Structure of the human eye » The Iris

The iris is the most forwardly protuberant part of the middle layer of the eye wall. It has a shape of annulus with a centrally located hole called a pupil. The outer edge of the iris changes into an algoid corpuscle. The inner edge sets bounds to a circular hole of a pupil. Pupil is not exactly centrally located, but it is shifted slightly medially. It partially separates the front and the rear chamber of the eyeball.

Structure of the iris

The iris do not have a homogeneous structure and due to this inhomogenity there are two annuluses formed on its front side: annulus iris major and annulus iris minor. The iris is created mainly by a tissued stroma and two smooth muscles. There is a large number of vessels in stroma. There also are sympathicus and parasympathicus nerve fibers.

Histologically, the iris consists of 6 layers. The most important is a muscular layer and a pigment layer.

There is a pupil sphincter (musculus sphincter pupille) in a muscular layer. Predominantly it has circular muscle fibers and a pupil extender (musculus dilataor pupille) with fibers spokewise arranged. A pupil constriction is known as miosis and a pupil extension as mydriasis.

The interplay of the two muscles determines the size of the pupil and reflectively regulates the amount of light passing through the eye. The pupil reflex (pupillary reflex) is invoked by changing light intensity and it is a result of the integration of sympathicus and parasympathicus. The pupil reflex center is in the midbrain.

Color of the iris

Color of the iris depends on the amount and the depth of a pigment location. If the pigment is missing ( e.g. albinism), color appears as pink because the red color of choroid shows through the iris. If the pigment is situated in bottommost layers in a small amount, the iris is blue or gray. Conversely, larger number of pigment gives the iris green or brown-green color. The presence of a large amount of pigment predicts dark brown color. Exceptionally, one person may have differently colored the iris in eyes - inequable iris color. A pigment layer of the iris is important for optical properties of the eye because it prevents light to come through the outside of the eye pupil.