Choroid is the middle layer located just after the sclera. Choroid is supplied with blood vessels and it forms the iris. There is a central opening in the choroid layer, the pupil. The main function of the choroid is to supply nutrients and oxygen to the sclera and the retina. The iris is a thin, circular membrane in the eye. It has a pigmented layer behind it, which imparts a specific colour to the eye. The iris muscles help relax and contract the pupil.