The eye functions a lot like a camera. When you take a picture the light passes through the lens in the camera that foucsses the light on the film in the back of the camera. when the light falls on the film, a picture image is formed.
The eye works in a
similar way. The front parts of the eye namely; the cornea, pupil
and lens are transparent that allows light rays to enter the eye.
The light is then focussed on the retina. The retina is like the
film in the camera. The focussed light forms images on the retina;
just like the film of the camera. The retina is the most delicate
part of the eye which is highly sensitive to light. The retina
consists of specilized cells called rods & cones which
differentiate various colors, shape & size of objects and form
visual images. The two eyes function together in a precise co-ordinated
manner and send this visual information in the form of signals via
the optic nerve to brain, where it is processed & analysed to
form a final sharp, single visual image of any object.
The two eyes lies in a bony socket called the orbit.
Externally, the eyelids
protect the eye from any external injury or any foreign body by
action of reflux closure of lids. It also serves as an important
function of blinking & helps the tears to lubricate the ocular
surface to prevent it from drying.
sclera is the white, protective outer coat of the eye made up of
tough fibrous tissue. The sclera forms the posterior(five/sixth)
part of the outer coat of the eye.
Any loss of transparency
or opacity of the cornea due to discase process, injury or
congenital cause can lead to scattering of light or block the entry
of light rays into the eye in severe cases and cause diminution of
vision. It is the cause of
conjunctiva is a thin mucous membrane lining the under surface of
eyelids and reflected onto the anterior portion of sclera. It
contains goblet cells that secrete mucin and accessory lacrimal
Function: Secretion of tears.
Any rise of intraocular pressure beyond the physiological limits of the eye can cause progressive damage to the optic nerve resulting in progressive loss of vision. This disease process is called glaucoma.
Choroid: It is highly vascular tissue that extends from the posterior part of the ciliary body. It is made layer of choriocapillaries which is formed by branches of short posterior ciliary arteries. The innermost surface of choriocapillaries is lined by a thin elastic membrane called Bruch's membrane.
Lens: It is transparent, biconvex, avascular structure. The lens lies suspended behind the iris by delicate fibres called suspensary ligament of the lens. Structurally, the lens is enclosed in an elastic capsule. The adult lens is composed of fibres arrenged such that the older fibres get densely packed in the centre forming the nucleus, white the peripheral fibres are soft called as cortex.
Function: It's function is similar to function of lens in a camera.
Any loss of transparency or opacity of the lens and or its capsule due to any cause is called cataract.
Vitreous humour: It is a clear jelly-like substance that fills the posterior compartment of the eye between the lens and retina.
Retina: It is the most delicate, transparent membrane similar to film in a camera that captures images. It is made up of millions of nerve fibres and also light sensitive photoreceptors called rods & cones. The centre of retina called the fovea centralis is most sensitive to light stimulus that forms sharp retinal images. Microscopically, the retina is made up of ten layers
Optic nerve: It is formed of nerve fibres as continuation of axons of ganglion cells of retina.
The optic nerve is covered by three sheaths which are continous with the meninges of the brain.
Visual pathway: The light stimulus evokes a visual sensation to the photoreceptors in retina namely rods & cones and is transmitted via three sets of neurones to be processed and analysed by the visual centre in the occipital region of the brain. The nerve fibre layer of retina form the optic nerve. The light stimulus is converted into electrical signals which are transmitted via the optic nerve, optic chiasma, optic tract to reach the lateral geniculate body. Here the third order neurons carry the visual impulse along the optic radiation to the visual cortex situated in the striate calcarine cortex in the occipital lobe of the brain. Here the visual sensation is processed & analysed into a visual image.
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