Generality
The iris is a thin membrane, of variable color and anular shape, visible from the front through the transparency of the cornea.
In addition to determining the color of our eyes, in fact, the iris acts as a muscular diaphragm, regulating the amount of light reaching the retina.
Relationship with other eye structures
The iris is located in the anterior chamber of the eye, posterior to the cornea and in front of the crystalline lens, which acts as a lens, allowing the light rays to be focused on the retina.
The iris surrounds the pupil and is laterally related to the sclera (white part of the eyeball) thanks to the sclerocorneal rim (or limbus). Between the cornea and the iris there is a transparent liquid, consisting of water, salts and protein substances, which is formed by secretion from the ciliary body: the aqueous humor .
The iris is divided, by convention, into two circular portions:
- ciliary margin (peripheral; it continues with the ciliary body, which helps to form the vascular tunic of the eye)
- pupillary margin (circumscribes the pupil).
The zone of passage between the anterior aspect of the iris and the posterior part of the cornea has the name of iridocorneal angle and corresponds to the periphery of the anterior chamber of the eye, situated behind the limbus.
Structure
The iris appears as a circular laminar disc, which establishes a boundary between the anterior and posterior chamber of the eyeball; it has a diameter of about 10-12 mm and an average thickness of 0.3 mm. The center is occupied by the pupillary hole.
Together with the ciliary body (which contains the muscles that allow accommodation) and the choroid (rich in blood vessels), the iris constitutes the vascular habit (uvea).
The iris is formed by three layers: endothelium, stroma and epithelium.
- The endothelium lines the anterior aspect of the iris and is in continuity with that of the posterior aspect of the cornea (endothelium of the anterior chamber).
- The stroma is the fundamental layer of the iris; it consists of fibrillar connective tissue and pigmented cells (melanocytes). The color of the eyes is determined by the density and distribution of these cells. In the subjects with blue eyes the pigment is missing in the body of the iris, which is thus crossed by the light, which bounces off the inner surface of the pigment epithelium. On the other hand, individuals with brown and black eyes have more pigmented cells.
In the stroma, then, there is the constrictor muscle (or sphincter) of the pupil, a flattened ring made up of muscular bundles that run parallel to the pupillary margin; its contraction determines miosis (narrowing of the pupil). The stroma is traversed by blood vessels and nerves.
- The epithelium is formed by an inner layer consisting of polyhedral cells, which contain small granules rich in dark pigment, and an outer layer, in direct continuation with the ciliary part of the retina.
The dilator pupil muscle is just anterior to the pigmented epithelium of the iris; this muscle is formed by myoepithelial cells and extends radially from the ciliary edge of the iris to the pupillary margin, without reaching it. Its contraction causes dilation of the pupil (mydriasis).
The contraction of the constrictor muscle of the pupil is regulated by the parasympathetic fibers of the third pair of cranial nerves, while the dilator muscle is innervated by the sympathetic system. Due to the effect of this system of muscles, the pupil allows the passage of light: it expands to let in more light ( mydriasis ) and shrinks when it has to let in a smaller quantity ( miosis ), just like a photographic lens.
Appearance and Color
The front face of the iris is visible through the cornea, like a colored disc with a black pupillary hole in the center.
The word "iris" comes from the Latin "iris", which means rainbow. This structure, in fact, consists of pigmented fabric that gives color to the eyes, varying from subject to subject. The iris can be clear (from blue to green) or brown (from brown to black), based on the amount of pigment, melanin, present in the iridium stroma (the greater the amount of pigment, the more the iris will take on a color dark next to brown) and by optical reflection and light diffraction phenomena.
The color of the iris is genetically transmitted. Brown eyes are more common (dominant), while light ones are recessive. However, being the color of the eyes a polygenic character, the transmission does not follow the simple Mendelian rules.
The anterior surface of the iris is irregular due to the presence of numerous folds and small depressions (crypts) with a radial course and can be distinguished even with the naked eye, especially when the pupillary hole is narrow. This aspect is conferred by the continuous stresses due to the dilation and contraction of the pupil.
The back face, instead, rests slightly on the crystalline lens, with which it helps to limit the posterior chamber of the eye anteriorly; this part of the iris is characterized by a uniform velvety dark brown-black lamina.
Each iris is unique
The chromatic nuances and the crypts of the iris have a high degree of individuality and provide discriminating information comparable to that of fingerprints. For this reason, iris scanning can be useful for identifying a subject.
In daily practice, the recognition of the iris could find application in airport controls and in the search for missing persons.
Iridology
Natural prevention medicine
Iridology is a non-diagnostic analysis system based on the study of the iris. According to this approach, the iris would reproduce, in its own small way, the detailed map of the human body, including information on the anatomy and functions of the various organs.
By analyzing the spots and the chromatic nuances of a person's iris, one could determine the existence of an energy deficit related to some organism or function of the organism, but not defining with certainty what is the possible underlying disease. In the field of natural prevention medicine, therefore, iridological observation can be a useful indication in the orientation towards further diagnostic investigations.
Functions
Adjustment of the amount of light entering the eye
The main function of the iris is to regulate the amount of light that penetrates inside the eye, varying the diameter of the pupil, which widens or shrinks depending on the brightness of the surrounding environment.
- In low light conditions, in the dark or at night, the iris dilator muscle (arranged radially) causes dilation of the pupil ( mydriasis ) and allows a greater amount of light to reach the retina.
- When the environment is too bright, instead, the pupil ( miosis ) shrinks through the sphincter muscle of the pupil. This allows a smaller amount of light to reach the retina, allowing for better vision.
Iris diseases
Iritis
Irritation is inflammation of the iris; may develop following a local infection or rheumatic disease. It manifests itself with ocular pain, redness, miosis and dark iris (the inflamed iris is darker than the other).
Iridocyclitis (or anterior uveitis)
Iridocyclitis is an inflammation of the iris and ciliary bodies that can cause adhesions with the crystalline lens and corneal precipitates. The symptoms of iritis are accompanied by intense photophobia, eye pain and decreased visual acuity.
Coloboma of the iris
The coloboma is a congenital defect characterized by the absence of a portion of iris, which is not perfectly welded during embryonic development.
The coloboma of the iris, in general, is not a serious pathological condition. However, very large defects can cause difficulty in modulating light and monocular diplopia (double vision of objects from one eye). In rare cases, the coloboma of the iris is a consequence of surgical procedures.
aniridia
Aniridia is characterized by the complete or partial absence of the iris. This alteration can be hereditary, sporadic or traumatic. Often, aniridia is associated with a series of ocular complications present from birth or late onset: reduction of visual acuity, macular hypoplasia and optic nerve, nystagmus, amblyopia, opacity of the lens and glaucoma.
