The iris is the screen between the front chamber and the rear area of the eye. Small muscles in the iris are decisive for diameter of the central opening (the pupil).

The term pupil stands for ‘little man’, since people see themselves in the cornea reflex of someone else’s eye. The size of the pupil opening depends on the light intensity. The pupil opening also allows the circulation of the aqueous humour, which is created behind the iris screen near the corpus ciliare (ciliary body).