Layers of the eyeball
• The most pathetic person in the world is
someone who has sight, but has no vision.
– Helen Keller
Eye and camera
• Both deal with similar sets of issues
• Maintaining a stable relationship between a
focusing apparatus and a focusing apparatus
• Focusing on near and far objects
• Regulating the amount of light reaching the
photosensitive surface
• Recording the pattern of incoming light
General features
• Image focusing system, composed of
1. Cornea
2. Lens
3. Refractive media
• Internally black- prevents ‘scatter effect’
• In front of iris – anterior chamber
• Behind the iris – posterior chamber
• Refractive media enclosed in 3 coats
1. Fibrous [sclera, cornea]
2. Vascular/uveal coat [choroid, ciliary body, iris]
3. Nervous [retina]
Sclera[‘white of the eye’]
• Posterior 5/6ths
• Opaque - composed of dense collagen and
elastic fibres
• Thinnest at equator
• Pierced by recti muscles
• Thickest at back, except where pierced by fibres
of CN II [lamina cribrosa]
• ‘cupping’ of optic disc= posterior bulging of disc
in sustained ↑in intraocular pressure
• Blends with dura mater
• Continues posteriorly as the sheath of CN II
• Site of muscle insertion
• Pierced by ciliary nerves and arteries, venae
vorticosae
• Almost avascular, except where connected to
fascial sheath of eye and bulbar conjunctiva
Cornea
• Limbus is a transition zone between sclera and
cornea
• Beginning from limbus, cornea forms anterior
1/6th of fibrous coat
• Transparent fibrous tissue laminae
• Avascular [no transplant rejection]
Layers
1. Corneal epithelium
2. Bowman’s membrane /anterior limiting
membrane; scattered collagen fibrils and
ground substance
3. Corneal stroma/substantia propria
• 200 collagen fibril lamellae
• Scattered fibroblasts
• Transparency because of lattice arrangement
4. Descemet’s membrane /posterior limiting
membrane
5. Corneal endothelium
Nerve supply
• Short and long ciliary nerves
• Mainly short ciliary
• Corneal reflex pathway; short ciliary nerves→
trigeminal ganglion→ main CN V sensory
nucleus→ reticular formation→ both CN VII
motor nuclei [both orbicularis oculi muscles
act]
Uvea/uveal tract
• Heavily vascularised
• Similar to arachnoid and pia
• Principal route through which blood vessels and
nerves [other than CN II]
• Components \ choroid, ciliary body, iris
CHOROID
• Thin, pigmented
• Outer layer separated from sclera by
suprachoroid lamina [delicate connective tissue]
• Inner layer firmly attached to pigmented layer of
retina
• Rods and cones nourished by choroidal
capillaries
• Venae voricosae [4-5] drain choroid- exit
through sclera
Ciliary body
•
•
•
•
Continuous with choroid behind and iris in front
Like a flat ring applied to inner scleral surface
Thick in front, thin behind
Triangular ;2 lond sides in contact with sclera
and vitroeus
• Attachment of iris halfway along flat anterior
short base
• Ciliary muscle in scleral surface
• Vitreous surface – bilayered epithelium [outer
pigmented, inner nonpigmented]
• Layers represent pigment and nervous layers of
retina
• Scleral surface projected into70-80 ciliary
processes that lie in reciprocal grooves on
anterior surface of vitreous body
Iris
• Attached at periphery to anterior surface of
ciliary body and a narrow rim of sclera to form
iridocorneal angle of anterior chamber
• Perforated centrally by pupil
• Main bulk- vascular connective tissue connective
tissue
• Amount of melanin granules increases from
anterior to posterior
• Amount of pigment increases with age
• Color is variable in different individuals
Sphincter pupillae
• Circular smooth muscle
• Supplied from Edinger – Westphal nucleus of
CN III
Dilator pupillae
• Radial smooth muscle
• Supplied by cervical sympathetics
• Preganglionic neurons lie in T1 segment of
spinal cord
Trabecular meshwork and scleral
venous sinus
Lens
•
•
•
•
Transparent, biconvex
More convex posteriorly
Transparent ,elastic capsule
Posteriorly rests on vitreous, anteriorly in
contact with iris
• 10 mm dia., 4 mm thick
• Centrally , single layer of cubical cells
• Peripherally , cells elongate to produce fibres
• Increase in length leads to increase in lens
substance
Suspensory ligament/zonule
• Series of delicate fibrils attached to ciliary
processes and through the furrows between
them, further back on ciliary body
• Most fibres attach themselves to the lens- mostly
in front and a few behind the circumference
• Holds lens flattened under tension
• Contraction of ciliary muscle→ forward
displacement of choroid and ciliary body
• This relieves some tension exerted by zonule on
the lens; makes it more globular→ increased
refractive power [Accomodation]
•
•
•
•
•
Delicate
Outer surface in contact with choroid
Inner surface in contact with vitreous
Ora serrata- anterior limit of light ssensitive area
Beyond ora serrata- thin light insensitive layer
continues as epithelial layers of ciliary body and
iris
Retina - components
1. Retinal pigment epithelium
2. Neural retina
Retinal pigment epithelium
• Outer layer
• Simple cuboidal melanin-containing cells
• Firm attachment to choroid via Bruch’s
membrane [thin refractile layer –multilaminar]
Neural retina
• Contains light – sensitive receptors [ rods and
cones] + complex neuronal networks
• Potential space exists between neural retina and
RPE
• Layers can be separated mechanically
• Eye disease or trauma also leads to separation
[Retinal detachment]
Components of neural retina
• Nonvisual part; anterior to ora serrata- lines
inner aspect of ciliary body and posterior surface
of iris
• Photosensitive /visual part; lines inner
surface of eye posterior to ora serrata, except
where it is pierced by CN II
Optic disc
•
•
•
•
1.5 mm dia.
Site of entry of CN II
Overlies lamina cribrosa of sclera
Deepened to a variable degree to form a
‘physiological’ cup
• Insensitive to light – ‘blind spot’
Fundus
• Disc and whole of surrounding area at the back
of the eye seen with ophthalmoscope
Macula lutea –
• yellowish shallow depression, avascular
• 3mm lateral to optic disc
Fovea centralis
•
•
•
•
•
Shallow central pit in macula
Thinnest area of retina
Avascular
No rods
High concentration of cones=site of most acute
vision
Arrangement
• Outer layer- pigmented cells attached to choroid
• Not a firm attachment
• In retinal detachment- pigmented cells remain
in position; rods and cones and other layers
displaced onwards
Physiological arrangement
• Similar to any sensory pathway
• 1st order neuron – bipolar cell – peripheral
process connected to rods and cones
• Synapses with 2nd order neurons – ganglion cell
• Passes to thalamus [lateral geniculate body]
which has 3rd order neurons
• Axons pass through retrolentiform part of
internal capsule to visual cortex