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Preksha Dhyana :
viscous fluid called the vitreous humour, lies between the lens and the retina and makes up most of the volume of the eyeball. The optic and other nerves as well as arteries that supply the eye-muscles pass through two openings at the back. Six external muscles connect the eyeball to the orbital cavity and provide movement and support. Physiology of Sight
Light is necessary for seeing. Light is reflected into the eyes by the various objects within the field of vision. The eye uses refraction' to focus the light rays it receives from the object to the retina. The four refracting media of the eye are, the cornea, the aqueous humour, the crystalline lens and the viterous humour. The process of focussing begins when the light passes through the cornea and is refracted. This is referred to as the coarse focus. The acqueous humour which is a clear dilute solution of salts (mainly sodium chloride) is renewed every few hours. It has hardly any effect on the light rays as it is of a similar density to the cornea. Its main function is to nourish the internal structures of the eye that do not possess a blood supply of their own.
The pupil determines the amount of light let in as a result of the contraction and expansion of the muscles of the iris. The crystalline lens lies immediately posterior to the iris. It is a semisolid body with biconvex surface consisting of 2000 thin layers of transparent tissue enclosed in a thin elastic capsule. To keep the object focussed on the retina the lens thins for distant vision and thickens for the near vision. In old age, it becomes denser and less elastic. As a result, old people need glasses for reading etc. In cataract it loses its transparency and blocks the passage of light rays resulting in a loss of vision Cataract is treated by removing the opaque lens and fitting an artificial intra ocular lens or compensating with external glasses.
The vitreous body is a jelly-like material enclosed in a thin membrane and fills the posterior four-fifths of the eyeball. Its function is to nourish and support the retina (which will collapse inward otherwise) and maintain the 1. Light rays which normally travel in straight lines through air, art refracted or change direction when they enter a denser medium such as water or glass.
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