The Eye

The eye 

The sclera is the tough, white outer coating. The front part of the sclera is clear and allows light to enter the eye. This part is called the cornea. The conjunctiva is a thin epithelium, which lines the inside of the eyelids and the front of the sclera and is continuous with the epithelium of the cornea. The eye contains a clear liquid whose outward pressure on the sclera keeps the spherical shape of the eyeball. The liquid behind the lens is jelly-like and called vitreous humour. The aqueous humour in front of the lens is watery. The lens is a transparent structure, held in place by a ring of fibres called the suspensory ligament. Unlike the lens of a camera or a telescope, the eye lens is flexible and can change its shape. In front of the lens is a disc of tissue called the iris. It is the iris we refer to when we describe the colour of the eye as brown or blue. The iris controls how much light enters the pupil, which is a hole in the centre of the iris. The pupil lets in light to the rest of the eye. The pupil looks black because all the light entering the eye is absorbed by the black pigment in the choroid. The choroid layer, which contains many blood vessels, lies between the retina and the sclera. In the front of the eyeball, it forms the iris and the ciliary body. The ciliary body produces aqueous humour. The internal lining at the back of the eye is the retina and it consists of many thousands of cells that respond to light. When light falls on these cells, they send off nervous impulses, which travel in nerve fibres, through the optic nerve, to the brain and so give rise to the sensation of sight. The part of the retina lying directly in front of the optic nerve contains no lightsensitive cells. This region is called the blind spot. Tear glands under the top eyelid produce tear fluid. This is a dilute solution of sodium chloride and sodium hydrogencarbonate. The fluid is spread over the eye surface by the blinking of the eyelids, keeping the surface moist and washing away any dust particles or foreign bodies. Tear fluid also contains an enzyme, lysozyme, which attacks bacteria. 

https://www.khanacademy.org/science/in-in-class10th-physics/in-in-the-human-eye-and-the-colourful-world/in-in-the-human-eye/v/structure-of-human-eye