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