Medial palpebral ligament dividing into a superficial and a deep band; MS—Muscle sheaths; TA—Anterior part of Tenon's capsule; TP—Posterior part of Tenon's
- The subperiosteal space: Between the bones of the orbital wall and the periorbita (periosteum).
- The peripheral orbital space: Between the periorbita and the extraocular muscles which are joined together by fascial connections.
- The central space: A cone-shaped area enclosed by the muscles (the muscle cone).
- Tenon's space: Around the globe.
BLOOD SUPPLY OF THE EYE (FIG. 1.23)
ARTERIES (FIG. 1.24)
The eye and the orbital contents receive their main blood supply from the ophthalmic artery. The eyelids and conjunctiva have an anastomotic supply from the branches of both external carotid and ophthalmic artery.23
Ophthalmic Artery
It arises from the fifth bend of the internal carotid artery. It enters the orbit through the optic foramina, below and lateral to the optic nerve.
Branches
- The central retinal artery
- The short posterior ciliary arteries (10–20 in number)
- Two long posterior ciliary arteries
- Recurrent meningeal artery
- Lacrimal artery
- Variable number of recurrent arteries
- Muscular branches to each of the extraocular muscles.
The anterior ciliary arteries are the forward continuations of muscular arteries. Each rectus muscle has two muscular arteries except the lateral rectus, which has one.
External Carotid Artery
The blood supply to the eyelids and conjunctiva from the branches of the external carotid artery, originates from the external maxillary artery, the superficial temporal artery and the internal maxillary artery.
VEINS
Mainly through the superior and inferior orbital veins and they empty into the cavernous sinus (Fig. 1.25).
- Superior orbital vein communicates with the angular vein, and then to facial vein.
- Inferior ophthalmic vein communicates with the pterygoid venous plexus, and also to the cavernous sinus directly or via superior ophthalmic veins.
- Two or more superior vortex veins drain into the superior orbital vein and inferior vortex veins into the inferior orbital vein.
- The central retinal vein enters the cavernous sinus separately, or to the superior ophthalmic veins.
Cavernous Sinus
It is an irregular-shaped, endothelium lined venous space, situated on either side of the body of the sphenoid bone.
Connections (Figs 1.26A and B)
- The superior and inferior ophthalmic veins enter it from the front.
- The superior and inferior petrosal sinuses leave it from behind.
- It connects directly with pterygoid plexus, and indirectly via inferior ophthalmic veins.
- To the opposite sinus by two or three transverse sinuses which surround the pituitary stalk.
Structures passing through it (Fig. 1.27):
- Internal carotid artery with sympathetic plexus via medial wall.
- Abducens nerve is just lateral to the artery.
- Ophthalmic and maxillary divisions of the trigeminal nerve, are lateral and below the artery.
- Oculomotor and trochlear nerves are on its lateral wall, on the superior aspect.
One or more nerves may be affected by disease of the cavernous sinus, e.g. thrombosis of the sinus, rupture or aneurysm of the internal carotid artery.
NERVES OF THE EYE
- Visual optic nerveThe visual pathway—optic nerve → optic chiasma → optic tract → lateral geniculate body → optic radiation → visual cortex.
- Motor
- Oculomotor (NIII)
- Superior division → Superior rectus and LPS.
- Inferior division → Medial rectus, inferior rectus, and inferior oblique.Figs 1.26A and B: Tributaries of the cavernous sinus. A. Lateral view. B. Above viewAng.—Angular vein; Cav. sinus—Cavernous sinus; Com.—Communicating vein; Fac.—Facial vein; Fr.—Frontal vein; I.oph.—Inferior ophthalmic vein; I.petr.,—(infr. petr. s), Inferior petrosal sinus; Jug. v.—Jugular vein; Lab.—Labyrinthine veins; Lat. sinus—Lateral sinus; Mas. Em.—Mastoid emissary vein; Mid. Men.—Middle meningeal veins; Nasal—Nasal veins; Pt. px.—Pterygoid plexus; S.o. (oph)—Superior ophthalmic vein; Supra-orb—Supraorbital vein; S. (Supr.) petr.—Superior petrosal sinus; Tr.—Transverse sinus; Sps. sinus—Sphenoparietal; C. ret. V—Central retinal vein
- Short root to ciliary ganglion → ciliary and sphincter papillae muscle.
- Trochlear (NIV) → Superior oblique.
- Abducens (NVI) → Lateral rectus.
- Mixed motor and secretoryFacial (NVII) → (i) Motor to face, especially to the orbicularis oculi; (ii) Secretory to the lacrimal gland.
- SensoryTrigeminal (NV) → Via ophthalmic and maxillary divisions.
- Autonomic
- Para sympathetic supply
- Oculomotor nerve (from the EdingerWestphal nucleus) → inferior division → branch to inferior oblique → short root of ciliary ganglion → ciliary and sphincter papillae muscles
- Facial nerve (from salivary nucleus) → lacrimal gland.
- Sympathetic supply: Postganglionic fibers from the superior cervical ganglion → around the internal carotid artery (carotid plexus) → cavernous plexus → via the ophthalmic division of 5th nerve → nasociliary nerve → long ciliary nerve avoiding the ciliary ganglion (some via ciliary ganglion without a relay → short ciliary nerves) → along with long ciliary arteries into the suprachoriodal space → enter ciliary body and iris, to supply dilator pupillae.
LYMPHATIC DRAINAGE (FIG. 1.28)
From the eyelids, conjunctiva and from the orbital tissues.
- Medial group drains into the submandibular lymph nodes.
- Lateral group drains into the preauricular lymph nodes and sometimes, into the postauricular lymph nodes.
