Layers of the eyeball
•The most pathetic person in the world is someone who has sight, but has no vision.
– Helen Keller
General features
•Image focusing system, composed of 1. Cornea2. Lens3. Refractive media• Internally black- prevents ‘scatter effect’• In front of iris – anterior chamber• Behind the iris – posterior chamber
•Refractive media enclosed in 3 coats1. Fibrous [sclera, cornea]2. Vascular/uveal coat [choroid, ciliary
body, iris]3. Nervous [retina]
Fibrous coat
Sclera
•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•Site of muscle insertion
•Pierced by ciliary nerves and arteries, venae vorticosae
•Almost avascular, except ehere connected to fascial sheath of eye and bulbar conjunctiva
Cornea
•Forms anterior 1/6th of fibrous coat•Transparent fibrous tissue laminae•Avascular [no transplant rejection]
Layers
1. Corneal epithelium2. 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]
Vascular coat/uveal tract
•Components \ choroid, ciliary body, irisCHOROID•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]
Nervous coat - retina
•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 epithelium2. 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
•i.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