The cornea
- Innervated by CNV
- Nerve fibres in the cornea are unmyelinated
- Highly sensitive to touch, temperature, dryness, toxic chemicals or fumes
Corneal layers
Epithelium
- Regenerates
- Innervated
Bowman’s membrane
- Resistance to injury and infection
- Does not regenerate
Stroma
Descemet’s membrane
- Produced continuously throughout life
- Resistant to trauma, chemicals, pathological processes
- Regenerates
Endothelium
- Adjacent to anterior chamber - actively pumps water out of the stroma
- Does not regenerate
- No innervation
Embryology of the cornea
- Embryologically the cornea is the continuation of other structures
- Epithelium & Bowmans = conjuctiva
- Stroma = sclera
- Descemet’s membrane & endothelium = uveal tract (choroid, iris, ciliary body)
Corneal O2 and nutrient requirements
- The oxygen demands of the cornea are high
- Oxygen supply in the open eye:
- Mainly from the atmosphere via tears
- Aqueous humour
- Limbal and tarsal structures
- Nutrient supply
- Limbal vasculature
- Aqueous humour
- Tear film
- Oxygen in the closed eye
- Mainly supplied by tarsal vasculature
- Oxygen demand increases due to:
- The rise in temperature
- the increased rate of metabolism in the epithelium
- Overnight, normally the cornea swells
- Appears to be related to relative corneal hypoxia
- Corneal swelling subsides within 1hr of waking
Corneal-aqueous interface
- The single cell corneal endothelial layer has important function in regulating the osmotic pressure in the cornea
- Acts as a barrier, limits the entry of aq. fluid into the cornea
- While actively transporting nutrients and oxygen from the aq. to the cornea
- Also has a pump which removes excess fluid from the cornea
The crystalline lens
and
Outer capsule
It’s primary function is to focus light on the retina and maintain transparency.
Outer capsule
The outer capsule allows the transfer of nutritional material between the aq. and the lens interior. It is elastic and semi-permeable membrane, and hold and allows molding of shape of the lens.
The outer capsule of crystalline lens and accommodation
Epithelium of crystalline lens
- Lies under the capsule on anterior surface only
- Centrally, the cells are involved in the active transport of substances from the aq.
- Towards the equator cells become longer and form lens fibres
Vitreous humour
Primary function
- Transmits and refracts light
- Physical support to hold retina in place
- Storage area for metabolites
- Allows movement of substance
- Viscoelastic properties –> shock absorber
Metabolic activity
- Recent evidence suggests that it may consume oxygen
- Gel, water and fibrils
The sclera
It is a tough, white outer coat of the eye. It covers approx. 2/3rds of the eye and extends from the optic nerve to the cornea.
- Composed of connective tissue (collagen) and elastic fibres in dense bands
- Bands criss-cross parallel to the surface
- Forms a tough woven but flexible structure
- Thinnest at the optic nerve head, forming lamina cribrosa through which the axons of retinal cells pass
The iris
- The iris protects the internal eye from light by constriction of the pupil (miosis)
- Miosis also occurs during accommodation
- The action of the sphincter pupillae is mediated by stimulation of the parasympathetic nervous system
Eyelids
- The primary role of the eyelids is protection of the eye
- Protection against:
- light
- foreign objects
- loss of moisture
- action:
- blink reflex - tears
- lid closure/sleep
- They have an important role in nourishing the eye
Tears: “Tear film”
- Smooth optical refracting surface
- Nourishes and oxygenates
- Moistens and lubricates (conjuctivial goblet cells)
- Protective substances
- washes foreign bodies away
- removes metabolic waste
- dilutes noxious substances
- prevents build-up of microbial material
Function of the tear film layers
