The color of the eye depends on the position and amount of pigment cells
in the iris. In the dark brown eye there is an abundance of pigment
scattered through the substance of the iris as well as in the front
layers nearest the surface. In the blue eye the pigment cells are buried
deep in the iris and are fairly plentiful in amount. The colorless eye
of the albino is the result of a deficiency of pigment in the iris.
The iris is fixed at its outer circumference, but its inner rim, which
makes the border line of the pupil, is free, so that when the iris
contracts the pupil becomes larger, since its inner free margin is drawn
outwards toward the fixed outer margin. Close up against the deeper
surface of the iris comes the crystalline lens.