Iris (anatomy)

iris

is the greenish-yellow area. The other structures visible are the transparent pupil (showing as black), the white outer sclera, of which the central part, the cornea, is transparent.]]

In anatomy, the iris (correct classical plural: irides) is the most visible part of the eye. It is a circular pigmented tissue containing a stroma, itself containing the sphincter around the pupil of the eye, and two layers of epithelia.

Philogeny

In the animal kingdom, only longitudinally symmetric taxa, or chordates, have an iris.

Function

This part of the anterior pole of the eye allows the pupil to open and almost close on a continuous basis. This happens in order for the iris to control the amount of light striking the central retina.

Physiological properties

Sagittal section of the pars pupillaris of a human iris (near the pupillary border), using false colors to highlight the epithelia (red.)

In fact, despite pigment content in its stroma and epithelia, the iris is slightly transparent, especially in its peripheral part (pars ciliaris) and at the level of stromal crypts (of Fuchs). Moreover, iris-transmitted light, which is a few orders of magnitude less intense, because of iris stromal absorption, is totally reflected on the anterior surface of the lens, and directed towards the processi dentati of the ora serrata, a region collectively named the "peripheral retina" which processes light only for non-visual purposes.

The iris is the only living tissue always visible naturally, with open eyes.

Color

The iris is brightly pigmented, with colours ranging from red (no pigment at all, like in oculo-cutaneous albinism), to gray, green, blue, basic brown, black, and hazel. There is only one pigment that allows the human iris to absorb almost all the colours of the rainbow, and this is the black pigment called melanin. Structurally, this huge molecule is only slightly different from its equivalent pigment found in skin and hair.

Genetics and determination of iris color

Despite periodical reports of eye color genes "discovered" the iris color remains a highly complex, combined effect of texture, pigmentation and other histological characteristics of an individual's epigenetic constitution. A person's "eye colour" is actually the colour of her or his iris, since the cornea is transparent and the sclera is rarely if ever of any other colour than white. However, the color of the iris results from only one pigment, which is black and only slightly different from melanin in hair and skin for that matter. Sometimes lipofuscin, a yellow "wear and tear" pigment also enters into the net eye color "account", especially in green eyes.

Grosso modo, the physical phenomena described by Tyndall, Raleigh and even Sir Chandrasekhar Venkata Raman, which explain the color of the sky, also explain preferential dispersion of light in the iris. Constructive interference, like in the feathers of birds, has a place in the physical theory. This, as well as full understanding of physical and constitutional characterics of the iris stroma, especially in the anterior border layer cannot be left aside in explanations of iris color. For instance, like in eye spots of butterfly wings, the same chemical component of a given color can yield dramatically different physical components of color depending only of angles of view. Last, but not least, melanin in the iris stroma is maintained in highly mobile and perfectly controlled melanosomes which radically change the diffraction pattern for light entering the iris. In conclusion, reported EYCL3 (Eye color 3 - Brown/Blue Eye Color on HC-15) and EYCL1-GEY (Eye Color 1 - Green/Blue Eye Color on HC-19) are only obvious misnomers, wishful thinking or both.

Faking the iris color

Certain eye colours are sometimes seen as being especially attractive and motif-expressing contact lenses can be worn to mask one's natural eye colour with another. They are rarely needed and almost never recommended by serious medical doctors, unless the patient's retina needs extra protection, as in aniridia.

Iris color as paternity test

As stated above, although there has been much fuss about finding the genes for eye colour, there is no simple genetic determinism for such a complex trait, as there is more to iris colour than pigmentation. Overall, there is no simple Mendelian inheritance in iris color. Consequently no serious test of paternity can be exclusively based on evaluations and even "measurements" of iris colour.

Different colors in the two eyes

An example of sectorial heterochromia iridis. In this case, a green iris with several brown areas of varying size

The occurrence of two irides with different colours is a rare situation which could also be perfectly normal and which is named heterochromia iridis. Circular sectors of strikingly different colours in the same iris happen more often and are sometimes described as heterochromia iridium. Alexander the Great and Anastasios the First were dubbed δικορος ("with two pupils") for their patent heterochromias. This particular dicoria, which is only heterochromia iridis is often confused with real polycoria (more than one pupil in the same iris, a classical evil eye). Some breeds of dog are often found with different colored irides (e.g., Siberian Huskies) which is an enhanced expression of the Waardenburg inheritance due to interbreeding.

"Red eye"

When photographed with a flash, the iris only reacts to protect the retina, and not fast enough to avoid the red eye effect. — Ophthalmologists only refer to the "red eye" as a common denomination for yet-to-be-diagnosed Iridocyclitis (and other local forms of uveitis), conjunctivis, acute glaucoma and even various forms of ocular albinism.

Iris features in biometrics

The photographic image of an iris can be used as an identifying feature. Unlike most physical characteristics, the pattern of microscopic tears in the tissue of the iris, which occur randomly as the iris grows in utero, are unique in every iris - so my left iris is as different from my right as it is from (say) my sibling's or my parent's. Proponents of this type of biometric claim an error rate so small that it is possible to identify a person (not merely verify a claimed identity) from a population of millions. Whether the technique can live up to such claims remains to be seen; much public mistrust has been caused by the unfounded rumour that the technique requires the use of laser light.

Sensory system - Visual system - Eye	Edit
Retina - Cornea - Iris - Pupil - Lens - Macula - Sclera - Optic fovea - Blind spot - Vitreous humour - Aqueous humour - Choroid - Ciliary body - Conjunctiva - Angle structure - Tapetum lucidum

Sensory system - Visual system	Edit
Eye - Optic nerve - Optic chiasm - Lateral geniculate nucleus - Visual cortex