cornea, conjunctiva and sclera

the anatomy and physiology of cornea, conjunctiva and sclera

1. Cornea,conjunctiva and sclera Fawaz H. Alzweimel, MD King Hussein hospital Ophthalmology department

2. The Cornea transparent avascular tissue. smooth , convex outer surface and concave inner surface. To meet the diverse functional demands the cornea must be: -Transparent -Refract light -Contain the intraocular pressure -Provide a protective interface

3. Anterior surface: -Elliptical, about 11.7 mm horizontally and 10.6 mm vertically Posterior surface: -Circular, about 11.7 mm in diameter

4. Thickness : - Centrally about 0.52 mm -Peripherally about 0.67 mm Surface area: About 1.3 cm² (one-sixth of the globe) Optical zone: Cornea is almost a sphere, the central 1/3 rd is called optical zone about 5.4 mm. Radius of curvature: -Anterior surface – about 7.8 mm -Post. Surface – about 6.5 mm Refractive power: +43.1 D (Air-tear = +43.6 D, Tear-cornea = +5.3 D, Cornea-aqueous = -5.8 D) Refractive index: 1.376 . Curvature varies from apex to limbus, greater flattening is nasally and superiorly. Cornea is flatter in men than in women. Cornea flattens slightly on convergence.

5. Composition of human cornea Water : 78 % Collagen: 15 % Type-I : 50-55 % Type-III : 1 % Type-IV : 8-10 % Type-VI : 25-30 % Other protein: 5 % Keratan sulphate : 0.7 % Condroitin /dermatan sulphate : 0.3 % Hyaluronic acid Salts: 1 %

6. Embryonic origin of Cornea The formation of cornea is induced by the lens and the optic cup at the 7th weeks. •Corneal epithelium – Surface ectoderm •Bowman’s membrane – Mesenchyme •Stroma – Mesenchyme and Neural crest •Descemet’s membrane – Synthesized by endothelium •Endothelium – Neural crest

7. Structure 1.Epithelium 2.Bowman’s layer 3.Stroma 4.Descemet’s membrane 5.Endothelium

8. Epithelium : -Stratified, squamous and non-keratinized -Continuous with conjunctival epithelium at limbus but having no goblet cells -50-90 μ m in thickness -Consists of 5 or 6 layers of nucleated cells resting on a basal lamina : a . Basal cells b . Wing cells c. Surface cells

9. Basal cells: -Deepest cell layer -Stand in a palisade manner on basal lamina - Germinative layer of the epithelium -Columnar with rounded heads and flat bases -Nucleus is oval and oriented parallel to the cells long axis Wing or umbrella cells: -Second epithelial cell layer (1-2 layers of cells) -Polyhedral cells -Convex anteriorly forming cap over basal cells and send processes between them -Nucleus is oval and oriented parallel to corneal surface

10. Surface cells: -Most superficial 2-3 layers of cells -Also polyhedral and become wider & flattened towards the surface -Flattened nuclei project backwards leaving the surface perfectly smooth -Most superficial cells are mostly hexagonal in shape and exhibit surface microvilli or microplicae Ultrastructural features of epithelium: -Epithelial cells shows usual organelles like other actively metabolizing cells -Moderately abundant mitochondria in wings & middle layer cells but small and scarce in basal cells -Wing & superficial cells have high glycogen content

11. Tonofibrils: cells contain a cytoplasmic meshwork of electron dense intermediate filaments composed of cytokeratins -The plasma membrane of contiguous cells interdigitated to each other -Adhesion is achieved by: •Tight junctions & desmosomes – surface cells •Desmosomes – wings & superficial cells •Desmosomes & Hemidesmosomes – in basal cells In the basal cells there are anchoring filaments which pass through the hemidesmosomal structure to be inserted into basal lamina Langerhans cells (cells of immune recognition system) present near periphery. They are almost absent at central cornea but aggregate in response to infection

12. Basal lamina: -The basal lamina is secreted by the basal cells -0.5 - 1 μ m wide - Ultrastructurally it is distinguished in to two parts – i . Lamina lucida (superficial) ii. Lamina densa (deep osmiophilc ) The lamina consist of collagen and glycoprotein constituents which integrated with Bowman’s layer by array of short anchoring filaments -Lipid solvent, stromal edema and inflammation may loosened the cohesion between Bowman’s zone and lamina -With old age, in diabetes and in some corneal disorders it becomes thickened and multilamellar

13. Epithelial Turnover: -Early studies suggested that the epithelium replaced approximately weekly by division of basal cells and the oldest shed from the surface -It is now recognized that the germinative region lies at the limbus (the stem cells) and cells migrate at a very slower rate (123 μm/week) to the center of the cornea which may be as long as a year The XYZ hypothesis: 1.Thoft R. and Friend J. (1983) proposed on the basis of experimental evidence that both limbal basal and corneal basal cells are the source for corneal epithelial cells, and there is a balance among division, migration & shedding. The corneal epithelium is maintained by a balance among sloughing (Z) of cells from the corneal surface, cell division (X) in the basal layer and renewal of basal cells by centripetal migration (Y) of new basal cells originating from the limbal stem cells .

14. Epithelial Repair: Injury (abrasion) Cells at wound edge retract, thicken and lose attachment Travel in an amoeboid movement to cover the defect Cells at wound edge ruffle and send out filopodia and lamellipodia towards the center of wound Migration process is halted by contact inhibition They then anchor and Mitosis resumes to re-establish epithelial thickness Surface tight junctions re-established Adhesion with Bowman’s layer within 7 days (if basal lamina intact )

15. The healing process occurs rapidly, rate of cell migration is 60 – 80 μm / hr In case of total epithelial loss including total limbus, cornea is covered with vascularized conjunctival type of epithelium by adjacent conjunctiva If a small part of limbus with stem cell is retained then conjunctival type of epithelium is gradually disappear and metabolic behavior of corneal epithelium re-established very slowly

16. Bowman’s layer: ( Anterior Limiting lamina) Modified region of anterior stroma Acellular homogeneous zone 8 – 14 μ m thick Anterior surface is smooth & parallel with corneal surface It delineates the anterior junction between cornea and limbus Ultrastructural features: Ultra structurally it is a felted meshwork of fine collagen fibrils of uniform size in a ground substance Posteriorly it becomes blended & interweaving with fibrils of ant. stroma Compact arrangement of collagen gives it great strength and relatively resistant to trauma both mechanical and infective Convex ridges may generate over surface if its tension is relaxed during indentation, hypotony or manipulation causes anterior corneal mosaic, polygonal or chicken-wire pattern over surface No regeneration and replaced by coarse scar tissue It is perforated many places by nerve to epithelium

17. Stroma: (Substantia propria ) About 500 μm thick (about 90% of corneal thickness) Consists of regularly arranged lamellae of collagen bundles, lie in proteoglycan ground substance with: 200 – 300 bundles – centrally 500 bundles – peripherally Width about 9 – 260 μm Height about 1.15 – 2 μm Small population of cells – keratocytes present Lamellae are arranged in layers, parallel with each other & with corneal surface In deeper stroma the lamellae form strap-like ribbons which run approximately at right angles to those in consecutive layers At the periphery this right-angular arrangement is slightly changed where it gets scleral fibres At the limbus the bundles appeared to take a circular course

18. Ultrastructural features: Each lamellae comprises of a band of collagen fibrils arranged in parallel with each other Fibrils show typical 64 nm periodicity of connective tissue collagens with a micro period of 6 nm There is a unique uniformity of fibril diameter, it is 22 (±1) nm from ant. to post. There is remarkable regularity of separation both within and between lamellae The keratocytes occupy 2.5 – 5 % of total stromal volume and is responsible for synthesis and maintaining of collagen & proteoglycan substance of stroma Keratocytes : Long , thin, flattened cells (maximally 2μm thick) running parallel to corneal surface Having long flattened nuclei, sparse cytoplasm but contains full component of organelles Position – between the lamellae There stellate processes extended for great distance and frequent contacts are made with other keratocytes in same horizontal plane forming gap junctions Lymphocytes, macrophages and polymorphonuclear leukocytes also found in stroma occasionally

19. Stromal repair: stroma after small injuries Keratocytes activation Migration & transformation into fibroblasts Production of scar tissue Initial fibrils are large & irregular Remodeling of scar tissue occurs, it ensues: 1.Thinning of fibrils 2.Reformation of lamellae over months 3.Increase in transparency Larger wounds provoke rapid vascular response and leaving vascularized scar along with lymphatic channels

20. Descemet’s membrane: ( Posterior Limiting layer) It is the basal lamina of corneal endothelium First appears at 2nd month of gestation and synthesis continue throughout adult life Thickness: at birth :- 3 – 4 μm at childhood :- about 5 μm at adult :- 10 – 12 μ m There is a distinct structural difference between fetal & postnatal components It is a strong resistant sheet -It thickens with age and in some corneal degenerative conditions -Major protein of DM is Type IV collagen

21. In adult human, the anterior 1/3rd of DM corresponds to fetal part, which is like a laminated structure and shows an irregular banded pattern in cross section In tangential section it appears to consist of superimposed plates forming a lamellar pattern Posterior 2/3rd formed after birth and consist of a homogeneous fibrogranular material The zone adjoining the endothelium is the most recently formed Modified hemidesmosomes attachment present in between DM & endothelium

22. Hassal-Henle warts: -It is the peripheral excrescence produced by focal overproduction of basal lamina like material in aging cornea -No clinical abnormality in corneal function Descemet’s warts of central cornea is called Cornea Guttata , it is associated with increased permeability of endothelium Cornea Guttata in Fuchs’ dystrophy

23. Schwalbe’s line: •The peripheral rim of DM is the internal landmark of corneal limbus and also it is the anterior limit of drainage angle, is called Schwalbe’ line. Posterior embryotoxon : •Schwalbe’s line may hypertrophied in congenital anomalies and appears as visible shelf on gonioscopy , is called posterior embryotoxon

24. Repair of Descemet’s layer : After traumatic interruption of DM (Path./Mech.) Endothelium spread its cells to resurface the defect Synthesis of fresh basal lamina which is structurally identical to normal Descemet's layer

25. Endothelium: -It is a single layer of hexagonal, cuboidal cells attached posterior aspect of DM. -It is nuroectodermal in origin. -at birth, the human endothelium compromises a monolayer of up to 500,000 cells. Endothelial cells density -about 6000 cells/mm² at birth. -26% lost in 1st year. -Further 26% lost over next 11 years. -Rate of cell loss slows and stabilizes around middle age and then it is about 2500 cells/mm². -If cells density falls below 500 cells/mm², corneal edema develops and transparency reduced. Normal endothelium

26. At birth cells are 10 μm in height, with age it becomes flattened to 3-5 μm and 18-20 μm width Single oval nucleus located centrally Cells shape is hexagonal in youth with age it become polymorphic The anterior cell membrane (Basal) is attached with DM by modified hemidesmosomes The posterior cell membrane (Apical) facing Anterior chamber shows 20-30 microvilli Lateral borders produce a complex interdigitation with neighboring cells Cell junctions with surrounding cells at lateral surfaces: Ant . 2/3rd – maculae adherents Post . 1/3rd & apicolateral edges – macculae occludentes Endothelium is rich in subcellular organelles – large number of mitochondria, both rough and smooth endoplasmic reticulum, free ribosomes, these reflects that endothelium is extremely active metabolically

27. Nutrition to endothelium: -Endothelium gets its nutrition & O₂ from aqueous -Essential nutrients (such as glucose & amino acids) pass across its surface to supply the cellular needs of all the corneal layers Fluid regulation: -The state of relative deturgescence of stroma is maintained by this delicate monolayer of cells by two ways : Providing a barrier function to the ingress of salt and metabolites to the stroma Actively reducing the osmotic pressure of stroma by metabolically pumping the bicarbonate ions out of the stroma to aqueous Endothelial Repair: -Physical & chemical damage to endothelium results in loss of cells -Neighboring cells move over to fill the gap by sliding process and enlargement of cells occur -Thus, after injury, the endothelial cell density falls, the cell area increases and the cell height decreases

28. Blood supply of Cornea: -The cornea is an avascular structure -Small loops derived from the anterior ciliary vessels invade its periphery for about 1 mm. -Actually, these loops are not in cornea but in the subconjunctival tissue which overlaps the cornea. sbj

29. Nerve supply of Cornea: Cornea is rich in sensory nerve supply derived from ophthalmic division of trigeminal nerve via anterior ciliary nerves and nerves to the surrounding conjunctiva: Anterior ciliary nerve enter the pericoroidal space a short distance behind the limbus Connect with each other & conjunctival nerve and form pericorneal plexus 60-80 myelinated branches pass into cornea After 1-2 mm lose myelin sheaths and divide into anterior and posterior groups Anterior nerves (40-50) pass through stroma and form plexus subjacent to Bowman’s layer Nerve fibers then penetrate Bowman’s layer and form subepithelial plexus Fibers then divide dichotomously to form a parallel network which run for up to 2 mm And give rise to fine free nerve terminals to superficial epithelial layers The posterior groups of nerves (40-50) pass posteriorly to innervate the posterior stroma excluding Descemet’s membrane.

30. Corneal Nutrition & Metabolism -Cornea requires energy for normal metabolic activities as well as for maintaining transparency and dehydration -Energy is generated by the breakdown of glucose in the form of ATP -Most actively metabolizing layers are epithelium & endothelium Sources of Nutrients : Oxygen : mainly from atmosphere through tear film, with minor amounts supplied by the aqueous and limbal vasculature Normal Po₂ in tears is 155 mm Hg In aqueous is about 40 mm Hg Minimum 25 mm Hg Po₂ is needed for maintaining deturgescent state and transparency Epithelium consumes O₂ 10 times faster than stroma Glucose, amino acid, vitamins, and other nutrients supplied to cornea by aqueous humor, a lesser amounts from tears or limbal vessels Glucose also derived from glycogen stores in corneal epithelium

31. Metabolic pathways: Three processes or pathways: 1.Penntose shunt (Hexose monophosphate shunt ): occurs both In hypoxic and normoxic condition (Forms NADPH and Pentose (Ribose 5-P) from glucose which are used in nucleic acid synthesis). 2.Glycolysis ( Embden meyerhof pathway): anaerobic process, glucose/glycogen converted to pyruvate yielding 2 ATPs. 3.TCA or Krebs or citric acid cycle: in aerobic conditions pyruvate is oxidized to yield 36 ATP, water and CO₂.

32. Corneal Transparency The cornea transmits nearly 100% of the light that enters it. Transparency achieved by: 1.Arrangement of stromal lamellae: Two theories – i ) Maurice (1957): The transparency of the stroma is due to the lattice arrangement of collagen fibrils. He explained, because of their small diameter and regularity of separation, back scattered light would be almost completely suppressed by destructive interference ii) Goldman et al. (1968): He suggest, a perfect crystalline lattice periodicity is not always necessary for sufficient destructive interference. He explained, if fibril separation and diameter is less than a third of the wavelength of incident light, then almost perfect transparency will ensue. This is the situation which obtains in normal cornea . 2.Corneal epithelium & tear film: •Epithelial non-keratinization •Regular & uniform arrangement of corneal epithelium •Junctions between cells & its compactness and also tear film maintain a homogenicity of its refractive index. 3.Relative deturgescence state of normal cornea. 4.Corneal avascularity. 5.Non myelinated nerve fibers.

33. Factors affecting corneal Hydration : Stromal swelling pressure exerted by GAGs. Barrier function of epithelium and endothelium. Hydration controlled by active pump mechanisms of the corneal endothelium : The enzyme pump systems are: Na ⁺/K⁺ ATPase pump system Bicarbonate dependent ATPase Carbonic anhydrase enzyme Na ⁺/H⁺ pump IV. Evaporation of water from corneal surface. V. Intraocular pressure.

34. Drug permeability across the Cornea: Factors affecting drug penetration through the cornea are: Lipid and water solubility of the drug. Molecular size, weight and concentration of drug. Ionic form of the drug. pH of the solution. Tonicity of the solution. Surface active agents. Pro-drug form.

35. The conjunctiva Vascularized mucous membrane that covers the anterior surface of the globe (bulbar and forniceal conjunctiva) and the posterior surface of the upper and lower eyelids (palpebral conjunctiva ). Conjoin = to join joins the eyeball to the lids. Functions of conjunctiva The conjunctiva helps lubricate the eye by producing mucus and tears, although a smaller volume of tears than the lacrimal gland. It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye.

36. Palpebral conjunctiva: It is richly vascular, extremely thin and strongly bounded to the tarsal plate. Marginal: Extends from the lid margin to about 2mm back of the lid up to the sulcus subtarsalis . Actually a transitional zone between skin and the conjunctiva proper Lacrimal puncta open in the marginal zone. 2. Tarsal: Thin, transparent and highly vascular Firmly adherent to the whole tarsal plate in the upper lid and only to half width of the tarsus in the lower lid The tarsal glands are seen through it as yellow streaks 3. Orbital: It lies loose between the tarsal plate and the fornix Orbital margin of the upper eyelid is loose and lies over the muller’s muscle

37. Bulbar conjunctiva: It is transparent and lies loose over the underlying structures and thus can be moved easily. It is separated from the anterior sclera by episcleral tissue and tenon’s capsule. The average thickness is 33 microns, It is also known as ocular conjunctiva. It is further of two types: Limbal A 3mm ridge of bulbar conjunctiva around the cornea. Strongly adherent to sclero -corneal junction. 2. Scleral Covers the eyeball above the anterior sclera. Thin , transparent & loosely attached to underlying sclera. Separated from the sclera by episcleral vessels and Tenon’s capsule.

38. Conjunctival fornix: It is thin, transparent , continuous circular cul de sac. It is broken (absent) only on the medial side by caruncle and the plica semilunaris . It joins the bulbar conjunctiva with the palpebral conjunctiva. It is further of four types: 1. Superior : Located at the level of superior orbital margin Extends from slightly upper border of the tarsal plate to a distance about 10mm from the upper limbus Here we can find the glands of Krause and Mullers’s muscle in the subconjunctival tissue. 2. Inferior : Extends from slightly below the lower border of the lower tarsal plate to a distance about 8mm from the lower limbus. Located near the inferior orbital margin. Helps in maintaining the recess of the inferior fornix during movements of the lower lid. 3. Lateral: Small in size like a cul de sac. Extends to just behind the equator of the eyeball It is 14mm from the lateral limbus and about 5mm from the lateral canthus 4. Medial: It is a shallow cul de sac in which lie the caruncle and plica seminlunaris dipped in pool of tears called as tear lake.

39. HISTOLOGY OF THE CONJUNCTIVA:

40. Epithelium The layers of epithelial cells in the conjunctiva vary from region to region and its different parts are: Marginal conjunctiva : Have 5 layers non keratinized stratified squamous type of epithelium o Superficial layer: squamous cell o Intermediate 3 layers: polyhedral cells o Deepest layer: goblet cells Tarsal conjunctiva: Has 2 layer epithelium in the upper eyelid o Superficial layer: cylindrical cells o Deep layers: cubical cells Lower tarsal conjunctiva is made of 3-4 layers of cells like the cubical, polygonal, elongated wedge shaped and cone shaped cells.

41. Fornix and bulbar conjunctiva 3 layered epithelium: o Superficial layer- cylindrical ells o Middle layer- polyhedral cells o Deep layer- cuboidal cells Limbal conjunctiva 8-10 layers of stratified squamous epithelium o Most superficial 1-2 layers- squamous cells o Intermediate several layers- polygonal cells o Basal layer- cylindrical or cubical cells

42. Cells Present In The Epithelium a. Goblet cells : Present between the epithelial cells in all regions of conjunctiva b . Melanocytes: Found in conjunctiva at limbus, fornix, caruncle and at the site of entry of anterior ciliary vessels c. Langerhans cells: Present in all parts of conjunctiva d. Conjunctival associated lymphoid tissue ( CALT ): Consists of T and B lymphocytes e. Mucosal associated lymphoid tissue(MALT) MALT of the gut and bronchi are also found in the conjunctiva

43. 2. Adenoid layer Also called as lymphoid layer Consists of fine connective tissue reticulum in the meshes of which lie the lymphocytes Most developed in the fornices and ends at the sub-tarsal fold Develops after 2-3 months of life 3. Fibrous layer Consists of a meshwork of collagenous and elastic fibers. Thicker than the adenoid layer, except in the tarsal conjunctiva where it is very thin. This layer consist vessels and nerves of the conjunctiva. The adenoid layer and the fibrous layer are collectively called as substantia propria .

44. Conjunctival glands

45. 1. Goblet cells Round or oval in shape with an eccentric flat nucleus. Unicellular mucous cells located abundantly within the epithelium of all regions of conjunctiva. These cells are formed from the deepest cells of the conjunctiva. Once discharging their contents (the mucin) they are destroyed. Density is more in children than adults. More in the bulbar conjunctiva and inferior fornix.

46. 2. Henle’s glands Not true glands but folds of mucous membrane present in the palpebral conjunctiva. These are tubular structures with lumen of 15-30 μ m. 3. Glands of manz Found in limbal conjunctiva in animals. 4. Glands of krause Microscopic glands that lie in the sub conjunctival tissue of the fornices. These are about 40-42 in the upper fornix and about 6-8 in the lower fornix. 5. Glands of wolfring Also called as the glands of Ciaccio These are microscopic glands present along the upper border of superior tarsus and lower border of inferior tarsus.

47. Blood supply Arteries supplying the conjunctiva are derived from 3 sources : 1. Marginal tarsal arcade of the eyelid 2. Peripheral tarsal arcade of the eyelid 3. Anterior ciliary artery The palpebral conjunctiva and the fornices are supplied by branches from the marginal and peripheral tarsal arcades of the eyelid. Bulbar conjunctiva is supplied by posterior conjunctival arteries and anterior conjunctival arteries.

48. Venous drainage The veins from conjunctiva drain into the venous plexus of eyelids which in turn drain into the superior and inferior ophthalmic veins . A circumcorneal zone of limbus drain into the anterior ciliary veins. Lymphatic drainage Lymphatics from the lateral side drain into the preauricular lymph nodes. The lymphatics from the medial side drain the submandibular lymph nodes. Nerve supply A circumcorneal zone of the conjunctiva is supplied from the long ciliary nerves. Rest of the conjunctiva is supplied by the branches from the lacrimal, infratrochlear, supratrochlear, supraorbital and the frontal nerves.

49. dense connective tissue that accounts for 5/6 th of the outer coat of the eyeball. composed of collagen bundles of varying diameter and less uniform orientation ( Opaque appearance ). Whole outer surface is covered by Tenon’s capsule. Anterior part is covered by bulbar conjunctiva. Functions: 1. protects intraocular components from trauma, light , and mechanical displacement. 2. withstands the considerable expansive force generated by the intraocular pressure maintaining the shape of the globe. 3 . provides attachment sites for the extraocular muscles. EMBRYOLOGY: Sclera derived from mesenchyme derived from neural crest. From anterior to posterior and from inside to outside. The sclera

50. Postnatal development and age related characteristics: • Postnatal : relatively thin, bluish, distensible , small, and translucent • Childhood and puberty: thicker , whiter, less distensible , larger, and more opaque • Adult: poorly distensible , opaque or translucent depending on water content • Elderly: less distensible , yellowish color and senile scleral plaques . Thickness of sclera: Differs from individual to individual also based upon age. Thinner in children than adult & female than male. Sclera is thickest posteriorly and gradually gets thin anteriorly. Posterior thickness is (1mm) Thinnest at insertion of EOMS (0.3mm) At equator (0.4-0.8mm) At limbus (0.8 mm)

51. Special regions of sclera : 1. Scleral sulcus It is a furrow on inner surface of anterior most point of sclera . It houses schlemm’s canal. 2. Scleral spur Circular rim in anterior most part of sclera lying deep to schlemm’s canal. Meridional fibers of ciliary body are attached to it . 3. Lamina cribrosa Sieve like sclera Optic nerve passes through it. During glaucoma in which IOP will rise lamina cribrosa will gradually increase in posterior curvature.

52. Tenon capsule fascial sheath of the eyeball. extends anteriorly from the limbus backward, envelopes the globe and fuses with the optic nerve dural sheath and with the sclera around the exit of the optic nerve. supports the eyeball within the orbit. permits the eyeball movement produced by the extraocular muscles .

53. Episcleral tissue Superficial aspect of sclera. It is thin, dense vascularized layer of connective tissue which covers sclera proper. thickest anterior to the rectus muscle insertions and becomes progressively thinner toward the back of the eye . Fine fibroblasts, macrophages and lymphocytes are also present in this layer. 2. Sclera proper It is an avascular structure which consists of dense bundles of collagen fibers. The band of collagen tissue cross each other in all direction. 3. Lamina fusca It is the innermost part of sclera which blends with supra choroidal and supraciliary lamina of the uveal tract . grooves for the passage of ciliary vessels and nerves (emissary canals) attached to the choroid by fine collagen fibers It is brownish in color due to presence of pigmented cells.

54. Scleral apertures 3 sets of aperture: 1. Posterior aperture- situates around Optic nerve. • Transmit long And short Ciliary nerves & vessels 2. Middle aperture- 4 in number. • Are situated slightly posterior to the equator • 4 vertex veins pass through this 3. Anterior aperture- situates 3-4mm away from limbus • Anterior ciliary vessels pass through this Scleral foramina: Anterior for the cornea Posterior for the optic nerve .

55. Vascular Supply Episclera : Anterior ciliary artery form a dense Episcleral plexus anterior to the insertion of rectus muscle . Long and short posterior ciliary artery supplies the posterior part . Scleral stroma: relatively avascular structure. Venous drainage Episcleral collecting veins Anterior ciliary veins. Vortex veins. Nerve supply Rich in nerve supply Anterior sclera : long posterior ciliary nerves which pierce it 2-4mm from the limbus to form a plexus Posterior sclera: short posterior ciliary nerves

56. Thank you