આંખ

**આંખ** એ સજીવનું જોવા માટેનું અંગ છે. આંખને નેત્ર, નયન, નેણ, લોચન તરીકે પણ ઓળખવામાં આવે છે.
આંખો પ્રકાશને ઓળખી તેનું ન્યુરોન્સમાં થતા ઈલેક્ટ્રોકેમિકલ ઈમ્પલ્સમાં રૂપાંતર કરે છે. ખૂબ જ સાદા ફોટોરિસેપ્ટર્સ પ્રકાશને જાગૃત દ્રશ્યમાં થતા હલનચલન સાથે જોડે છે. ઉચ્ચ કક્ષાનાં ઓર્ગેનિઝમ્સમાં આંખ એક કોમ્પલેક્ષ સિસ્ટમ છે જે આસપાસથી પ્રકાશ ભેગો કરી તેની તીવ્રતાને રેગ્યુલેટ કરી; સ્થિતિસ્થાપક લેન્સીસનાં આયોજન વડે કેન્દ્રિત કરી ચિત્ર બનાવે છે; આ ચિત્રનું ઈલેક્ટ્રિકલ સિગ્નલમાં રૂપાંતર કરી મગજને આંખની ઓપ્ટીક નર્વ સાથે જોડતાં કોમ્પલેક્ષ ન્યુરલ માર્ગો દ્વારા મગજનાં વિઝ્યુઅલ કોર્ટેક્ષ અને બીજા ભાગોમાં મોકલાવે છે. પ્રકાશનાં ચિત્રીકરણની ક્ષમતા ધરાવતી આંખો મુખ્યત્વે દસ પ્રકારની છે અને ૯૬% પ્રાણીસૃષ્ટિ સંકુલિત દ્રષ્ટિ યોજના ધરાવે છે. મોલ્યુસ્ક, કોર્ડેટ અને આર્થરોપોડ પ્રાણીઓ ચિત્રીકરણ કરતી આંખો ધરાવે છે.

સાદામાં સાદી આંખો, જેમ કે સુક્ષ્મજીવોની આંખો પ્રકાશની હાજરી અથવા અંધકાર આ બે જ સ્થિતિ ઓળખી શકે છે. આટલી માહિતી તેમની રાત અને દિવસ સાથે જીવનનો તાલ મેળવવા માટે પૂરતી છે. વધુ કોમ્પલેક્ષ આંખો તેમનાં રેટીનલ ફોટોસેન્સિટીવ ગેન્ગલીયન કોષો દ્વારા રેટીનોહાઈપોથાલ્મીક માર્ગનો ઉપયોગ કરી સુપ્રાકીયાસ્માટીક ન્યુકલી ને રાત અને દિવસનાં ચક્ર વિષે માહિતી પહોંચાડે છે.

## Overview 
[thumb|225px|Eye of the [wisent, the European bison]]

કોમ્પલેક્ષ આંખો આકાર અને રંગનો તફાવત પારખે છે. ઘણાં સજીવોના, ખાસ કરીને શિકારી જીવોના, દૃષ્ટિક્ષેત્રોમાં ઊંડાણની પરખ માટે વિસ્તૃત બાઈનોક્યુલર દૃષ્ટિ સંકળાયેલી હોય છે, જ્યારે બીજા જીવોમા આંખોનું સ્થન એવુ હોય છે જેથી ખૂબ જ મોટું દૃષ્ટિક્ષેત્ર મળે, જેમ કે સસલાં અને ઘોડા, જેમની મોનોક્યુલર દૃષ્ટિ છે.

૬૦ અબજ (૬૦૦ મિલિયન) વર્ષો પહેલા, કેમ્બ્રિયન વિસ્ફોટના ગાળામાં પ્રથમ પ્રોટોચક્ષુ ઉદભવ્યા હતા.  પ્રાણીઓનાં સૌથી જૂનાં પૂર્વજો જરૂરી દૃષ્ટિ માટે બાયોકેમિકલ સાધનો ધરાવતાં હતાં અને વધુ અગ્ર કક્ષાની આંખો ૯૬% પ્રાણીની ત્રીસથી વધુ પ્રજાતિઓમાંથી છ  મુખ્ય ફાયલામાં ઉદભવી હતી. અને જ્યારે પૂરતો પ્રકાશ હોય ત્યારે એબરેશન ઘટાડે છે.

મોટાં ભાગનાં સેફલોપોડ પ્રાણીઓ, માછલીઓ, દ્વિચર પ્રાણીઓ અને સર્પોની આંખોનાં લેન્સ ફીક્સ આકારનાં હોય છે અને લેન્સને કેમેરાની જેમ આગળ-પાછળ કરી દૃષ્યને કેન્દ્રિત કરે છે. 

આર્થરોપોડ પ્રાણીઓમાં જોવા મળતી કમ્પાઉન્ડ આંખો અનેક સાદાં પાસાંઓની બનેલી હોય છે જેની સંખ્યા શરીરની રચના પ્રમાણે નક્કી થયેલી હોય છે. આ કમ્પાઉન્ડ આંખો અનેક ટપકાંવાળી એક જ છબી બનાવે છે અથવા એક આંખની એક તે મુજબ અનેક આંખોની અનેક છબીઓ બનાવે છે. આંખનાં દરેક પાસાંનો પોતાનો લેન્સ અને પોતાનાં પ્રકાશસંવેદનશીલ કોષો હોય છે. કેટલીક આંખો ૨૮,૦૦૦ સુધીનાં ષટકોણાકાર માં ગોઠવાયેલાં પાસાં ધરાવે છે અને ૩૬૦ અંશનો દૃષ્ટિવ્યાપ ધરાવે છે. કમ્પાઉન્ડ આંખો દૃષ્યમાં થતા નાજુક ફેરફારને પણ પારખે છે. અનેક આંખો દ્વારા મળતી દૃષ્યની માહિતી મગજમાં જોડાઈને જૂદી જાતનું અત્યંત સ્પષ્ટ ચિત્ર બનાવે છે.

મેન્ટીસ નામ્ના ઝીંગા સોથી જટીલ રંગ-દ્રશ્ય રચના ધારાવે છે. તે ઝીણવટ ભરેલી આત્યંતિક રંગપટલ ભેદની માહિતીનું વિશ્લેષણ કરે છે. હાલના સમયમાં લુપ્ત એવા ટ્રાયલોબાઈટ પણ વિશિષ્ટ પ્રકારની સંયુક્ત આંખો ધરાવતા હતા. તેઓ તેમની આંખના લેન્સના નિર્માણ મટે સાફ કેલ્સાઈટાના સ્ફટિકો વાપરતા હતા. અહીં તેઓ મૃદુ આંખો ધરાવતા અન્ય સન્ધિપાદ વર્ગના પ્રાણીઓથી જુદા પડતા હતા. આ પ્રાણીઓમાં લેન્સની સંખ્યા ભિન્ન હતી. અમુક જાતિમાં એક લેન્સ હતું, જ્યારે અમુકમાં એક આંખમાં હજારો લેન્સ હોઈ શકતાં. 

સંયુક્ત આંખની સરખામણીમાં સાદી આંખો માત્ર એક જ લેન્સ ધરાવે છે. દા. ત. ઉછળતા કરોળિયા માર્યાદિત દ્રષ્ટિ પટલ ધરાવતી મોટા આકાર સાદી આંખો ની એક જોડ ધરાવે છે. આ સાથે પરિમિતિય દ્રષ્ટિ માટે નાની નાની આંખોની અન્ય જોડીઓ ધરાવટ છે

For example, jumping spiders have a large pair of simple eyes with a narrow field of view, supported by an array of other, smaller eyes for peripheral vision. Some insect larva]]e, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. Some of the simplest eyes, called ocelli, can be found in animals like some of the snails, which cannot actually "see" in the normal sense. They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. They can distinguish between light and dark, but no more. This enables snails to keep out ofજ direct sunlight.
In organisms dwelling near deep-sea vents, compound eyes have been secondarily simplified and adapted to spot the infra-red light produced by the hot vents–in this way the bearers can spot hot springs and avoid being boiled alive. The common origin (monophyly) of all animal eyes is now widely accepted as fact. This is based upon the shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye believed to have evolved some 540 million years ago. The majority of the advancements in early eyes are believed to have taken only a few million years to develop, since the first predator to gain true imaging would have touched off an "arms race". Prey animals and competing predators alike would be at a distinct disadvantage without such capabilities and would be less likely to survive and reproduce. Hence multiple eye types and subtypes developed in parallel.

Eyes in various animals show adaption to their requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. The different forms of eye in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry.

The very earliest "eyes", called eyespots, were simple patches of photoreceptor protein in unicellular animals. In multicellular beings, multicellular eyespots evolved, physically similar to the receptor patches for taste and smell. These eyespots could only sense ambient brightness: they could distinguish light and dark, but not the direction of the lightsource.

Through gradual change, as the eyespot depressed into a shallow "cup" shape, the ability to slightly discriminate directional brightness was achieved by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of dimly distinguishing shapes.

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the eyespot, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein.

The gap between tissue layers naturally formed a bioconvex shape, an optimally ideal structure for a normal refractive index. Independently, a transparent layer and a nontransparent layer split forward from the lens: the cornea and iris. Separation of the forward layer again formed a humour, the aqueous humour. This increased refractive power and again eased circulatory problems. Formation of a nontransparent ring allowed more blood vessels, more circulation, and larger eye sizes. and some annelids possess both.

### Non-compound eyes 
Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times in vertebrates, cephalopods, annelids, crustacea and cubozoa.

#### Pit eyes 
Pit eyes, also known as stemma, are eye-spots which may be set into a pit to reduce the angles of light that enters and affects the eyespot, to allow the organism to deduce the angle of incoming light.
This fast response has led to suggestions that the ocelli of insects are used mainly in flight, because they can be used to detect sudden changes in which way is up (because light, especially UV light which is absorbed by vegetation, usually comes from above).

### Compound eyes 
[thumb|right|An image of a house fly compound eye surface by using Scanning [Electron Microscope|200px]]
thumb|right|Anatomy of the compound eye of an insect
[thumb|Arthropods such as this [Calliphora vomitoria fly have compound eyes]]

A compound eye may consist of thousands of individual photoreceptor units or ommatidia (ommatidium, singular). The image perceived is a combination of inputs from the numerous ommatidia (individual "eye units"), which are located on a convex surface, thus pointing in slightly different directions. Compared with simple eyes, compound eyes possess a very large view angle, and can detect fast movement and, in some cases, the polarization of light. Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained (assuming that they do not function as phased arrays). This can only be countered by increasing lens size and number. To see with a resolution comparable to our simple eyes, humans would require compound eyes which would each reach the size of their head.

Compound eyes fall into two groups: apposition eyes, which form multiple inverted images, and superposition eyes, which form a single erect image. Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.

#### Apposition eyes 
Apposition eyes are the most common form of eye, and are presumably the ancestral form of compound eye. They are found in all arthropod groups, although they may have evolved more than once within this phylum. It is produced by certain retinal cells. It is of rather similar composition to the cornea, but contains very few cells (mostly phagocytes which remove unwanted cellular debris in the visual field, as well as the hyalocytes of Balazs of the surface of the vitreous, which reprocess the hyaluronic acid), no blood vessels, and 98-99% of its volume is water (as opposed to 75% in the cornea) with salts, sugars, vitrosin (a type of collagen), a network of collagen type II fibers with the mucopolysaccharide hyaluronic acid, and also a wide array of proteins in micro amounts. Amazingly, with so little solid matter, it tautly holds the eye.

## Relationship to life requirements 
Eyes are generally adapted to the environment and life requirements of the organism which bears them. For instance, the distribution of photoreceptors tends to match the area in which the highest acuity is required, with horizon-scanning organisms, such as those that live on the African plains, having a horizontal line of high-density ganglia, while tree-dwelling creatures which require good all-round vision tend to have a symmetrical distribution of ganglia, with acuity decreasing outwards from the centre.

Of course, for most eye types, it is impossible to diverge from a spherical form, so only the density of optical receptors can be altered. In organisms with compound eyes, it is the number of ommatidia rather than ganglia that reflects the region of highest data acquisition.
An extension of this concept is that the eyes of predators typically have a zone of very acute vision at their centre, to assist in the identification of prey. In deep water organisms, it may not be the centre of the eye that is enlarged. The hyperiid amphipods are deep water animals that feed on organisms above them. Their eyes are almost divided into two, with the upper region thought to be involved in detecting the silhouettes of potential prey—or predators—against the faint light of the sky above. Accordingly, deeper water hyperiids, where the light against which the silhouettes must be compared is dimmer, have larger "upper-eyes", and may lose the lower portion of their eyes altogether.

## Visual acuity 
[thumb|A [Red-tailed Hawk|hawk's eye]]

Visual acuity, or resolving power, is "the ability to distinguish fine detail" and is the property of cones. It is often measured in *cycles per degree* (CPD), which measures an angular resolution, or how much an eye can differentiate one object from another in terms of visual angles. Resolution in CPD can be measured by bar charts of different numbers of white/black stripe cycles. For example, if each pattern is 1.75&nbsp;cm wide and is placed at 1 m distance from the eye, it will subtend an angle of 1 degree, so the number of white/black bar pairs on the pattern will be a measure of the cycles per degree of that pattern. The highest such number that the eye can resolve as stripes, or distinguish from a gray block, is then the measurement of visual acuity of the eye.

For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD (1.2 arcminute per line pair, or a 0.35&nbsp;mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD. A horse has higher acuity through most of the visual field of its eyes than a human has, but does not match the high acuity of the human eye's central fovea region.

Spherical aberration limits the resolution of a 7&nbsp;mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3&nbsp;mm, the spherical aberration is greatly reduced, resulting in an improved resolution of approximately 1.7 arcminutes per line pair. A resolution of 2 arcminutes per line pair, equivalent to a 1 arcminute gap in an optotype, corresponds to 20/20 (normal vision) in humans.

## Perception of colours 
"Colour vision is the faculty of the organism to distinguish lights of different spectral qualities." All organisms are restricted to a small range of electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700&nbsp;nm.
This is a rather small section of the electromagnetic spectrum, probably reflecting the submarine evolution of the organ: water blocks out all but two small windows of the EM spectrum, and there has been no evolutionary pressure among land animals to broaden this range.

The most sensitive pigment, rhodopsin, has a peak response at 500&nbsp;nm. Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioural tests can this be deduced. The eyes of vertebrates usually contain cilliary cells with c-opsins, and (bilaterian) invertebrates have rhabdomeric cells in the eye with r-opsins. However, some *ganglion* cells of vertebrates express r-opsins, suggesting that their ancestors used this pigment in vision, and that remnants survive in the eyes. Likewise, c-opsins have been found to be expressed in the *brain* of some invertebrates. They may have been expressed in ciliary cells of larval eyes, which were subsequently resorbed into the brain on metamorphosis to the adult form. C-opsins are also found in some derived bilaterian-invertebrate eyes, such as the pallial eyes of the bivalve molluscs; however, the lateral eyes (which were presumably the ancestral type for this group, if eyes evolved once there) always use r-opsins.
Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group. Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.

## સંદર્ભ 
### નોંધ 
## બાહ્ય કડીઓ 
* 
* Evolution of the eye
* Diagram of the eye 
* Webvision. The organisation of the retina and visual system. An in-depth treatment of retinal function, open to all but geared most toward graduate students.
* Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows
* A collection of resources for eye anatomy. 
* Anatomy of the eye - flash animated interactive.
* Eyes  - Eyes are organs that detect light, and send signals along the optic nerve to the visual and other areas of the brain.
* Detection of weak optical signals by the human visual system : Perspectives in Neuroscience and in Quantum Physics
* Macro photo of Eye 
* Are Your Eyes Right  February 1944 Popular Science

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