Friday, 20 January 2012

eye structure

1.The eye has both internal and external muscles. The internal muscles control focusing and pupil size. The external muscles direct the eyes to the object in mind and keep the image in a constant slight motion.

2.The outer layer of the eye is called the sclera. It is the solid, white and opaque tissue to which the eye muscles are attached. It consists of connective tissue and gives shape to the eye while protecting
its inner tissues. It contains the greater part of the eye and connects with the transparent cornea at the front of the eye.

3.The choroid is the middle layer of the eyeball. It covers the sclera inside the eyeball. The choroid consists of small blood vessels which provide oxygen and nutrients to the outer layers of the retina.

4.The retina is a thin tissue that covers the inner part of the eye. It stretches from the place where the optic nerve enters the eyeball to where the choroid connects with the iris. The retina receives light, and transforms it into nerve impulses that are transported along the optic nerve to the brain, which interprets the images.

5.The optic nerve carries the impulses, or stimuli, generated by the retina towards the brain. The optic nerve contains more than a million nerve fibres. Each nerve fibre can convey several signals to the brain at the same time. At least 50% of all nerve stimuli received by the brain come from the eyes. The place where the optic nerve leaves the eyeball is called the papilla, better known as the blind spot.

6.The blind spot is about one and a half millimetres in diameter and corresponds to the place where the optic nerve leaves the eyeball. Here there is no retina, so both the cones and the rods are not able to produce ad send nerve stimuli to the brain. Therefore light that falls on that spot is not recognised, thus the name, the blind spot.

7.The macula is located in the centre of the retina, close to the blind spot. It is very small but contains a high concentration of cones and is one of the most sensitive part of the retina. By means of it we are able to perceive minute details.

8.The blood vessels bring nutrition and oxygen to the eye.

9.The vitreous body is a clear jelly-like substance, surrounded by a thin membrane. It fills the inner part of the eye and pushes the inner layers of the eye against the outer wall.

10.The ciliary body consist of strong muscles which contracts and relaxes to focus on objects at different distances. When the muscles contract, the lens becomes more convex (bulging or curving outwards), improving the focus for closer objects. When the ciliary muscles relax, it flattens the lens, improving the focus for farther objects. The suspensory ligaments consists of strong fibres that hold the lens in its position.

11.The posterior chamber of the eye is where the aqueous humour (a clear fluid containing nutritional substances for the lens, the cornea and the vitreous humour) is produced. This part of the eye is just behind the iris.

12.The anterior chamber is the space between the cornea and the iris. Because aqueous humor is constantly produced and evacuated, the pressure in the eyeball remains constant.

13.The pupil is the dark opening that is located in the middle of the iris. The pupil controls the amount of light that enters the eye. In bright light, the pupil narrows in order to protect the retina. In lesser light, such as in a dark room, the pupil widens to allow more light to enter the eye.

14.The iris is the coloured part of the eye lying behind the cornea. The colour of the iris is determined by the number of pigment cells. If there are many of these pigments, the iris is usually brown; if there are fewer, the iris is either blue or grey.

15.The cornea is the transparent, clear tissue at the front of the eye. Light enters the eye through the cornea’s wide surface and is directed towards the pupil. Two thirds of the refractive power of the eye is due to the cornea. It contains many nerve ends, therefore it is very sensitive. To avoid drying up, tears are spread by blinking over the surface of the cornea.

16.The crystalline lens is the clear lens which hangs behind the pupil. Together with the cornea it ensures that the light rays entering the eye are concentrated and directed so as to fall exactly on the retina. This is called refraction. The flexibility of the lens enables it to become flatter or steeper, by means of the internal muscle (mentioned above), depending on the distance from the object viewed. This way, an object, whether near or far away, can be seen as a sharp image. The image on the retina is an inverted picture of the object whose light rays are received by the eye.

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