Iris (eye)
The iris ( ancient Greek ἶρις " rainbow ", already in Galenos also "iris in the eye"; plural irides or irides ), also called iris , is the diaphragm of the eye colored by pigments . It is the front part of the second layer of tissue ( uvea ) of the eye at the transition ( limbus ) from the cornea to the white skin of the eye and separates the anterior from the posterior chamber . The inner edge of the iris forms the pupil , the eye hole, and lies on the front surface of the lens as the margin of pupillary . The smooth iris regulates the incidence of light in the eye by changing the pupil diameter ( adaptation ).
Muscles of the iris
The iris attaches its iris root to the ciliary body and leaves an opening, the pupil , in its center . Their width is regulated involuntarily by the contraction of muscles: near the edge of the pupil, the ring-shaped, parasympathetically innervated sphincter pupillae muscle leads to a narrowing of the visual opening. The sympathetically innervated dilatator pupillae muscle, which runs fan-shaped on the back of the iris, widens the pupil opening. Both cause the pupil play , the involuntary adjustment to the different lighting conditions and regulate the light radiation into the eye. The widening of the pupil is called mydriasis , the narrowing is called miosis .
Feinbau
The iris is made up of two layers. The front part - the stroma - with the exception of the pigment cells it contains, comes from the mesoderm , the rear part - the pigment sheet , also called pars iridica retinae - and the pigment cells of the stroma come from the ectoderm . The pigment contained in the pigment sheet ensures that scattered light is filtered and the intensity of the incident light is regulated, thereby improving the optics. The pigment contained in the stroma determines the color of the eyes : A high proportion of pigment in the stroma turns the iris brown, while a lower proportion makes it appear green to blue or gray.
Since the structure of the iris stroma is different in every person, the iris recognition is used for personal identification ( biometrics ) similar to the fingerprint .
The two muscles are derived from the outer layer of the embryonic eye cup and thus from the neuro ectoderm . They belong to the smooth muscles . The sphincter muscle lies in the stroma of the third of the iris near the pupil and consists of smooth muscle cells arranged in a circle around the pupil (see fig.). The dilator muscle is formed by myofilament-rich, radially arranged basal appendages of the pigment sheet.
The pseudoscientific iridology or iris diagnosis claims to be able to make statements about diseases or predispositions of the body by observing the state and the variability of the visible iris tissue. However, there is no empirical evidence that this method is effective.
Diseases
In albinism , the pigment is completely absent, so that the iris is translucent and appears reddish through the blood vessels of the eye fundus, which is also poor in pigment. The lack of pigment in this disease is also a reason for the visual impairment of albinos, since the iris cannot fulfill its diaphragm function here: Light also penetrates through the iris itself onto the rods, which are built for low brightness, and leads to glare and thus to impaired development the visual function in infancy and early childhood . In many forms of albinism, there is also a malformation of the retina with an absence of the fovea .
The (congenital or acquired) lack of the iris is called aniridia . Defects in the iris are called colobomas .
The iridocyclitis (inflammation of the iris and ciliary body) is a symptom of various rheumatic diseases such. B. Bechterew's disease , but also occurs as an independent disease.
An atrophy (iris atrophy) may also be part of the rare iridocorneal endothelial syndrome as Essential iris atrophy occur.
In order to counteract the glare sensitivity and the limitations in patients with congenital or acquired iris defects, artificial intraocular lenses with painted irises, and more recently artificial iris implants, have been used. If there are extensive defects in the iris and an artificial lens is required at the same time, implantation of an aniridia intraocular lens is an option.
See also
literature
- Theodor Axenfeld (founder), Hans Pau (ed.): Textbook and atlas of ophthalmology. With the collaboration of Rudolf Sachsenweger and others 12th, completely revised edition. Gustav Fischer, Stuttgart a. a. 1980, ISBN 3-437-00255-4 .
- Pschyrembel clinical dictionary. With clinical syndromes and nouns anatomica. = Clinical Dictionary. Edited by the publisher's dictionary editor under the direction of Christoph Zink. 256th, revised edition. de Gruyter, Berlin a. a. 1990, ISBN 3-11-010881-X .
Web links
- http://www.kaug.ovgu.de/unimagdeburg_mm/Downloads/Kliniken/KAUG/Uvea.pdf (PDF file; 146 kB)
Individual evidence
- ^ Franz Dornseiff : The Greek words in German. Berlin 1950, p. 54.
- ↑ Walther Graumann, Dieter Sasse (Ed.): Compact textbook anatomy. Volume 4: Sensory systems, skin, CNS, peripheral pathways. Schattauer, Stuttgart et al. 2005, ISBN 3-7945-2064-5 , p. 34.
- ↑ Michaele Hartmann, Maria A. Pabst, Rudolf Schmied, Hans Ch. Caluba: Cytology, Histology and Microscopic Anatomy. Light and electron microscopic image atlas. 4th revised edition. Facultas.wuv, Vienna 2009, ISBN 978-3-7089-0348-4 , p. 130.
- ^ Thomas Kohnen: Refractive Surgery. Springer, Berlin et al. 2011, ISBN 978-3-642-05405-1 , p. 22.
- ^ Review by Edzard Ernst on iris diagnostics in the Journal of the American Medical Association
- ↑ CS Mayer, AE Hoffmann: Surgical treatment with an artificial iris . In: Ophthalmologist . 2015, doi : 10.1007 / s00347-015-0123-6 ( tum.de [PDF]).
- ↑ K. Schmitz, A. Viestenz, D. Meller, W. Behrens-Baumann, K.-P. Steuhl: Aniridia intraocular lenses for eyes with traumatic iris defects . In: Springer Medizin Verlag (Hrsg.): Ophthalmologe . 2008, doi : 10.1007 / s00347-007-1683-x .