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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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