Dioptric apparatus

Schematic structure of a vertebrate eye with its optically effective components

The dioptric apparatus ( Greek: dioptron "means to see through") is understood in physiological optics as the refractive part of the visual system of living beings, under which the optically effective ( refractive ) components of an eye are summarized. They are also known as breaking media . This not exactly centered lens system is found with different structures in the lens eyes of vertebrates and cephalopods , as well as in the complex eyes of arthropods . The dioptric apparatus includes the cornea , the lens , the vitreous humor , and the aqueous humor of the anterior and posterior chambers of the eye. The type and extent of their refraction ensure the best possible image on the retina and thus visual acuity . Different causes can lead to imaging errors and thus to a reduced visual quality of perception.

Depending on the species, there are also different mechanisms for adjusting the eye to different object distances ( accommodation ). Furthermore, their refraction ratios are adapted to the optical density of the prevailing ambient media (air, water).

Basics

Graphic representation of the refractive media in an emmetropic eye

Incident light rays from observed objects are refracted several times when they enter the eye, first through the cornea, then through the aqueous humor, then through the lens and vitreous humor. The pupil functions as a diaphragm to regulate the incidence of light . In the case of a "right-sighted" ( emmetropic ) eye, an object being viewed is reduced in size and, conversely, projected exactly onto the retinal plane. The refractive indices of the individual media are different, as are the radii of curvature, their interfaces and their distances from one another. To illustrate this optical system, the Swedish doctor Allvar Gullstrand developed the model of a so-called normal eye or reduced eye , which was based on the evaluation of many normal-sighted eyes and helped to convert anatomical peculiarities into an optical system.

In the relaxed, accommodation-free state (remote setting), the eye has a refractive power of around 59 diopters (abbreviation: dpt ), which corresponds to a focal length of 17 mm. Of this, the cornea accounts for around 43 D and the lens between 14 and 30 D. The maximum possible change in refractive power is called the range of accommodation . This indicates the area between the individual near and far point between which the eye can see objects clearly.

Optical errors, changes in the refraction ratios

Diagram of the refraction of light by a myopic (above) and a far-sighted eye (below)

Deviations from the ideal form of emmetropia lead to optical ametropia (also: refractive errors or ametropia ). As a rule, they are not pathological, but represent a functional impairment. However, there are also pathological causes. The most common forms are nearsightedness and farsightedness . They are mostly due to a disproportion between the overall length of the eye and its refractive power, which is why they are also called axial refractive errors or axial ametropia . If the eyeball is too long, the focal point of the incident light is in front of the retina, if the eye is too short behind it. In both cases, the result is a blurred image.

Another refractive error is known as astigmatism, astigmatism, or astigmatism . As a result of structural or topographical changes in the cornea or lens, this results in a distorted image, since the light rays falling into the eye are not bundled in a focal point , but rather in a focal line.

The range of accommodation slowly decreases in the course of life due to the increasing loss of elasticity of the lens and ultimately leads to presbyopia . The result is a shift of the near point away from the eye and into the distance.

Surgical removal of the lens, for example in the case of a cataract , represents a massive intervention in the functioning of the dioptric apparatus . In most cases, it is therefore replaced by an artificial implant to compensate for the lack of refractive power of the natural lens. The same applies to removing the cornea and replacing it with a transplant .

The human eye is adapted to the refractive index of air . If the refractive index of the medium surrounding the eye changes, for example when diving in water, this also results in a deterioration in image quality and thus in visual acuity.

literature

• Robert F. Schmidt, Hans-Georg Schaible et al. (Ed.): Neuro- and sensory physiology. 5th edition. Springer Verlag, 2005, ISBN 3-540-25700-4 .
• A. Waldeyer, A. Mayet, D. Graf von Keyserlingk: Human anatomy - head and neck, eye, ear, brain, arm, chest. Volume 2, Walter de Gruyter, Berlin 1993, ISBN 3-11-013233-8 , pp. 209-231.
• H. Bartels, R. Bartels: Physiology - textbook and atlas. Urban & Schwarzenberg, Oldenburg 1995, ISBN 3-541-09055-3 .
• A. Rigutti (author), C. Schöninger, M. Eder (ed.): Physiology of humans - How our body works. Kaiser, Klagenfurt 2008, ISBN 978-3-7043-9019-6 , pp. 254-258.
• Th. Axenfeld (conception), H. Pau (ed.): Textbook and atlas of ophthalmology . With the collaboration of R. Sachsenweger u. a. Gustav Fischer Verlag, Stuttgart 1980, ISBN 3-437-00255-4 .
• Albert J. Augustin: Ophthalmology . Springer Verlag, Berlin 2007, ISBN 978-3-540-30454-8 .
• Franz Grehn: Ophthalmology. 30th edition. Springer Verlag, Berlin 2008, ISBN 978-3-540-75264-6 .
• Rainer Klinke, Stefan Silbernagl: Textbook of Physiology. 4., corr. Edition. Stuttgart 2003, ISBN 3-13-796004-5 .

Individual evidence

1. ↑ G. Seitz: Investigations on the dioptric apparatus of the firefly eye. In: Journal of Comparative Physiology A: Neuroethology, Sensory, Neural, and Behavioral Physiology. Volume 62, Number 1, 1969, pp. 61-74, doi: 10.1007 / BF00298042 .
2. ↑ A. Rigutti (author), C. Schöninger, M. Eder (ed.): Physiology of humans - How our body works. Kaiser, Klagenfurt 2008, ISBN 978-3-7043-9019-6 , p. 256.
3. ^ Online Journal of Ophthalmology . Retrieved June 3, 2012.
4. ↑ A. Waldeyer, A. Mayet, D. Graf von Keyserlingk: Human anatomy - head and neck, eye, ear, brain, arm, chest. Volume 2, Walter de Gruyter, Berlin 1993, ISBN 3-11-013233-8 , p. 225.
5. ↑ Th. Axenfeld (original), H. Pau (ed.): Textbook and atlas of ophthalmology . With the collaboration of R. Sachsenweger u. a. Gustav Fischer Verlag, Stuttgart 1980, ISBN 3-437-00255-4 .

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