NCERT 10th Class Science Chapter Human Eye

THE HUMAN EYE

    The construction and working of the human eye is similar to photographic camera in many respects. Human eye is almost a spherical ball, with a light bulge in the front. The structure and function of each part of the eye is given below :

8    Sclerotic : It is the outermost covering of the eye ball. It is made of white tough fibrous tissues. 
    Its function is to house and protect the vital internal parts of the eye.

8    Cornea : It is the front bulging part of the eye. It is made of transparent tissues. 
    Its function is to act as a window to the world, i.e., to allow the light to enter in the eye ball.

8    Choroid : It is a grey membrane attached to the sclerotic from the inner side. 
    Its function is to darken the eye from inside and, hence, prevent any internal reflection.

8    Optic Nerve : It is a bundle of approximately 70,000 nerves originating from the brain and entering the eye ball from behind. 
    Its function is to carry optical messages (visual messages) to the brain.

8    Retina : The optic nerve on entering the ball, spreads like a canopy, such that each nerve end attaches itself to the choroid. The nerve endings form a hemi-spherical screen called retina. These nerve endings on the retina are sensitive to visible light. On retina there are two important areas namely yellow spot and Blind spot.
    The function of retina is to receive the optical image of the object and then convert it to optical pulses. These pulses are then sent to the brain through optic nerve.

8    Yellow spot : It is a small area, facing the eye lens. It has high concentration of nerve endings and is slightly raised as well as slightly yellow in colour. 
    Its function is to form a very clear image by sending a large number of optical pulses to brain.

8    Blind Spot : It is a region on the retina, where the optic nerve enters the eye ball. It has no nerve ending and hence, is insensitive to light.     
    It does not seem to have any function. Any image formed on this spot is not visible.

8    Crystalline lens : It is a double convex lens made of transparent tissues. It is held in position by a ring of muscles, commonly called ciliary muscles. 
    Its function is to focus the images of different objects clearly on the retina.

8    Ciliary Muscles : It is a ring of muscles which holds the crystalline lens in position . When these muscles relax, they increase the focal length of the crystalline lens and vice versa. Its function is to alter the focal length of crystalline lens so that the images of the objects, situated at different distances, are clearly focused on the retina.


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