Iris (eye anatomy)

The iris is an eye colored membrane which controls the amount of light that enters the eyeball and reach the retina by the contraction of its sphincter and dilator muscles. The iris is flat and divides the front of the eye into an anterior chamber posterior chamber. It is made of pigmented fibrovascular tissue called stroma.

The iris is divided into two zones: 1) pupillary Zone, which is the inner part of the iris that forms the pupil's boundary; 2) ciliary Zone, which is the remaining part of the iris that extends into the ciliary body.

The iris contains twy types of muscles. The sphincter muscle, which lies around the very edge of the pupil, contracts in bright light, causing the pupil to constrict. The dilator muscle, which runs radially through the iris, like spokes on a wheel, dilates the pupillary orifice in dim lighting.