Iris, "eye color"

English: iris


The iris is the diaphragm of the optical apparatus of the eye. It has an opening in the middle that represents the pupil. The iris consists of several layers. The amount of pigment stored in the iris (dye) determines the eye color. The incidence of light on the retina is regulated by varying the size of the pupil. This is ensured by a complex interconnection of nerves and several muscles.


  1. Pigment sheet
  2. Iris stroma
  3. Ciliary body


The iris consists of the two leaves of the iris stroma and the pigment leaf. The iris stroma contains connective tissue and lies in front. There are also cells (Melanocytes) and blood vessels. This is followed by the pigment sheet, which in turn consists of two parts. At the back there is a layer of cells from the pigment epithelium that provides the color. This ensures that the iris becomes opaque. This part is responsible for the diaphragm function of the iris.
The pigment epithelium can be seen around the pupil as a pupillary fringe. If the pigment is missing, the iris appears reddish (e.g. in albinism), which is a reflection of the retina, which is reddish. The color of the pigment sheet is responsible for the color of the eyes. The anterior cell layers with their extensions form a muscle (Dilator pupillae muscle), which is responsible for enlarging the size of the pupil. There is also another muscle to constrict the pupil (Sphincter pupillae muscle).

The orris root is on the outside and merges into the ciliary body. This structure consists of two parts. The rear part (Pars plana) passes 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 over fibers (Zonular fibers) suspended from the ciliary body. The ciliary body also has processes, the cells of which (Epithelial cells) produce a liquid called aqueous humor. The iris separates the anterior eye into two chambers, i.e. the anterior and posterior chambers. Both chambers are connected through the hole in the middle of the iris, the pupil.

Illustration: Horizontal section through the left eyeball, seen from below
  1. Cornea - Cornea
  2. Dermis - Sclera
  3. Iris - iris
  4. Radiant Bodies - Corpus ciliary
  5. Choroid - Choroid
  6. Retina - retina
  7. Anterior chamber of the eye -
    Camera anterior
  8. Chamber angle -
    Angulus irodocomealis
  9. Posterior chamber of the eye -
    Camera posterior
  10. Eye lens - Lens
  11. Vitreous - Corpus vitreum
  12. Yellow spot - Macula lutea
  13. Blind spot -
    Discus nervi optici
  14. Optic nerve (2nd cranial nerve) -
    Optic nerve
  15. Main line of sight - Axis opticus
  16. Axis of the eyeball - Axis bulbi
  17. Lateral rectus eye muscle -
    Lateral rectus muscle
  18. Inner rectus eye muscle -
    Medial rectus muscle

You can find an overview of all Dr-Gumpert images at: medical illustrations


The iris has the function of a diaphragm and regulates the incidence of light in the eye. It has a hole in the middle that represents the pupil. The pupil size depends on the one hand on the time of day or brightness, and on the other hand on the activity of the autonomic nervous system.
The incidence of light is perceived by the retina, translated into electrochemical information and sent to the brain. The light information is perceived and evaluated in the brain. There the optic nerves are connected to the nerves that control the muscles, which in turn regulate the incidence of light. This interconnection is very complex and affects several nerves and muscles.
In addition, the autonomic nervous system regulates the size of the pupils. The two most important muscles for regulating the incidence of light include the pupil-expanding muscle (Dilator pupillae muscle) and the pupil-constricting muscle (Sphincter pupillae muscle). The dilating muscle is regulated by the sympathetic nervous system. This is particularly active during fight, flight, stress, fear, etc. The constricting muscle is controlled by the parasympathetic nervous system. This parasympathetic part of the autonomic nervous system predominates during rest, sleep and in the digestive phase. That is why the pupil size is small when tired and large when being active and stressed.
These mechanisms of regulation of the incidence of light are supplemented by the eyelids and their muscles. When there is very strong light, e.g. when looking into the sun, the eyelids are closed reflexively.
The color of the eyes depends on the amount of pigment. There is little pigment in a blue iris. Since the pigment does not form until the first few months after birth, newborns have blue eyes.

Function of the iris

The Function of the iris resembles the one Camera shutter. It encloses the pupil and certainly their diameter. Only the portion of the light that hits the pupil can reach the retina. Is the Iris set wide, a lot of light comes in, whereby sufficient exposure of the retina is still possible even in poor light conditions. However, the additional incident light makes the perceived image blurred. The reason for this is that the light is less bundled due to the larger opening. The depth of field decreases when the iris is wide. This means that the area in which the image is perceived as being in focus becomes smaller.

It is the other way around with one severely narrowed iris. Due to the smaller opening, light bundles fall less extensively into the eye. At the same time, less light reaches the eye overall, which makes the perceived image appear darker. The depth of field is shallower.

The The size of the iris becomes unconscious in humans about the autonomic nervous system controlled. Arbitrary control of the pupil width is therefore not possible. The width of the pupil is determined by the Lighting conditionswho looked at picture and ours emotional state certainly. If you want to look at an object up close, the pupil is narrowed, which increases the sharpness. On the other hand, if you look into the distance, the pupil is slightly widened so that more light can enter the eye. Even in the dark, the pupil is made wider, so that more light reaches the retina.

The iris can do that Amount of incident light by a factor of about ten to twenty change. Every day, however, the eye is confronted with significantly greater changes in lighting conditions (up to a factor of 1012). Therefore, further processes on the retina are necessary. This becomes clear in the morning after waking up. If you look into the bright light shortly afterwards, it blinds you. The pupil reacts to the new light conditions within milliseconds and becomes narrow. Since this is not enough on its own, the glaring light perception remains somewhat. Further processes on the retina are necessary until the eye is used to the bright light.
Also ours State of mind has an impact on the iris. The part of the autonomic nervous system that is responsible for dilating the pupil is mainly in emotionally exciting situations activated. Its messenger substances are adrenaline and noradrenaline. In exciting moments, the pupil therefore appears wide. The typical “bedroom view” is also created by widening the pupils when looking at a loved one.

How does the color of the iris come about?

The Color of the iris is through the dye Melanin certainly. This dye is used in Eyes and skin as light protection. Melanin is brownish in color and absorbs incident light. Humans do not produce a pigment of a different color. Originally therefore probably had all people initially have brown eyes.
Different colored eyes appear when im Eye less melanin is produced. Incoming light is scattered by tiny particles in the now more transparent iris. This is known as the Tyndall effect. The strength of the scattering depends on the wavelength of the light. Blue light has a particularly short wavelength and is therefore more strongly scattered than red light. Part of the scattered light is reflected. This makes the eye appear blue. It is similar with green eyes.
So the eye color depends not just the pigmentation, but also on the microscopic properties of the iris from. Since eyes of different colors are still very young in evolution, 90% of people worldwide have brown eyes. Green eyes are only represented in 2% of the world's population.


In the Heterochromia differs the Color of the iris of one eye is different from the color of the other eye. Sectoral heterochromia is also possible. Here is just a section of an iris affected. The cause is usually poor pigmentation in one of the eyes.
Since the eye color is genetic, the heterochromia can also be triggered by genetic causes. Often these are harmless variations. However, in addition to the harmless cases of heterochromia, there are also genetic diseases. These include certain pigmentation disorders. In the hereditary Waardenburg syndrome, there is one congenital heterochromia associated with hearing loss. However, heterochromia can also appear as a symptom of various diseases in the course of life.
Inflammation of the iris or adjacent tissues can cause depigmentation of the affected eye. Such inflammation of the iris can also spread to the lens. If this happens, the Cloud the lens, one speaks of grey star. A newly occurring heterochromia should therefore be examined by an ophthalmologist.