Ans.Structure of Eye Eye Orbit
Our eyes are located in small portions of skull known as the orbits or eye .sockets.
Eyelids
Eyelids wipe eyes and prevent dehydration. They spread tears on eyes, which contains substances for fighting bacterial infections.
Eyelashes
Eyelashes prevent fine particles from entering eye.
The structure of eye can be divided into three main layers.
Outer Layer .
The outer layer of eyeball consists of sclera and cornea.
Sclera gives eye most of its white colour. It consists of dense connective tissue and protects the inner components of eye and maintains its shape. In the front, sclera forms the transparent cornea. Cornea admits light to the interior of eye and bends light rays so that they can be brought to a focus.
Middle Layer
The middle layer is called choroid. It contains blood vessels and gives the inner eye a dark colour. The dark colour prevents disruptive reflections within eye.
Iris and Pupil
Behind cornea,
choroid bends to form
a muscular ring, called :
iris. There is round
hole, called pupil, in the centre of iris. After striking the cornea, light passes
through the pupil. The size of pupil is adjusted by the muscles of iris. Pupil
constricts in bright light when the circular muscles of iris contract. Similarly, pupil
dilates in dim light when the radial muscles of iris contract. Figure'12.7,12.8
Lens .
Behind iris, there is a convex lens, which focuses light on retina.
Ciliary muscles and suspensory ligament
Lens is attached to ciliary muscles of eye via a ring of suspensory ligaments. To clearly see an object far away, ciliary muscles are relaxed and lens becomes less convex. When ciliary muscles contract, lens becomes more convex and round.
Inner Layer e
The inner layer is sensory and is called retina.
Rods and Cones .
Retina contains the photosensitive cells called rods and cones and associated neurons.
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Rods are sensitive to, dim light while cones are sensitive to bright light and so distinguish different colours.
Fovea and Optic disc
Retina has two points i.e. fovea and optic disc.
Fovea is a dip in retina, directly opposite to lens and is densely packed with cone cells. It is largely responsible for colour vision and sharpness. Optic disc is a point on retina where the optic nerve enters retina. There are no rods and cones at this point, that is why it is also referred to as the blind spot.
Aqueous humour and Vitreous humour
The iris divides the cavity of eye into two chambers. The anterior chamber is in front of iris i.e. between cornea and iris; whereas the posterior chamber is between iris and retina. The anterior chamber contains a clear fluid known as aqueous humour while the posterior chamber contains a jelly-like fluid known as vitreous humour. It helps maintain the shape of eye and suspends the delicate lens.