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The cornea is a transparent, avascular, watch glass

like structure. It forms anterior 1/6 th of the outer fibrous coat of

the eyeball.

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o The anterior surface : elliptical with an average

horizontal diameter of 11.7 mm and vertical

diameter of 11 mm

o

The posterior surface of cornea is circular with anaverage diameter of 11.5 mm

o Thickness : in the center 0.52 mm

at periphery 0.7 mm

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It composed of the following 5 layers from

outside inwards.

1) Corneal epithelium

2) Bowmans membrane or anterior limitingmembrane

3) Substantia propria {corneal stroma}

4) Descemets membrane or posterior limiting

membrane

5) Endothelium

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It consists of non keratinized stratified squamousepithelium,{5 to 6 cells thick}At the sclera corneal junction

{limbus} epithelium continuous with epithelium of bulbar

conjunctiva.{ 10 cell thick}

The deepest layer is made up of columnar cells,

next 2 to 3 layers of wing or umbrella cell

Most superficial two layers of flattened cellThe surface

cells present microvilli which help retention of an

unbroken film of tear fluid to increase the refractive

surface of the eye.

The corneal epithelium regenrates rapidly &

is replaced continuously.

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2. Bowmans membrane

It forms an acellular, densely packed layer of fine

collagen fibres

12 m in thickness and binds

Covers the under lining substantia propri. It is not a true elastic membrane but simply a

condensed superficial part of the stroma.

It shows considerble resistance to infection. But

once destroyed, it does not regenerate.

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3. Stromao This layer is about 0.5 mm thickness and constitutes most of

the cornea [90% OF total thickness]o It consists collagen fibrils [ lamellae ] embedded in hydrated

matrix of proteoglycans.

o The lamella is arranged in many layers. In each layer they are

not only parallel to each other but also to the corneal planeand become continuos with scleral lamellae at the limbus.

o The alternating layers lamellae are at right angel to each

other.

o Among the lamellae keratocytes , wandering macrophages,

histiocytes, & a few leucocytes.

o All fibrils are of uniform size & embedded in a ground

substance rich in chondroitin sulphate and keratosulphate,

which helps to make the cornea transparent.

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4. Descemets membrane or posterior limiting membrane

It is a strong homogenous layer which bounds the stromaposteriorly.

It is very resistant to chemical agents, trauma and

pathological processes.Therefore, Descemetocele can maintain the integrity of

eyeball for long.

It consist of collagen and glycoprotineins. Unlike Bowmans

membrane it canregenerate.

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5. EndotheliumIt consists of a single layer of flat polygonal { mainly

hexagonal} cells .The cell density of endothelium is around 3000 cells /mm in

young adults,

Which decreases with the advancing age. This is a

considerable function rese

rve for the endothelium. Therefore, corneal

decompensation occurs only after

more then 75 % of cells are lost.

Because of the stroma tends to absorb water ,the

endothelium primary task isto pump excess water out of the stroma .

with out this pumping action, the stroma would swell with

water , become hazy, & ultimately opaque.

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Nutrition of cornea

Since the cornea is avascular, it gets nutrition by three

sources.1) Loop of capillaries at the periphery of conjuntivo corneal

junction .

2) Aqueous humor from the anterior chamber of the eye.

3) The lacrimal secretion spreading as fluid film over theanterior surface of cornea.

4) Solutes { glucose etc} enter the cornea by either simple

diffusion or active transport trough aqueous humor and by

diffusion from the perilimbal capillaries.

5) Oxygen is derived directly from air through the tear film.

This is an active process undertaken by the epithelium.

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Blood supply Cornea is avascular structure

Small loops derive from the anterior cilliary vessels invedeits periphery

For about 1 mm. actually these loops are not in the cornea

but in the sub

conjunctival tissue which overlaps the cornea.

Nerve supplyAnterior ciliary nerves { branches of opthalamic division of

5 th cranial n.

After going about 2mm in cornea the nerves lose theirmyelin sheath and divide dichotomously & form three

plexuses ..the stromal , subepithelial and

intraepithelial.

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Corneal transparencyIt is due to.

.the smoothness of the epithelium.the absence of blood vessel

.the uniform organisation of collagin fibrils of the

substantia propria

.the type of the ground substance.

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Note .New layer of human cornea discovered by researches in

university ofNottingham , by pro..Harminder Dua.

That is Duas layer

Between the corneal stroma & desment membrane.

Duas layer is a very thin layer of collagen which is

impervious to air.

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By :!!!