THE EYE :::
It is a sensory organ of vision that resembles purely a camera It is a globe or sphere of about 2.5 cm in diameter with a transparent membrane in the front It is composed of three distinct coats :Sclera choroid and Retina
Sclera ::
It is an opaque fibrous coat which is white in colour and forms a transparent membrane in front Known as cornea it protects delicate structures that lie inside and maintains the shape of the eyeball as well It is the thickest and strongest part of the eyeball Its inner part is pigmented and is called lamina fascia The optic nerve and blood vessels pass through the back part of the sclera
Cornea is the transparent horny coat of sclera and forms a prominent spherical surface it is circular in shape and has a small radius of curvature it is not supplied with vessels but is well supplied with sensory nerves cornea acts as a transparent window and protects the inner structures and also helps in focussing the image on the retina
Choroid ::
It is the middle vascular coat enriched with blood vessels it has pigmented layer forming inside of eye a dark chamber In the front part of the eye it forms the iris which has a central opening called pupil Attached to the choroid layer and suspended by suspensory ligaments is the lens for focussing the light rays on the retina
Iris ::
It is a muscular curtain situated in the front of the lens it helps to protect the retina it controls the amount of light entering the eye A powerful light can damage the sensitive structures and therefore iris reflexly contracts in response to bright light The iris is in continuation with the ciliary body and the choroid pigmented cells are freely scattered on the iris The pigmented layer of the iris determines the colour of the eye say blue brown or grey eyed person in the centre there is a circular aperture known as pupil The pupil is not a structure but a hole in the iris iris contains both the circular and radiating muscle fibres Contraction of the circular muscle fibres (sphinctre pupillae ) constrict the pupil and contraction of radial muscle (dilator pupillae ) dilates the pupil of the eye Circular muscle is innervated by parasympathetic fibres and radial muscle is innervated by sympathetic fibres Thus stimulation of parasympathetic fibres causes constriction of the pupil (miosis) where stimulation of sympathetic fibres causes dilatation of pupil (mydriasis)
Lens ::
It is a biconvex transparent body made up of a number of layers It is situated behind the iris and is attached to the ciliary body by means of suspensory ligaments The lens is a capsular structure that recoils when suspensory ligaments are slackened and flattens when it is taut Lens being both transparent and elastic helps in focussing the clear image on the retina for both near and distant objects by changing its covexity
Lens and iris alongwith suspensory ligaments and ciliary body divide the entire eye ball into two parts Aqueous chamber and vitreous chamber Aqueous chamber is again divided by the iris and pupil into two chambers : anterior chamber and posterior chamber The aqueous chamber consists of a watery fluid called aqueous humor This fluid gives an intraocular pressure that can be measured by tonometer This fluid from anterior chamber is drained out into the blood stream through the channel of schlem
Vitreous chamber is the space between lens and retina it is filled with jelly like albuminous fluid that keeps retina in contact with choroid and sclera
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