Eye

The Eye (Latin oculus, Greek ὤψ ops ) is a sensory organ for the perception of light stimuli. It is part of the visual system and allows animals to see. Receiving the stimuli takes place by means of photoreceptors, light-sensitive nerve cells whose energization state is changed due to the different wavelengths of electromagnetic radiation from the visible spectrum. In vertebrates, these nerve impulses pass via the optic pathways to the visual center of the brain, where they are finally processed into a visual perception.

The eyes of animals partially differ significantly in structure and functionality. Your biological significance and thus their performance is closely matched to the requirements of each organism. The number of eyes is an evolutionary result of life circumstances. Some animals whose orientation is determined less by visual impressions, just need a rough distinction between light and dark, others of contrast and movement patterns. More highly developed eyes serve the high-contrast image perception, whose quality increases with the ability to brightness differences highly differentiated perceive (minimum visibile ). This expresses itself reflected in a corresponding visual acuity (minimum separabile ), which can be very different by day, dusk or night. Still others require less contrast visibility rather than a large field or a differentiated perception of color in different wavelength ranges.

The performance of the vision and the complexity of the anatomy and the overarching areas for image generation and processing increases with the requirements of the life forms on the quality of visual orientation.

• 3.1 photoreceptor in unicellular
• 3.2 light-sensitive cells
• 3.3 flat eye
• 3.4 Pigment cup eye
• 3.5 mine eye
• 3.6 hole eye and bubble eye
• 3.7 Compound Eye ( compound eye )
• 3.8 lens eye
• 3.9 vertebrate eye 3.9.1 eyeball
• 3.9.2 Notes Boards
• 3.9.3 visual pathway

Evolution of the eye

→ Main article: Eye Evolution

There are estimates that eyes of different designs have been developed about the course of evolution 40 times again. Nevertheless, Pax - 6 gene plays a role in the early development of the initiative of the eye in both the squid as well as in mammals (mice ), and insects. In the fruit fly (Drosophila melanogaster), the gene homologous thereto " eyeless " has the same function. Therefore, it stands to reason that all of these eye types have a common origin. Orthologs of PAX -6 are found in many chordates ( phylogenetic origin in the Precambrian ). Fossil findings also show that there are already 505 million years ago in the Cambrian geological era was early eyes (eg, the pinhole camera eye of the nautilus ). The first lenses were trilobites in compound eyes before 520-500 million years ago.

The primary characteristics

As a result of a visual stimulus processing properties towards vision, visual acuity, visual field and color vision are mentioned. The requirements of the life forms on these properties are pronounced very differently. In addition, many species are able to adapt their eyes with different precision to different object distances (accommodation).

Direction Watch

Some eye types are due to their anatomical and physiological development only able to identify the direction, turns out the light on their sensory cells. This feature allows for a little visual orientation, however, is over the mere perception of light and dark represents a higher possibility of differentiation

Visual acuity

Visual acuity with the ability of an organism is referred to detect contours and patterns in the outside as such. Their quality depends on

• The resolution of the eyeball
• The image quality on the retina caused by the refractive media of the eye - is determined - the cornea, aqueous humor, lens and vitreous body
• The optical properties of the object and its environment (contrast, color, brightness )
• The shape of the object: the retina and central nervous system are capable of certain forms (horizontal and vertical straight lines, right angles ) to resolve higher than that corresponding to the resolution of the eyeball alone.

To quantify one has defined various parameters. The angular visual acuity ( angular visual acuity ) is the resolution, in which two visual objects are still perceived as separate (minimum separabile ). The resolution of 1 ' ( one minute of arc ) corresponds to a spatial resolution of about 1.5 mm at 5 m distance. The smaller the angle of visual acuity, the visual acuity is better. The dimensionless characteristic visual acuity is defined by the reference 1 ' is related to the individual angular visual acuity.

Visual acuity = 1 ' / (individual angular visual acuity )

The greater the visual acuity, the better is the visual acuity. Example: if a person can only separate points at an angular distance of 2 ', it has a visual acuity of 0.5. Instead of angles, distances can be determined. If the distance D is chosen as a reference, in which ' provides two points at an angle of 1, then:

Visual acuity = individual distance / d

Example: if a person only at a distance of 6 m, the points can be seen separately, which have an angular distance of 1 m have at 12 ', it has a visual acuity of 6/12 = 0.5.

Visual field

With visual field is any central and peripheral points of the outer space, which can be visually perceived in a quiet, straight head position and straight directional, motionless look, and make different by varying the size and luminance requirements on the quality of visual perception. One distinguishes the monocular visual field of one eye of each of the sum of the fields of all the eyes of a living being. Its dimensions will be specified in the rule in degrees and partly differs very significantly depending on the particular organism. Examples of the dimension of a horizontal field of view:

• Flying almost 360 ° ( compound eyes )
• Frog 330 °
• Kestrel 300 °
• Crocodile 290 °
• Human 175 °
• Barn Owl 160 °
• Snails ( bowl and hole eyes) about 100 ° to 200 °
• Jellyfish and worms ( flat eyes) 100 ° to 180 ° greater in several eye

Color vision

Color perception is the ability to distinguish the electromagnetic wave of different wavelengths in intensity. This ability is widespread throughout the animal kingdom. The absorption spectrum of the perceived and distinct species-specific wavelengths characterizes the quality of this ability. To the perceptual system must have at least two different types of light receptors to detect the compositions of the light can.

Designs

The simplest of the "eyes" are photosensitive cells along the skin which function as passive optical systems. You can only detect whether the surroundings are light or dark. This is called skin light sense.

Insects and other arthropods have eyes that are composed of many individual eyes. This compound eyes provide a grid-like image (not multiple images, as one might suspect ).

In addition to the described types of eyes with refractive lenses can be found in nature occasionally mirror eyes. In the eyes of the scallop ( Pecten ) the image is produced by a concave mirror positioned behind the retina. The lens located directly in front of the retina is used for optical correction of the highly distorted mirror image. The mirrors are built according to the principle of reflective glass plates. More than 30 layers of finest guanine crystals are densely stacked, each layer enclosed in a double membrane. Other animals also have mirror eyes, including the deep sea Gigantocypris cancer, the lobster and crayfish. This form has apparently enforced where it is less important to the image quality and more on the light output.

Photoreceptor in unicellular

Already the unicellular Euglena has a photoreceptor for light-dark perception, which forms a bulge at the base of the flagellum. He is shielded on one side, so that even a simple direction of vision is possible through the pigments of the eye patch. This allows the cell to move towards light ( phototaxis ).

Photoreceptor cells

More sophisticated creatures like earthworms have on the body end, or scattered individual photoreceptor cells.

Flat eye

Jellyfish and starfish have a lot of light-sensitive cells juxtaposed, the inside can connect to a layer of pigment cells. The concentration of the sensory cells in such a flat eyes improves the light-dark perception.

Pigment cup eye

In pigment cup eyes the visual cells are located away from the light (inverse situation ) in a cup of opaque pigment cells. The light can only penetrate through the opening of the cup in order to stimulate the retinal cells. Therefore, since only a small part of the visual cells is stimulated, besides the brightness and the incident direction of light can be determined. Such eyes have among other flatworms and snails.

Mine eye

The mine eye is different from the pigment cup eye by the light-facing ( everse ) Location of the sensory cells and the fact that the pit is filled with secretion. In the pit, the photoreceptor cells form a cell layer that connects the inside of a layer of pigment cells. It is a further development of the flat eye. It also allows the determination of the intensity and the incident direction of light.

Eye hole and bubble eye

Hole eyes or pinhole eyes are advanced mine eyes, and operate according to the principle of the pinhole camera. Of the pit is a bubble-like indentation, the opening narrows to a small hole, and the cavity is completely filled with secretions. Due to the increased number of photoreceptor cells in a Sehzellenepithel ( retina) and image seeing is now possible. However, the image is faint, small, and stands like a camera obscura on the head. The sharpness of the image on the retina depends on the number of excited photoreceptor cells. Since it also depends on the distance from the Sehloch the subject, the eye hole a limited distance vision is possible. This eye type comes tart ago in primitive cephalopods such as the Perlbooten. A hole eye with improved performance is the bubble eye, in which the opening is covered by a transparent skin. Bladder eye may result from a invagination of the epidermis, which is lined with a pigment and a Sehzellenschicht. It occurs in coelenterates, snails and annelids. Depending on the diameter of the Sehöffnung creates either a brighter but blurrier or a darker but sharper image. The secretion can not solidified to a simple type of lens (in case of snails ). These modifications improve the image slightly.

Compound eye ( compound eye )

Compound eyes are made up of a variety of simple eyes ( ommatidia ), each of which contains eight sensory cells. Every single eye sees only a tiny part of the environment, the overall picture is a mosaic of all frames. The number of individual eyes can range from a few hundred to tens of thousands .. The resolution of the compound eye is limited by the number of eyes and is therefore far less than the resolution of the lens eye. However, the temporal resolution of compound eyes be significantly higher than that of lens eyes. It is about flying insects at 250 frames per second (that is, 250 Hz), which is about four times of the human eye at 60 to 65 Hz. This gives them a high speed response. The color sensitivity of the compound eye is shifted into the ultraviolet range. In addition, species have compound eyes on the largest field of view of all known living organisms. To find these eyes in crustaceans and insects.

In addition, many arthropods have ocelli, smaller eyes, which often are located on the center of the forehead and can be constructed very differently. In simple ocelli are mine eyes. Particularly powerful ocelli have a lens or, as in the arachnids, also a glass body, so it is small lens eyes.

Eye lens

The simplest lens eye has not been the complicated structure, which we know from the vertebrate eye. It consists of nothing more than the lens, pigment cells and retina. An example of this is the lens of the eye box jellyfish Carybdea marsupialis. In addition, the eyes look at the four sense objects at the screen edge of jellyfish in the shade inside. Nevertheless, it can thus see well enough to avoid rowing, where they could get hurt.

Also, some ocelli of arthropods are simple lens eyes.

Although similar to the eyes of vertebrates and cephalopods in construction, they have developed this very similar function independently. This can be seen in the formation of the eye in the embryo: While the eye in vertebrates developed by a protrusion of the cells that form the brain later, the eye of molluscs caused by an invagination of the outer cell layer that forms the skin later.

A toad eye has been most of the parts, which also has the human eye, only the eye muscles are absent. Therefore, a toad when seated himself quietly, to see the dormant objects, since they are not on active eye movements is capable of and the image on the retina fades in when it is stationary.

In the most highly developed lens eyes a multistage dioptric apparatus collects the light and throws it on the retina, which now contains two types of sensory cells, rods and cones. The eye to close and distance vision is made possible by a flexible lens which is stretched or compressed by zonules. The best lens eyes are found in vertebrates.

Thus, for example, in birds of prey developed the ability to see objects in a region of the retina greatly increased, which, when circling at high altitude while lurking for prey is particularly advantageous.

Nocturnal animals such as cats, owls and deer, but also sheep realized by a retro-reflecting layer ( usually green or blue) behind the retina, a gain in sensitivity, which will benefit them as nocturnal animals ( predators as prey ). See: tapetum lucidum.

In cats, one finds in addition a so-called slit diaphragm, which allows greater differences than the pinhole aperture ratio. When tags marriages but peripheral beam less than pinholes are suppressed at slit apertures, so that visual acuity at day marriages is less than optimal.

In relation to body size, the eyes in nocturnal animals are significantly larger than the diurnal.

For the performance of an eye next to the shape of the eye and the number and type of rods and cones also evaluating the perceptions through the nerve cells in the eye and in the brain and the eye movements and the position of the eyes on the head is very essential.

The analysis in the brain can vary greatly from species to species. Thus man has a lot more different areas of image processing and image recognition in the brain as a tree shrews.

Vertebrate eye

The eyes of vertebrates are very sensitive and sometimes highly developed sense organs. They are protected and are embedded in a muscle, fat and Bindegewebspolster in the bony orbits (orbit ) of the skull. In terrestrial vertebrates, the eye is protected externally by the eyelids, the blink reflex prevents damage caused by foreign bodies and other external factors. It also houses the sensitive cornea by constant wetting with tears from drying out. Also the eyelashes are designed to protect from debris, dust and smaller particles.

The organ of vision (Organon visus ) of vertebrates can be divided into three sub-units:

• The eyeball - eyeball (Latin ) or Ophthalmos (Greek )
• The appendages of the eye and
• The visual pathway.

With some exceptions, the structure of the vertebrate eye to that of humans. Nevertheless, there are in some birds, reptiles and aquatic vertebrates in some major differences in terms of functionality and performance. Externally visible, only the cornea, sclera and conjunctiva, iris and pupil, as well as the eyelids and part of the efferent tear ducts ( puncta ).

Eyeball

The eyeball ( eyeball ) is a nearly spherical body whose shell consists of three concentric layers, the sclera, choroid and retina, is that all have different tasks. The interior of the eyeball contains the vitreous ( vitreous body ), and the lentil (Lens) and is divided into anterior and posterior chamber (camera anterior and posterior bulbi). In addition, the eyeball has an optical system, called the dioptric apparatus, which makes a clear vision possible. This system is in addition to the lens and the vitreous body from the aqueous humor and the cornea.

Adnexis

The appendages of the eye are the lacrimal apparatus, the eye muscles, the conjunctiva and eyelids.

The lacrimal apparatus of terrestrial vertebrates consists of the authority responsible for the production of tears lacrimal gland, as well as the to - and efferent vessels and ducts, the lacrimal ducts that transport the tear fluid. The entire organ is used to supply the anterior segment, their purification and their protection.

In order to move the eyes, the vertebrate eye has over seven ( six in humans ) external eye muscles. They are divided into four straight and two oblique muscles of the eye that can attract the eye each in the different directions. Depending on the position of the eyes have muscles more or less pronounced main and sub-functions, which are expressed in the raising, lowering, turning sideways or lateral rotation of the eyeball. The so- induced eye movements are on the one hand, with the aim of being able to fix objects in outer space, on the other hand, to enlarge the field of view. In addition, they are a prerequisite for the development of spatial perception in some species.

The conjunctiva, also called conjunctiva is a mucous membrane in the anterior segment of the eye. It begins at the edge of the eyelid and covers the rear -facing surface of the eyeball of the eyelids. This mucous coating acts like a soft cloth and distributed during blinking the tears over the cornea, without hurting them.

The eyelid is a thin, composed of muscle, connective tissue and skin fold that can completely cover one eye to protect it, among others, by means of a reflex ( blink reflex ) against external influences and foreign bodies. It is distributed with each blink tear fluid accumulates in the form of a tear film over the front Augapfelfläche and keeps the sensitive cornea clean and moist. Fish do not have eyelids.

Visual pathway

As a visual pathway is any transmission lines and neuronal circuits of the optical system of the eye to the brain. These include the optic retina in the eye, the optic nerve to his profile at the optic chiasm, and the adjoining tract. In the lateral geniculate body of the diencephalon ( lateral geniculate body ) find the first interconnections of the visual pathway rather than outside the retina. It continues to be a so-called Gratioletsche optic radiations to the primary visual cortex.