Iris

Synonyms

iris, “eye color

Definition

The iris is the diaphragm of the optical apparatus of the eye. It has an opening in the center which represents the pupil. The iris consists of several layers.

The amount of pigment (dye) incorporated into the iris determines the color of the eye. By varying the size of the pupil, the incidence of light on the retina is regulated. This is achieved by a complex interconnection of nerves and several muscles.

Classification

  • Pigment Sheet
  • Irisstroma
  • Ciliary body

Anatomy

The iris consists of the two leaves Irisstroma and pigment leaf. The iris stroma contains connective tissue and is located in the front. There are also cells (melanocytes) and blood vessels.

Behind it is the pigment leaf, which in turn consists of two parts. In the back is a layer of cells from the coloring pigment epithelium. This ensures that the iris becomes opaque.

This part is responsible for the iris’ aperture function. Around the pupil, the pigment epithelium is visible as a pupillary fringe. If the pigment is missing, the iris appears reddish (e.g. in albinism), which is a reflection of the retina that appears reddish.

The color of the pigment sheet is responsible for the color of the eye. The anterior cell layers with their extensions form a muscle (Musculus dilatator pupillae), which is responsible for the dilation of the pupil size. In addition, there is another muscle (Musculus sphincter pupillae) which is responsible for the narrowing of the pupil.

The iris root lies on the outside and merges into the ciliary body. This structure consists of two parts. The rear part (pars plana) merges into the choroid.

The front part (pars plicata) contains the ciliary muscle. This muscle is responsible for the curvature of the lens and thus for the refractive power, i.e. sharp vision near and far. The lens is suspended from the ciliary body by fibers (zonula fibers).

The ciliary body also has extensions whose cells (epithelial cells) produce a fluid, the so-called aqueous humor. The iris separates the anterior eye into two chambers, i.e. anterior and posterior chamber of the eye. Both chambers are connected by the hole in the middle of the iris, the pupil.