Draw a neat labelled diagram of the structure of the human eye and mention the functions of any four parts.

(N/A) Cornea: $A$ thin transparent membrane that allows light to enter and refracts it to focus on the crystalline lens.
$(b)$ Crystalline lens: $A$ double convex lens that changes its focal length with the help of ciliary muscles and suspensory ligaments,enabling light rays to focus on the retina.
$(c)$ Iris: The coloured portion of the eye that regulates the size of the pupil and controls the amount of light entering the eye.
$(d)$ Retina: Consists of light-sensitive cells,known as rod and cone cells,which enable us to see in dim and bright light.