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The world seen in the eyes of the animals - The structure and the functions of the visual apparatus

biology

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 The world seen in the eyes of the animals

The structure and the functions of the visual apparatus

The visual apparatus, the most important sensitive organ, informs the central nervous system about all changes that happen in the surrounding environment.

It functions on the principle of cybernetic system, meaning that in the exterior, it has the ocular globe, named “the camera”, then message transmission paths and cortical centers of image interpretation.

The ocular globe has a spherical form, in general, and it has a structure made of three membranes:

Ø      the external membrane, named the sclerotic, nacreous-white, filamentous and resistant, inextensible at adults, but slightly extensible in the first childhood, it is also known as the sclera shell, because it maintains the form of the ocular globe and it has been compared to the bone system from other parts of the body. It is made of elastic-conjunctive fibers, knitted in four strata, in a balloon network system that gives its resistance and opaque, not transparent state. These fibers are made by the secretion of collagen substances and mucoplysacacchrides of some cells named fibercytes. If genetically these cells do not have a normal synthesizing message of these substances, reduced resistance fibers appear and because of the pressure of the contents of the ocular globe, they lengthen or modify their form, leading to refraction disturbances as short-sightedness, astigmatism, keratocone.

The physiologic role of the sclerotic is to protect the other ocular components.

The external membrane, the sclerotic, at the prior pole modifies its structure by placing the fibers in a parallel system, making this part to be transparent. This zone is named the transparent cornea, through which light penetrates, the specific eye stimulant. Cornea could be compared to the window of a room.

Cornea has its main optic role to allow luminous radiation penetration, and by its 40 dyoptric refraction power to deviate light trajectory, in order to get into the retina.

Ø      the central membrane is named the uveea and it is divided into the prior uveea and the posterior uveea. The prior uveea has two elements: the ciliary body and the iris.

The ciliary body is made of ciliary muscles and ciliary processes.

The ciliary muscles are smooth, disobedient to will, have a reflex function and very fine relation with the transparent crystalline lens. They have the most important optic ocular function: correcting the image that we see from any distance. This is the function of visual accommodation, necessary to clear eyesight, from any distance that we like. The muscles act by contracting or relaxing the crystalline lens.

The ciliary processes, very well vascularized, secrete the watery humour, necessary to the maintenance of normal intraocular pressure, as well as the nutrition of the formations that do not have vessels, as cornea or the crystalline lens.

The iris, the diaphragmatic membrane, situated vertically in front of the crystalline lens, is differently colored from one subject to another and from one race to another. In the middle of it, the iris has an orifice named the pupil.

The pupil can reduce or enlarge its diameter, according to the light from the exterior environment, having reflexively the role to doze the light quantity that penetrates the eye interior, up to the retina.

The posterior uveea, known as the choroide, is similar to a vascular sponge, because it contains, almost entirely, only vessels of different sizes, having the role to feed the retina and the other ocular components.

The choroide contains a pigment, which realizes the so-called obscure chamber of the eye.

Ø      the third membrane of the eye is the retina, of nervous type, made of ten strata, in which there are three types of neurons: the first ones are the cone and the stick, the second one is the bipolar cell and the third one is the ganglionary cell.

At this level, it is made the transformation of light radiation into electrical energy that transmits the visual message to the cerebral cortex. The cones and the sticks are the most important neurons and contain the photosensitive substances: iodopsin and rodopsin, substances hat have in their composition as essential element A vitamin. The cones are located in the center back of the eye, in the so-called zone “optical macula” (optical spot).

There are in general up to 8000000 cells that perceive the form of the elements (sense of forms) and distinguish monochromatic light, the colors (chromatic sense).

The sticks are over 60000000 elements and have the property to perceive light intensities more and more reduced, in other words they allow orientation in reduced light or dark (luminous sense).

The stimulant of these neuron cells is electromagnetic radiation that is white composed light, formed of very fine particles, called photons or light quantum.

Light quantum penetrates the eye through transparent and refracting media, as cornea, the watery humour, the crystalline lens, or the vitreous body. When it gets to the cones and sticks, it determines a micro bombing, because it has mass and velocity, cutting the photosensitive substance molecule (iodopsin and rodopsin). It follows a photophysicalchemical transformation, made by paramagnetic resonance and the electrons are thrown on external orbits, determining a potential difference. This potential difference makes the transmition, through the other neurons (bipolar and ganglionary cells), optical nerves and optical paths, up to the cerebral cortex, where the image is formed by psychical mechanism.

This piezo-electronic property of the retinal neurons gives the possibility of light transformation into electrical energy, taking the visual message to the cerebral cortex.

So, the visual image, the way it reflects the surrounding environment, is complete and it is formed in an optical step (transparent and refracting media), a physiological step (the mechanisms inside the retina neurons) and a psychical step (message interpretation in the cerebral cortex).

The optical ocular component, formed by the transparent cornea, is situated in the prior pole of the ocular globe, 40 dyoptric power refracting, can direct luminous rays. The watery humour, which is only transparent, is situated behind the cornea, in the so-called prior room of the eye, having the role only to lead luminous rays.

The third and very important element is the convex crystalline lens, transparent and 20 dyoptric power refracting.

This lens is connected with very fine fibers to the ciliary muscles. The crystalline is situated behind the iris and it has the possibility, by reflex contraction of the ciliary muscle, to increase its refracting power (it bulges), allowing in this way close eyesight, or to diminish its refracting power (it flattens), allowing in this way distant eyesight.

This property is the property of visual accommodation, which allows us to see clearly from any distance that we look.

Behind the crystalline lens and the rest of the ocular content is the vitreous body, or the glassy humour, a transparent element because of a chemical collagenic edifice, with a very fine structure, without any other structural element or vessels. The role of the vitreous body is to allow the rays of light to get to the retinal neurons.

Annexes of the ocular globe

In order to function properly, the ocular globe, or “the camera”, has additional apparatus, or annexes.

The orbit is a pyramidal cavity, four-angular, with the vertex to the posterior, and slightly oblique from the inside to the outside. At the top, there is an optical hole, through which the optical nerve enters the skull and gets into the brain by optical paths.

The base of this pyramid is prior at the face level, on both parts of the median line of the brain.

The orbit protects the ocular globe against different external aggressions. The ocular globe is situated on the orbit; the rest of the orbit is occupied with extrinsic muscles. These muscles determine the movement of the eye, vessels, nerves and adipose tissue.

The second important annex is made of the two eyelids, membranous muscle cutaneous formations. They protect the ocular globe against dust, smoke, foreign bodies.

The lachrymal apparatus lubricates the cornea and the conjunctiva, by the secretion of tears that participate to some nutritive exchanges and the oxygenation of the prior pole of the eye. Tears contain a substance called lysozyme, which is a bacteriostatic that maintains the bacteriologic equilibrium at the prior pole of the eye.

The conjunctiva is a very fine thin paper, transparent-pink, which covers the posterior part of the eyelid, and then, at their base, they reflect, forming a bottom of a sack, and pass through, up to the cornea. It is a very vascularized and excited membrane, protecting the ocular globe against any foreign body (dust, smoke, etc). Four of the extrinsic muscles of the eyes are straight (superior, inferior and external) and the other two are oblique. All of them take part in eyes movement.

The so organized eye transmits by the optical nerve, the message from the retina by optical paths, which partially cross in the optical chiasm and get into the optical bands, external geniculated bodies, then in optical radiations, ending up in the cerebral cortex, in the calcareous cleavage, in Brodmann 17, 18 and 19 zones.

The human eye is not the only type of eye. The eyes of almost all vertebrates are mainly like the human eye, but in the case of inferior animals, there are many other types of eyes: in the form of spots, or sensitive cavities, as well as less sensitive organs. In the case of invertebrates, there is another very well developed type of eye: the eye of the insects: the compound eye (most insects with big compound eyes, have more simple eyes).

The visual analyzer is the morpho-functional assembly that forms and analyses luminous sensations. Cornea, the watery humour, the crystalline lens, the glassy humour are transparent media, with different consistencies, fact that leads to various refraction grades and realizes a dyoptrical system adequate to the image formation on the retina; when at rest, this dyoptric system has aproximatively 60 dyoptries.

We name a visual field, the space that can be seen by an eye with a fixed position, all points in this space being visible in the same time. The size of the space perceived by the eye in such conditions is determined by the visual possibility of the peripheral retina.

The spreading of reception elements from the retina, sensitive to white light stimulant and the stimulation of other colors, is not the same, fact that determines variations of the visual field surface for white color and for other colors.

Although we see with two eyes, the visual sensation is unique, due to the fact that retinal images for one object are realized in symmetrical retinal areas of the two eyes, from which nervous impulses converge centrally in the same cortical area. These retinal areas, situated on the same part (right or left) are symmetrical; for example, the retinal area on the right part of the two eyes corresponds to the temporal retinal area of the right eye and to the nasal retinal area of the left eye.

Movement sensation is produced in different conditions, as:

Ø      the eye perceives the movement of a mobile, when its position is fixed and the image moves on the retina;

Ø      the eye follows the moving mobile; then the movement sensation depends on the reflex with departure point in the moto-ocular muscles, determined by the eye orientation modifications.

In the animal world, the eye is integered in the body in three ways:

Ø      eyes included in the body, as in the case of arthropods, where there is no relative movement between the eyes and the body;

Ø      eyes included in the body, with reduced width of the optical chord, allowing an angular degree of freedom;

Ø      chordate eye, with a more reduced optical connection, allowing a two-degree angular freedom.

Biological eyesight implies the senses of some species – specific parts of electromagnetic spectrum. In the human eye, light is translated by reception cells that contain Rhodospin, the visual pigment. At micro molecular level, a photon hits a Rhodospin molecule, which is excited at a higher level of energy and vibrates stronger. This leads to a Rhodospin diminishing in retinol (A vitamin) and of opsin, then to the closing of an ion channel and to the generation of an electrical signal, sent on the optical nerve in order to be processed by the brain.

This process of chemical transformation from photons into electrical energy is very common in the animal world. The eye has evolved independently and in different forms. Even the most primitive life forms are sensitive at irradiation. For example, planaria uses a curved row of photoreception cells, along each part of the body, near another row of cells, forming a barrier.

From simple sensitive spots, the more complex visual sensors have evolved up to the compound eye (made of ommadium) and the complex eye (subdivided, used by the animals with direct or inverse retina, mollusk order and chordate).

It is difficult to explain the structure form of the eye for each species. Eye evolution appears in tandem and it is influenced by the psychological characteristics of the animal. The used visual system type is very much related to the animal life medium and its mobility.

How does the animals see the world?

Is their vision the same as ours?

All animals see by gathering light rays, and although to a gorrila we appear very different, our eyes capture light in exactly the same way. But for both of us the image received on the light sensitive retina is not what the brain ultimatly sees, it is upside down and only the center shows any real detail.

On the retina this area contains a high concentration of light sensitive cells. Full colour and definition are confied to this inner circle. The brain not only inverts the image but also fills in all the missing information. Also like ours, a gorrila’s brain is envolved in judging distance. The right eye has a slightly different view from the left. From two different images the brain constructs a three dimensional picture. Judging distance becomes precise. The brain is envolved in the vision of all animals. But away from our close relatives we can only guess at how less familiar eyes finnaly interprete the view they see.

The starling’s eyes are far more mobile then our own. They can be brought together to increase the overlap, and so improve the central image, or be moved apart to wighten the view. The eyes also converge when searching for food, and moved apart to spot predators. Like most prey animals, the starling’s eyes are placed to the side of its head to offer a panoramic view. Eye movements add to the coverage.

No animal has a wider view than the woodcock. Without moving its eyes it even can see behind itself. The eyes placed centraly on either side of its head, give total wrap-around vision. With this three hundred degree view of the world, the camouflaged woodcock shows no intermovement as it watches for danger.

The eyes of a predator face forward and concentrate on the scene ahead. A fox’s view may be narrower but it has little to fear from danger behind. Like many mammals the fox has limited colour vision, but like us, it sees true detail only in the center circle, the brain fills in the rest.

Cats, being night hunters, developed very much eyesight in the dark. They have vertical iris, which can narrow up to the thinnest stripe during daylight, or can open, occupying 90% of eye surface, giving the pupil the possibility to capture the smallest ray of light. Moreover, cats have a bright membrane behind the eyes, called tapetum lucidum, which help them reflect back the light through the retina, in this way being able to see very well in insufficient light. However, they can’t see in total dark.

The eyes of cats have more sticks and less cones then humans, meaning that humans have a better perception of colors, while cats can detect better the movement, but they can’t see very well close objects. Still, they can distinguish well colors as: gold, green, orange, yellow, blue, violet.

Also, cats have a third eyelid that protects the eye when they hunt in the grass or bushes.

This membrane is situated in the internal corner of the eye.

The lion too has the forward facing eyes of a predator, but its vision differs from a fox’s in a way common to animals of the savannah. On a flaten landskape prey animals always appear to be on the horizon. To take advantage of this, the lion’s detailed view is innergated into a strip. This lateral strip gives greatly increased definition and again the brain constructs the missing details in the surrounding areas.

Like the eyes of all animals those of the wildbeests respond best to movement so the lion moves as stelthly as it can, making the most of any cover.     The kill will also provide food for scavengers.

The eagle has perhaps the keenest eye sight of any animal. As the sun heats the ground, the birds rise up on currents of warm air. These termals will provide uplift of thousands of metres or more. When they reach this hight, the vultures will be able to survey  many kilometres of savannah. With thousands of those flying eyes scowering the ground no carcas will be left for long. Their renowned eye sight relies on a remarcable adaptation. The central portion of their view is magnified two and a half times. On the retina, this enlarged area has a high concentration of light sensitive cells, these resolve the finest detail. The vultures not only scan for carrion, they are also guided by the behaviour of other animals. A gathering of other scavangers is a certain sign of food. Eyes adapted to darkness often have a mirrored lear at the back of the retina to reflect light. This gives the eyes a second change to absorbe even the fastest blimp. Most nocturnal mammals have this mirrored vision. The net casting spider needs neighter mirrors nor artificial light to see at night. Its enormous lenses let through every available ray and its retinas have huge light gathering cells. In addition its eyes have a totally overlaping view dubling sensitivity.

The domestic fly relies on movement information in order to survive. For this reason, its compound eye is capable to detect frequencies up to 300 Hz (compared to the human eye of 20-30 Hz). The characteristics of domestic fly visual system are different from the ones of the human eye (from the point of view of limit frequency, movement sense, color perception, perceived field).

            The spider can see in one tenth of the light we need. So, on the darkest night, it can still hunt. At first it constructs an unique kind of web, one that it’s held by its legs. Posed like a miniature gladiator, the spider relies on good vision to entrap its victim. Its eyes are not only sensitive to light, but they also react to the slightest movement. Although far simpler, the spiders’ eyes are similar in design to our own, but there are many animals with remarcably different vision. The rising sunlight filtered by the atmosphere appears to us red.

The bee is an insect, whose eyesight capability has been studied very attentively. The eyesight properties of bees are easily to be studied, as they are attracted to honey; there can be made experiments in which honey is identified by being placed on blue paper or red paper, and noticing where the bees go. By this method, some important and interesting things have been discovered about bees’ eyesight.

Firstly, trying to measure how accurate bees see the color difference between two pieces of white paper, some researchers found that they don’t see that well the difference, while other researchers found that they see extraordinary well the difference. The researchers used zinc white for one paper and plumb white for the other one. Although these nuances seem identical to us, bees can make the difference because they reflect differently in ultraviolet. In this way, it has been discovered that the eye of the bees is sensitive on a larger spectral interval than human eye. Our eyes function from 7000 angstroms up to 4000 angstroms, from red to violet, but bees’ eyes can see in ultraviolet up to 3000 angstroms. This fact leads to a series of interesting effects. Bees can make the difference between many flowers that seem identical to us, so the color of flowers has been invented for the eyes of the bees and not for ours, being a signal for the bees to certain flowers. We all know that there are many white flowers. The white color doesn’t seem to be interesting for the bees, as it has been discovered that all white flowers reflect differently in ultraviolet; they don’t reflect in ultraviolet not even at least 1% true white. Not all of the light returns from the flower; the ultraviolet is missing and this represents a color for the bee, in the same way in which the absence of blue means yellow for the human eye. In this way, all flowers are colored for the bees. We also know that red is invisible for the bees. An attentive study of red flowers shows in the first place that with our human eyes we can see that most red flowers have a blue shade; they reflect a certain blue light quantity, which is visible to the bees. Furthermore, experiments show that flowers ultraviolet reflection from one part of the petal to another one varies. If we saw the flowers, the way bees see the flowers, they would be prettier and more various.

It has been proven that there are some red flowers, which don’t reflect in blue or ultraviolet, and these flowers will seem black to bees. This fact became a preoccupation for those interested in these problems, because black doesn’t seem to be an interesting color, being hard to be noticed from a shadow. Researchers showed that these flowers are not visited by the bees; they are visited by humming birds who see red color.

            Insects like bees, are blind to this colour, but they see parts of the spectrum invisible to us. Our colour vision is sensitive to green, blue and red, but the bees’ is sensitive to green, blue and ultraviolet. With only three basic colours we both create a full colour picture, but the bees’ world looks very different. The spectrum is shifted towards the ultraviolet and its compound eye provides a far closer view. Seen through a bees’ eye, flowers become strangely unfamiliar. The flower colours we see have no real relevance. These hidden views have evolved to atract insects. It’s not only bees that have this ultraviolet vision. To our eyes, male and female butterflies look the same, but to a bee or another butterfly the ultraviolet courtship flashing of the male is strickingly visible. Insects are also sensitive to another kind of light invisible to us, the patterns created as the atmosphere polarises the suns’ rays. These patterns create a sky map which the bee can use to navigate. Although normally invisible to us these patterns can be seen with polarised sun glasses. The bee needs to see only a small portion of the sky map to find its way home.

Another interesting aspect of bees’ eyesight is that it seems that they seem to be able to determine sun direction only by looking at a region of blue sky, without seeing the sun. Humans can’t easily do the same thing. If we look on the window at the sky and we see that it is blue, can we tell in which direction the sun is? The bee can answer, because it is very sensitive to light polarity, and the spread light that gives color to the sky is polarized. There are anyway different opinions about how this sensitivity functions. Until now, it wasn’t sure whether it was the light different reflection in different circumstances or the eye of the bee was directly sensitive. Recent data show that the eye of the bee seems to be directly sensitive.

There are also suppositions that a bee can distinguish up to 200 oscillations a second, while humans can distinguish up to 20. The movements of bees in the beehive are very rapid, but these movements are hard to be noticed by us. The fact that their eyes react so quickly, is very important for them.

Visual accuracy of bees

The eye of bees is a compound eye, being made of a great number of special cells called ommatides, radially disposed on the surface of a sphere. On the exterior part of the head of the bee, in the superior part, there is a transparent region, similar to the crystalline lens, which in reality is more like a filter or a pipe that channels the light along the narrow fiber, where the absorption probably takes place. From that end of the cell, the nervous fiber begins. The central fiber is surrounded by six cells, which in fact secrete the fiber. These cells are conical objects, a lot of such cells being disposed one cell near another one, on all the eye surface of the bee.

Eye resolution of bees

It can be effectively calculated how large an ommatide is, by appealing to ration. If we have a big ommatide, we have reduced resolution: one cell receives information from one direction, the neighboring cell receives information from other direction, and so on, and the bee can’t see very well the objects situated between these directions, so the incertitude of visual accuracy will certainly correspond to an angle between the end of the ommatide and the curvature center of the eye. This angle from one ommatide to the next one is equal to the diameter of the ommatide, divided by the eye surface radius:

g=/r

Therefore, we can say that the smaller  is, the greater the visual accuracy is. Then, why doesn’t the bee use very fine ommatides? The answer is the fact that if we try to send light through a very narrow opening, we can’t see clearly because of the diffraction effect. Light received from many directions could enter due to diffraction. Light received under an angle d, such that:

d=/

Only now, we can say that if  becomes very small, each ommatide doesn’t look in only one direction. If  becomes very large, each cell looks into only one direction, but we don’t have enough directions in order to have a good image. So the distance d must be adjusted in order to reduce to the minimum the two cumulated effects. By adding up the two angles, we find where their sum is minimum:

d(g+d)/d=0=1/r-/d2

Giving us a distance:

2=r

            If we consider r about 3mm and wavelength of light seen by the bee is 4000 angstroms, and by extracting square root, we get:

=(3x10-3x4x10-7)1/2               m=3.5x10-5                  m=35

The real diameter is 30, so the result is very satisfactory, and we are able to understand which factors determine the size of bees’ eyes. Then, it is easy to introduce in the formulae, the above value of  and find how good the angular resolution of bees’ eyes is. Compared to the human eye, it is very poor. We can see objects with an apparent diameter 30 times smaller than those seen by the bees. The bees have an unclear image. However, this is maximum clarity they can realize. We wonder why bees don’t develop better eyes, similar to ours, with crystalline lens, and all other elements.

There are more reasons that are interesting. First, the bee is too small: if it had eyes like ours, at its size, the opening would be 30 and diffraction would be so important that it couldn’t see well. An eye like ours, at small size, is not so good. On the other hand, if the eye of the bee were larger, it would occupy its entire head.

The advantage of compound eye is that it doesn’t occupy space: it is just a very thin stratum at the surface of the bee. Therefore, instead of asking why bees didn’t develop their eyes the way we did, we should remember that they have their own problems.

Beside bees, there are many other animals can see colors. Fish, butterflies, birds and reptiles can see colors, but it is believed that most mammals can’t. The primates can see colors. Birds can certainly see colors, which explain their coloration. There would be no reason for male birds to be so brightly colored, if female birds couldn’t see this. The sexual instinct of birds is related to the fact that females can see the colors.

All invertebrates have very poor developed eyes, or even compound eyes, and all vertebrates have the eyes similar to our eyes, with one exception: zoologists agree that the most evolved animal is the octopus. It is very interesting that beside brain development, reactions, which are very good for an invertebrate, the octopus developed independently an eye different from the other invertebrates. The octopus doesn’t have a compound eye or a sensitive spot; it has a cornea, eyelids, an iris, a crystalline lens, two regions filled with watery humour and retina. It is in its essence the eye of vertebrates, but the retina in the case of the octopus, is part of the brain, part that got outside during embryonic development, in the same way as for vertebrates, but interesting fact, different from what can be seen at vertebrates: light sensitive cells are at the interior, and the cells that perform the transformations are behind them, and not the other way around, as in the case of human eye. The biggest eyes are the eyes of the giant calamari: they are 40cm diameter.

                                                                                                                                       

            Like the bee, the water boatman has a compound eye. This consists of an array of tiny lenses. Although each lense views only a fragment of the scene, they combine to create a single view. The water boatmans’ eyes are tuned to the polarised light reflected from the water. Our polarised glasses cut down these reflections, but the water boatmans’ eyes enhance them. They help it find new breading pools. The dragonfly has the ultimate compound eye. Its vision is four times better than the water boatmans’. Good vision si essential for this aerobatic marble, for the dragonfly not only hunts, but it also fights on the wind. Fourty thounsand lenses gather enough information for skilled manoeuvres, but this view is still thirty times poorer than our own. To match our eye sight, its compound eyes will have to measure a metre across.

            Like us, the dragonfly has eyes with areas of high resolution. These are used to spot flying insects in the sky, either food, mates or rivals. Territorial disputes are resolved by air to air combat. The victor returns to patrol its territory. Birds have the most complex colour vision of any animal. The light sensitive cells of their eye contain up to five different colour pigments. These pigments detect many more colour hues that we can see. The cells of their eye also contain coloured oil droplets. These act like miniatural filtres and reveal even more colours. While many birds appear to use oil droplets to improve their colour vision, sea birds use them for a different purpose. The eyes of turns have a high concentration of red oil droplets. It is believed that these act as rays filtres cutting out the reflected blue light of the sea. The turns that feed comunely these haze filtres may help individuals locate feeding flocks. The turns concentrate over schools os sand eals. As they hover, they use sights to single out a fish from the shore. Eyes cannot focus on the water so they simply tahe aim, them dive. Although low turns cannot cope with the change of focus needed below water, there are some fishing birds that can.

            Like most animals, a cormorand focuses light using both a lenze inside the eye and the cornea on the outside. In air, the cornea starts to focus light wich is then fine focused by a lenze that can change shape. Under water the cornea can no longer focus wich is why we only see a blur. But the cormorands’ vision is still cristal sharp.bu distorting its lenze it can compensate the now useless cornea and focus better than any other animal. A cormorands’ yes may outwit a fish, but in Mexico there are fish with vision to outwit birds. The great barrier reef is home to perhaps the strangest eyes of the animal world. Their vision is as bizzare as their appearance. They belong to a creature known as the mantris shrimp. The central band of the eye is the most complex colour analizer of the animal world. As it swips the sea, it not only scans foe visible colours, but also for ultraviolet and polarised light. The eyes are searching for anything that might be alive. Once a possible food source is found, an analiser is swept across it. It then brings the second analiser into play, lining up the scaning lines like the cross wires of a gun sight.

Owls have a very well developed eye: big eyes, well adapted to poor and very poor light, help them hunt at night. It is believed that owls can perfectly see in total darkness, but in strong light, they are blind – none of these is true. As owls have the eyes oriented in front, they have spatial eyesight, just as humans do. Their visual field is larger than ours, because they can turn their heads with almost 180 degrees in both directions.

Dolphins and sea pigs hunt usually in the superior stratum of the seas, illuminated by sunrays, in this way using their eyes, too. The eye lens is characterized by the possibility of form changing, so that in water and air, they fix very well the objects. Species with elongated muzzle, have a great binocular view (three-dimensional view), being able to approximate accurately the distance where objects are. Beside all these, though eyesight is very important, it has nothing to do with dolphins’ survival: trained dolphins can execute commands even with the eyes covered, because they also have other qualities, by which they perceive extraordinary well the surrounding environment.

In the case of seals, the eyes modified according to light diffraction in water. On earth, at strong light seals can see well enough, but when light intensity diminishes and their pupil dilates, their eyesight becomes opaque and they can see only very big objects, or very rapid moving objects. On the other hand, in water, the situation is completely different: due tot the eyes adapted to the optical water properties, seals can see very well even in the dark. Their eyes are very sensitive, especially in green light, fact that advantages them

            In the dark and lurky waters of the Amazon live fish with eyesight as formidable as their predatory reputation. In water, coloured red by organic dekay, the piranha has eyes that can pierce the gloom. Its eyes can see razes of light invisible to us. There the light we see it’s rapidly absorbed, but for red light still penetrates. As they hunt for prey, this killing machine rely on far red light to get through the merc. For the cat fish there is no escape. Even a mior gold fish has the visual powers of a piranha. In our high tech world we use far red lights for our own protection. Many security sistems use this light sometimes, known as infrared to iluminate garded parametres. Invisible to human eyes but picked up by special cameras, the far red light shows up any intruders. As the truck dissapears into darkness it is still seen by the security cameras. The gold fishes’eyes can see it too, lit by the far red lamps. But the gold fish has other visual powers. It not only sees far red, it can also see ultraviolet, a colour at the other end of the spectrum. A creature we take for granted can see a greater arrange of colours than any other animal.

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