It surrounds the contractile center, or pupil.
Light enters through the pupil and is focused by the lens into an image on the retina.
The nerves of the iris are connected to a part of the brain known as the hypothalamus. The left eye is believed to correspond with the left side of the body and the right eye with the right side of the body. In reading the iris to gain insight to the body there are several characteristics that are studied. These include, but are not limited to layers, colors, rings, and spots. A primary theory of Iridology is that the iris is constructed in layers that represent the four stages of tissue activity.