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 1/6 th 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 1 st order neuron – bipolar cell – peripheral process connected to rods and cones Synapses with 2 nd order neurons – ganglion cell Passes to thalamus [lateral geniculate body] which has 3 rd order neurons Axons pass through retrolentiform part of internal capsule to visual cortex