HUMAN EYE: STRUCTURE AND FEATURES

Eye is one of our most wonder sense organ in our lives. By the help of eyes we can see our surroundings, they are trees, clouds, rainbows, rivers, ponds, plants, people and birds flying in the blue sky, at night moon and stars and many other things. And it is the sense of sight that helps us to read and write etc. But how the blind persons read or write we will discuss later. So it is important to understand its structure and working.  We see things only when light coming from them enters ours eyes. The eye has a roughly spherical shape. The outer coat of the eye is white. It is tough so that it can protect the interior of the eye from accidents. Its transparent front part is called cornea. Behind the cornea, a dark muscular structure is present called iris. In the iris, there is a small opening called pupil.

·         The size of pupil is controlled by the iris.

·         The iris is that part of eye which gives it, its distinctive color.

·         When we say that a person has green eye, we refer actually to the iris.

·         The iris controls the amount of light entering into the eye.

Behind the pupil if the eye is a lens which is thicker in the centre. The lens is convex ones and it focuses light on the black of the eye, on a layer called retina. There are many functions of retina, following are some of them:

·         It contains several neurons (a type of cell that carries information within brain and between the brain and other parts of body and it is also called nerve cells).

·         The sensations felt by the nerve cells are then transmitted to the brain through the optic nerve, as you can see in the diagram.

·         There are two kinds of cells-cones, which are sensitive to bright light and rods, which are sensitive to dim light.

 

Cones sense color. At the joint of the optic nerve and the retina, there are no sensory cells, so no vision is possible is at that spot. This is called blind spot. The appearance of an image does not clear immediately from the retina. It persists there for about 1/16 of a second. So, if still images of a moving object are flashed on the eye at a rate faster than 16 per second, then the eye perceives this object as moving. The movies that we see are actually a number of separate pictures in proper sequence. They are made to move across the eye usually at the rate of 25 images per second (at the speed which is faster than 16 per second). So, we see a moving picture.

                                                                                                                                                                                                                         As we have discussed earlier that eyes are very important for our survival. The Mother Nature has provided eyes with eyelids to protect eyes and from any possible dangers. Eyelids shut when required. Eye is essential organ because normally, we can clearly see distant objects as well as obstacles which are nearby. There are variations in the minimum range of distances with ages of persons. The most comfortable distance at which a person can read or write with normal eye is about 25 centimeters.

                                                                                                                                                                                                                                     Sometimes, especially in old age, eyesight becomes foggy. It is due to the eye lens becoming cloudy. When it happens, persons are said to have cataract. There is a loss of vision, sometimes extremely severe. It is possible to treat this defect. The opaque lens is inserted. Modern technology has made the procedure simpler and safer.

Do you know?

If food is deficient in some components eyes may suffer. Lack of Vitamin-A in foodstuff is responsible for many eye troubles. Most common amongst them is night blindness. Raw carrot, broccoli and green vegetables (such as spinach) and cod oil are rich in Vitamin-A. Eggs, milk, crud, cheese, butter, and fruits such as papaya and mango are rich in Vitamin-A.

 

What is the Braille system?

 A system of printing for blind people in which the letters of the alphabet and the numbers are printed as raised dots that can be read by touching them. Louis Braille, himself a visually challenged person, developed a system for visually challenged persons and published it in 1821. The present system was adopted in the year 1932. There is Braille code for common languages, mathematics and scientific notation. Many Indian languages can be read using the Braille system. Braille system has 63 dot pattern or characters. Each character represents a letter, a combination of grammatical sign. Dots are arranged in cells of two vertical rows of three dots each.  

                        Patterns of dots to represent some English letters and some common words. These patterns when embossed on Braille sheets help visually challenged persons to recognize words by touching. To make them easier to touch, the dots are raised slightly. Braille texts can be produced by hand or by machine. Typewriter-such devices and printing machines have now been developed.