Anatomy of Sclera & Cornea. Clinical Evaluation.
Cornea & Sclera form the outer fibrous coat or the eyeball:
- Cornea 1/6 of outer coat
- Sclera 5/6 of outer coat
Cornea:
- avascular, transparent tissue
- 2/3 of refractive power
- very sensitive
- protective function to infection & trauma
- 78% water, 15% collagen, 5% other, 1% GAGs
—> 5 layers (Anterior—>Posterior):
- Anterior Epithelium
- Bauman’s layer
- Central Stroma
- Descements membrane
- Endothelium
- 1) Anterior Epithelium
- stratisfied, sq. & non-keratinized
- a) Basal cells: single layer of columnar cells at bottom
- b) Wing cells: 2-3 layers of cells with elongated processes
- c) Surface cells: 3 layers of flattened epithelial cells with microvilli & optic activity
- 2) Bowman’s membrane
- thin, superficial stromal layer of collagen fibrils
- does not regenerate and is replaced by scar tissue
- 3) Corneal stroma
- 80% of corneal thickness
- 75% composed of water
- 4) Descement’s Membrane
- highly elastic, acellular, produced by endothelium
- can regenerate
- 5) Endothelium
- monolayer of hexagonal cells
- takes nutrients from aq. humour
- cannot regenerate
- most important for corneal transparency
Blood supply:
—> conjunctival & episcleral capillary network
Nerve Supply:
—> nasociliary nerve originating from CN V
Sclera:
=> white outer covering of the eye, except for area of cornea
- occular limbus is transition between sclera & cornea
- it is covered by bulba conjunctiva
- inner surface is intact with the choroid
- function: protection and maintenance of shape
- white in color
- thickest near optic nerve
- extraocular muscles are attached to it
- —> scleral rupture due to trauma usually occurs at these insertions
3 layers of Sclera:
- 1. Episcleral tissue
- —> thin, dense vascularized connective tissue
- 2. Sclera proper
- —> avascular, dense bundles of collagen fibres
- 3. Lamina fusca
- —> brownish in color, due to presence of pigmented cells
Scleral apertures:
—> penetrated by various arteries and veins
- Posterior aperture
- —> around optic nerve
- Middle aperture
- —> posterior to the equator
- Anterior scleral aperture
- —> 3-4 mm away from the limbus
Blood Supply:
—> episclera by anterior ciliary artery
—> anterior to insertion of rectus muscles, by the long & short posterior ciliary arteries
Nerve Supply:
—> branches of the long ciliary nerves anteriorly & short ciliary nerves behind the equator