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Thursday, 14 July 2016

The Human Eye

The eye is the sense organ for sight or for vision.
It is found in the eye socket above the nose.

The parts of the human eye and their functions
1) Eyelid and Eyelash: eyelid opens and closes under voluntary or reflex action to protect the eye from injury.
_ blinking (rapid reflex) serves to distribute tears over the surface of the eye to keep the conjunctiva clean and moist.
_ eyelashes traps large air-Borne particles.

2) Conjuntiva: A thin transparent epithelium covering exposed parts including the front of the sclera.
It is a self-repairing skin kept moist and clean by tears and the reflex blinking movement.

3) Sclerotic later (sclera): A tough , non-elastic fibrous coat on the outside of the eyeball.
It is opaque and helps to maintain the shape of the eyeball by exerting an opposing force against the outward pressure exerted by the aqueous and vitreous humour.

4) Cornea: A transparent region in the front part of the Sclerotic coat.
The curved surface of the cornea refracts light passing through and causes light rays to converge at the lens.

5) Choroid layer (curious): A layer of tissue lining the inside of the Sclerotic coat.
It contains black pigment which prevents internal reflection of light in the eye and Carries a network of blood vessels which supply oxygen and nutrients to the eye.

6) Retina: A light sensitive layer of the made up of specialized nerve cells (photoreceptors): rods(for black an white vision) and the cones( for bright light and colour vision)

7) Yellow spot (fovea centralis): the region of retina where most cones are located most light sensitive region for sharp colour vision.

8) Blind spot: A non-photosensitive disc that is devoid or light sensitive cells.
It is the region where nerve fibres connecting the rods and cones leave the eyes as the optic nerve.

9) Optic nerve: Made up of sensory nerve fibres from rodes and cones in the retina.
It transmits impulse initiated by different wavelength of light to the visual region of the brain where the information is processed to form an image.

10) Lens: Made up soft transparent and elastic material which refracts light entering eye onto the retina.
It can become more or less November to focus light from near or far objects on retina.

11) Ciliary body(muscle): A ring of muscle fibres which alter the shape of the lens for focusing.

12) Suspensory ligaments
They hold lens in place and connect the lens to the ciliary body.

13) Iris: The coloured part of the eye which regulates the amount of light entering the eye.
It consist of radial and circular involuntary muscles which act antagonistically to change the size of the pupil.

14) Pupil: The round hole at the center of the iris through which light passes through from the front to the back of the eye.

15) Acqueous humour: A transparent watery liquid in the front cavity of the eye.
It act as a medium for the difusion of oxygen and nutrients from the network of blood vessels in the choroids layer to the cornea and lens.

16) Vitreous humour: A transparent jelly(mainly protein) in the rear cavity of the eye that exerts an outward pressure on the eyeball to maintain the shape of eye.
It helps to refract light which enters the eye.

17) Tear gland: they are exocrine glands which secrets water substance known as tears.
The tears which is been secreted are protective anti-bacterial fluid to lubricate the conjunctiva and cornea of the eye.

18) Eye muscles
- Holds eyes in place in their orbits.
- Allows coordinated movement of both eyes in the socket.

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