Structure and construction of The Human Eyes
The main part of the human eyes are Cornea , Iris , pupil , ciliary muscles , Eye lens , Retina and Optic nerve. The eyes ball is a approximately spherical in shape having a diameter of about 2.5 cm . It essential parts are described below :
- Cornea : cornea is the transparent Spherical membrane that covers the front of the eyes. The outer surface of cornea is convex in shape.
- Iris : iris is the coloured region between the cornea and lens. The iris controls the amount of light entering the eyes.
- Pupil : The pupil of the eye is the black circle in the centre of the iris.
- Crystalline lens : it is a double convex lens immediately behind iris. This is made of transparent concentric layers whose optical density increase towards the centre of the lens.
- Cililary Muscles : The lens is connected of the sclerotic by the ciliary muscles . These muscles change thickness of the lens by relaxing and exerting pressure.
- Aqueous humour : Anterior chamber is filled with the transparent liquid of refractive index 1.335. The liquid is called the aqueous humour.
- Vitreous humour : posterior chamber is filled with a transparent waterly liquid with little common salt having some refracted index.
- Retina : Retina is the back surface of the eyes. It's a thin layer of tissue that covers approximately 65 percent of the back of the eye . It consists of a thin membrane which is rich in nerve fibres , containing two kinds of vision cells called rods and cones and blood vessels . It is sensitive to light , for it is a continuation of the optic nerves . It serves the purpose of a sensitive screen for the reception of the image formed by the lens system of the eyes.
- Blind spot : The blind spot . It is the spot where the optic nerves enter the eyes . It is also slightly raised and insensitive to light , because it is not covered with choroid and retina.
- Optic Nerve : It is located in the back of the eye. The optic nerves relay message from our eyes to ours brain to create visual images.
Damage to an optic Nerve can lead to vision loss in one or both eyes.
Rods more in number than cones in human eyes :
There are approximately 120 million rods and 6 million cones in the human eye
rods are spread all over the rest of the retina, and are useful for Visual field as well as for dim light vision. Rods are active for vision at night
While Cones are only for color and contrast vision. They are present only in the central small portion of the retina, called Macula.cones are for daylight and colour reception.