Human Eye- Part 2

vi) Ciliary muscles- These are the muscles which are attached to eye lens and can modify the shape of eye lens which leads to the variation in focal lengths, so that we can see the object clearly that placed at different positions. 

      Function- It helps to see the object clearly that placed at different positions by changing the shape of eye lens. 

vii) Retina- The screen on which image is formed in the eye is called retina. It is present behind the eye lens and at the back part of the eye. It is a delicate membrane in which large number of sensitive cells (rod and cone cells) are present. 

Note:- The nature of image formed on the retina is real and inverted. 

   Optics nerve -> The nerves that carries messages from the retina to the brain are called optics nerve. 

        These are of two types:- 

* Cones- Cones are the optic nerve cells that are more sensitive to bright light. 

    They help in colour vision. They are called cone cells because of its cone shaped like structure. 

* Rods- Rods are the optic nerve cells that are more sensitive to dim lights. They help in peripheral vision. They are called rod cells because of its rod shaped like structure. 

Note:- Nocturnal animals- Animals which sleep during the day and comes out at night are called nocturnal animals. 

e.g. Owls, Beaver, Bat, etc.

 -> Owls have large number of rod cells in the retina of its eye. These cells help them to see properly even in the dim light during the night. 

viii) Blind Spot- At the junction of optic nerve and retina in the eye, there are no light sensitive cells are present due to which no vision is possible at that spot. 

   This spot is known as blind spot. It is insensitive to light. 

Note:- If image of the object falls on the blind spot then it can’t be seen by the eye. It should be noted that there is an eye-lid in front of eye which act like a shutter of a camera. 

    * Eyelid (पलक)- The lid or cover of the eye or a movable fold of skin and muscle, that can be closed over the eyeball or opened at will. 

       Function- * Its main role is to protect the eyes. 

                       * It is responsible for spreading the tear film evenly across the surface of cheek. 

                       * They keep the cornea from drying out when we sleep during night by blocking the lights of surrounding. 

ix) Aqueous Humour- The space between cornea and eye lens which is filled with clear watery fluid is called aqueous humour. Since the lens and cornea have no blood supply, the aqueous humour performs the blood's job of carrying nutrients to those structures. So,

         Function- * It performs the blood's job of carrying nutrients to those structures. 

                         * It maintains the pressure within the eye.

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