A human eyeball is made-up of three layers which are - 1) outermost sclera, 2) middle choroid and 3) innermost retina.
1) Sclera: It is made-up of dense connective tissue, anterior portion of sclera is transparent and termed as cornea.
2) Choroid: It is bluish in colour and contains many blood vessels. Its anterior part is thick and forms ciliary body which gives rise to iris. Iris is the visible portion of the eye and the portion surrounded by iris is called pupil.
3) Retina: It is the innermost layer of eyeball. It consists of three sub layers which are - ganglion cells, bipolar cells and photo-receptor cell. Photo-receptor cells are rods and cons.
Rods are sensitive for deem light and black and white vision (i.e. Scotopic) while cons are sensitive for bright light and coloured vision (i.e. Photopic).
Image source: http://www.freedomscientific.com/resources/vision-anatomy-eye.asp
