It has come to my attention


Moderator: Moderators

It has come to my attention

Postby Eddie » Sun Dec 20, 2009 2:50 am

That there was a murder clue in a dead man's eye.
This also make no sense, because;
From Wikipedia, the free encyclopedia
Jump to:navigation, search
For other uses of "death" and "deceased", see Death (disambiguation).
Universal symbol for death: a human skull

Death is the termination of the biological functions that define a living organism. It refers both to a particular event and to the condition that results thereby. The true nature of the latter has for millennia been a central concern of the world's religious traditions and of philosophical enquiry. Belief in some kind of afterlife or rebirth is a central aspect of many religious traditions. Within the scientific community, many suppose death to terminate mind or consciousness. The effect of physical death on any possible mind or soul remains for many an open question. Cognitive science has yet to explain fully the origin and nature of consciousness; any view about the existence or non-existence of consciousness after death remains speculative.[1][2]

Humans and the vast majority of other animals die in due course from senescence. Remarkable exceptions include the hydra, and the jellyfish turritopsis nutricula, which is thought to possess in effect biological immortality.[3]

Intervening phenomena which commonly bring about death earlier include malnutrition, disease, or accidents resulting in terminal physical injury. Predation is a cause of death for many species. Intentional human activity causing death includes suicide, homicide, and war. Roughly 150,000 people die each day across the globe.[4] Death in the natural world can also occur as an indirect result of human activity: an increasing cause of species depletion in recent times has been destruction of ecosystems as a consequence of the widening spread of industrial technology.[5]


* 1 Signs and symptoms
* 2 Diagnosis
o 2.1 Problems of definition
o 2.2 Legal
o 2.3 Misdiagnosed
* 3 Causes
o 3.1 Autopsy
* 4 Prevention
* 5 Society and culture
* 6 In biology
o 6.1 Natural selection
o 6.2 Extinction
o 6.3 Evolution of aging
* 7 See also
* 8 References
* 9 Further reading
* 10 External links

[edit] Signs and symptoms

Signs of death, or strong indications that a person is no longer alive are:

* Ceasing respiration
* The body no longer metabolises
* Pallor mortis, paleness which happens in the 15â€“120 minutes after the death
* Decomposition, the reduction into simpler forms of matter

[edit] Diagnosis
[edit] Problems of definition
What is death? A flower, a skull and an hour-glass stand in for Life, Death and Time in this 17th Century painting by Philippe de Champaigne

For those who define death as a state following the state of life, one of the challenges in defining death is in distinguishing it from life. Death would seem to refer to either the moment at which life ends, or when the state that follows life begins. However, determining when death has occurred requires drawing precise conceptual boundaries between life and death. This is problematic however because there is little consensus over how to define life. Some have suggested defining life in terms of consciousness. When consciousness ceases, a living organism can be said to have died. One of the notable flaws in this approach is that there are many organisms which are alive but probably not conscious (for example, single-celled organisms). Another problem with this approach is in defining consciousness, which remains a mystery to modern scientists, psychologists and philosophers. This general problem of defining death applies to the particular challenge of defining death in the context of medicine.

Other definitions for death focus on the character of cessation of something.[7] In this context 'death' describes merely the state where something has ceased, e.g., life. Thus, the definition of 'life' simultaneously defines death.




Many have challenged the idea that brain death is equivalent to the cessation of consciousness. Critics point out that much of human consciousness is embodied in numerous body parts and that the end of electrical impulses in the brain does not necessarily indicate that this embodied consciousness has also ceased. Given this possibility, brain death does not necessitate the end of consciousness, and thus brain dead people may still be alive. Furthermore, some have argued, even if brain death does mean the end of consciousness for a human being, the whole notion that cessation of consciousness indicates death is problematic. Critics note the existence of many single-celled organisms such as bacteria that we consider to be alive but which many doubt are conscious. If life does not require consciousness, defining death in terms of "brain death" is a dubious procedure, even if the brain is the seat of consciousness. Thus while legal concerns surrounding death force us to develop a working definition of death, it is not at all clear that the current American definition, according to brain death, coincides at all with a definition that can be reasonably endorsed.[who?]

Those people maintaining that only the neo-cortex of the brain is necessary for consciousness sometimes argue that only electrical activity there should be considered when defining death. Eventually it is possible that the criterion for death will be the permanent and irreversible loss of cognitive function, as evidenced by the death of the cerebral cortex. All hope of recovering human thought and personality is then gone given current and foreseeable medical technology. However, at present, in most places the more conservative definition of death â€” irreversible cessation of electrical activity in the whole brain, as opposed to just in the neo-cortex â€” has been adopted (for example the Uniform Determination Of Death Act in the United States). In 2005, the Terri Schiavo case brought the question of brain death and artificial sustenance to the front of American politics.

[edit] Legal
See also: Legal death

In the United States, a person is dead by law if a Statement of Death or Death Certificate is approved by a licensed medical practitioner. Various legal consequences follow death, including the removal from the person of what in legal terminology is called personhood.

The possession of brain activities, or ability to resume brain activity, is a necessary condition to legal personhood in the United States. "It appears that once brain death has been determined â€¦ no criminal or civil liability will result from disconnecting the life-support devices." (Dority v. Superior Court of San Bernardino County, 193 Cal.Rptr. 288, 291 (1983))
[edit] Misdiagnosed

There are many anecdotal references to people being declared dead by physicians and then 'coming back to life', sometimes days later in their own coffin, or when embalming procedures are just about to begin. Owing to significant scientific advancements in the Victorian era, some people in Britain became obsessively worried about living after being declared dead.[9]

In cases of electric shock, CPR for an hour or longer can allow stunned nerves to recover, allowing an apparently dead person to survive. People found unconscious under icy water may survive if their faces are kept continuously cold until they arrive at an emergency room.[10] This "diving response", in which metabolic activity and oxygen requirements are minimal, is something humans share with cetaceans called the mammalian diving reflex.[10]

As medical technologies advance, ideas about when death occurs may have to be re-evaluated in light of the ability to restore a person to vitality after longer periods of apparent death (as happened when CPR and defibrillation showed that cessation of heartbeat is inadequate as a decisive indicator of death). The lack of electrical brain activity may not be enough to consider someone scientifically dead. Therefore, the concept of information theoretical death has been suggested as a better means of defining when true death actually occurs, though the concept has few practical applications outside of the field of cryonics.

There have been some scientific attempts to bring dead organisms back to life, but with limited success.[11] In science fiction scenarios where such technology is readily available, real death is distinguished from reversible death.
[edit] Causes
See also: List of causes of death by rate and List of preventable causes of death

The leading cause of death in developing countries is infectious disease. The leading causes of death in developed countries are atherosclerosis (heart disease and stroke), cancer, and other diseases related to obesity and aging. These conditions cause loss of homeostasis, leading to cardiac arrest, causing loss of oxygen and nutrient supply, causing irreversible deterioration of the brain and other tissues. Of the roughly 150,000 people who die each day across the globe, about two thirds â€” 100,000 per day â€” die of age-related causes.[4] In industrialized nations, the proportion is much higher, reaching 90%.[4] With improved medical capability, dying has become a condition to be managed. Home deaths, once normal, are now rare in the developed world.

In developing nations, inferior sanitary conditions and lack of access to modern medical technology makes death from infectious diseases more common than in developed countries. One such disease is tuberculosis, a bacterial disease which killed 1.7 million people in 2004.[12] Malaria causes about 400â€“900 million cases of fever and approximately one to three million deaths annually.[13] AIDS death toll in Africa may reach 90-100 million by 2025.[14][15]

According to Jean Ziegler, who was the United Nations Special reporter on the Right to Food from 2000 to March 2008; mortality due to malnutrition accounted for 58% of the total mortality rate in 2006. Ziegler says worldwide approximately 62 million people died from all causes and of those deaths more than 36 million died of hunger or diseases due to deficiencies in micronutrients."[16]

Tobacco smoking killed 100 million people worldwide in the 20th century and could kill 1 billion people around the world in the 21st century, a WHO Report warned.[17][18]

[edit] Autopsy
Rembrandt turns an autopsy into a masterpiece: The Anatomy Lesson of Dr. Nicolaes Tulp



[edit] Prevention
Main article: Life extension


[edit] Society and culture
Main article: Death and culture
Death haunts even the beautiful: an early 20th century artist says, "All is Vanity"

Death is the center of many traditions and organizations, and is a feature of every culture around the world. Much of this revolves around the care of the dead, as well as the afterlife and the disposal of bodies upon the onset of death. The disposal of human corpses does, in general, begin with the last offices before significant time has passed, and ritualistic ceremonies often occur, most commonly interment or cremation. This is not a unified practice, however, as in Tibet for instance the body is given a sky burial and left on a mountain top. Proper preparation for death and techniques and ceremonies for producing the ability to transfer one's spiritual attainments into another body (reincarnation) are subjects of detailed study in Tibet.[20] Mummification or embalming is also prevalent in some cultures, to retard the rate of decay.




Suicide in general, and particularly euthanasia are also points of cultural debate. Both acts are understood very differently in contrasting cultures. In Japan, for example, ending a life with honor by seppuku was considered a desirable death, whereas according to traditional Christian and Islamic cultures, suicide is viewed as a sin. Death is personified in many cultures, with such symbolic representations as the Grim Reaper, Azrael and Father Time.
[edit] In biology
[edit] Natural selection
Main articles: competition (biology), natural selection, and extinction

[edit] Extinction
Main article: Extinction
Dead as a dodo: the bird that became a byword in English for species extinction [22]


Microorganisms also play a vital role, raising the temperature of the decomposing matter as they break it down into yet simpler molecules. Not all materials need be decomposed fully, however. Coal, a fossil fuel formed over vast tracts of time in swamp ecosystems, is one example.
[edit] Evolution of aging
Main article: Evolution of ageing

Inquiry into the evolution of aging aims to explain why so many living things and the vast majority of animals weaken and die with age (a notable exception being hydra, which may be biologically immortal). The evolutionary origin of senescence remains one of the fundamental puzzles of biology. Gerontology specializes in the science of human aging processes.
[edit] See also

* Afterlife
* Bardo Thodol (Tibetan Book of the Dead)
* Burial
* Black Death
* Cadaveric spasm
* Death, Desire and Loss in Western Culture by Jonathan Dollimore
* Dead bell
* Death drive
* Death erection
* Death messengers
* Death (personification)
* Death rattle
* DÃ­a de los Muertos (Day of the Dead)
* Dying declaration
* International Necronautical Society

* Karōshi
* Last rites
* Leading preventable causes of death
* Legal death
* List of causes of death by rate
* List of natural disasters by death toll
* List of wars and disasters by death toll
* Mortician
* Near-death experience
* Post Mortem Interval
* Pseudocide
* Thanatology
* Vampire
* World War I casualties
* World War II casualties
* Zombie

[edit] References

1. ^ "The Big Questions: What is consciousness?". New Scientist. 18 November 2006. ... ?full=true. Retrieved 2009-09-04.
2. ^ "Facing up to the problem of consciousness".
3. ^ Guerin, John C. (2009). Emerging Area of Aging Research: Long-lived Animals with "Negligible Senescence". Retrieved August 21, 2009.
5. ^ Human Activities Cause of Current Extinction Crisis, accessed 7 April 2009
6. ^ Crippen, David. "Brain Failure and Brain Death". ACS Surgery Online, Critical Care, April 2005. Archived from the original on 24 June 2006. ... cs0812.htm. Retrieved 2007-01-09.
7. ^ Oxford English Dictionary
8. ^ FG Miller "Death and organ donation: back to the future" Journal of Medical Ethics 2009;35:616-620
9. ^ As reflected from at least one article of literature by authors like Edgar Allan Poe, where subjects were buried alive.
10. ^ a b Limmer, D. et al. (2006). Emergency care (AHA update, Ed. 10e). Prentice Hall.
11. ^ Blood Swapping Reanimates Dead Dogs
12. ^ World Health Organization (WHO). Tuberculosis Fact sheet NÂ°104 - Global and regional incidence. March 2006, Retrieved on 6 October 2006.
13. ^ USAIDâ€™s Malaria Programs
14. ^ Aids could kill 90 million Africans, says UN
15. ^ AIDS Toll May Reach 100 Million in Africa, Washington Post
17. ^ Tobacco Could Kill One Billion By 2100, World Health Organization Report Warns
18. ^ Tobacco could kill more than 1 billion this century: World Health Organization
19. ^ SJ Olshanksy et al. (2006). "Longevity dividend: What should we be doing to prepare for the unprecedented aging of humanity?". The Scientist 20: 28â€“36. Retrieved 2007-03-31.
20. ^ Mullin (1999).
21. ^ "Shot at Dawn, campaign for pardons for British and Commonwealth soldiers executed in World War I". Shot at Dawn Pardons Campaign. Retrieved 2006-07-20.
22. ^ Diamond, Jared (1999). "Up to the Starting Line". Guns, Germs, and Steel. W. W. Norton. pp. 43â€“44. ISBN 0-393-31755-2.

[edit] Further reading

* Appel, JM. Defining Death: When Physicians and Families Differ. Journal of Medical Ethics Fall 2005.
* Child AM (1995) J Archaeolog Sci 22: 165-174it funny
* Piepenbrink H (1985) J Archaeolog Sci 13: 417-430
* Piepenbrink H (1989) Applied Geochem 4: 273-280
* Pounder, Derrick J. (2005-12-15). "Postmortem changes and time of death". University of Dundee. ... edeath.pdf. Retrieved 2006-12-13.

[edit] External links
Search Wikiquote Wikiquote has a collection of quotations related to: Death
Search Wikimedia Commons Wikimedia Commons has media related to: Death
Search Wiktionary Look up death in Wiktionary, the free dictionary.

* Death at the Open Directory Project
* Death (Stanford Encyclopedia of Philosophy)
* Doctors Change the Way They Think About Death
* Causes of Death
* Causes of Death 1916 How the medical profession categorized causes of death a century ago.
* Before and After Death Interviews with people dying in hospices, and portraits of them before, and shortly after, death

Preceded by
Old age Stages of human development
Death Succeeded by

From Wikipedia, the free encyclopedia
Jump to:navigation, search
For information specific to humans, see Human eye.
For other uses, see Eye (disambiguation).
Schematic diagram of the human eye en.svg
Schematic diagram of the vertebrate eye.
Compound eye of Antarctic krill

Eyes are organs that detect light, and send electrical impulses along the optic nerve to the visual and other areas of the brain. Complex optical systems with resolving power have come in ten fundamentally different forms, and 96% of animal species possess a complex optical system.[1] Image-resolving eyes are present in cnidaria, molluscs, chordates, annelids and arthropods.[2]


* 1 Overview
* 2 Evolution
* 3 Types of eye
o 3.1 Simple eyes
+ 3.1.1 Pit eyes
+ 3.1.2 Pinhole eye
+ 3.1.3 Spherical lensed eye
# Weaknesses
+ 3.1.4 Multiple lenses
+ 3.1.5 Refractive cornea
+ 3.1.6 Reflector eyes
o 3.2 Compound eyes
+ 3.2.1 Apposition eyes
+ 3.2.2 Superposition eyes
+ 3.2.3 Parabolic superposition
+ 3.2.4 Other
* 4 Relationship to lifestyle
* 5 Acuity
* 6 Color
* 7 Rods and cones
* 8 Pigment
* 9 See also
* 10 References
* 11 External links

[edit] Overview
Eye of the wisent,
the European bison

Complex eyes can distinguish shapes and colors. The visual fields of many organisms, especially predators, involve large areas of binocular vision to improve depth perception; in other organisms, eyes are located so as to maximise the field of view, such as in rabbits and horses, which have monocular vision.

The first proto-eyes evolved among animals 540 million years ago, about the time of the Cambrian explosion.[citation needed] The last common ancestor of animals possessed the biochemical toolkit necessary for vision, and more advanced eyes have evolved in 96% of animal species in 6 of the thirty-plus[note 1] main phyla.[1] In most vertebrates and some molluscs, the eye works by allowing light to enter it and project onto a light-sensitive panel of cells, known as the retina, at the rear of the eye. The cone cells (for color) and the rod cells (for low-light contrasts) in the retina detect and convert light into neural signals for vision. The visual signals are then transmitted to the brain via the optic nerve. Such eyes are typically roughly spherical, filled with a transparent gel-like substance called the vitreous humour, with a focusing lens and often an iris; the relaxing or tightening of the muscles around the iris change the size of the pupil, thereby regulating the amount of light that enters the eye,[3] and reducing aberrations when there is enough light.[4]

The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens â€” similar to how a camera focuses.[5]


Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system.[6] Trilobites, which are now extinct, had unique compound eyes. They used clear calcite crystals to form the lenses of their eyes. In this, they differ from most other arthropods, which have soft eyes. The number of lenses in such an eye varied, however: some trilobites had only one, and some had thousands of lenses in one eye.

[edit] Evolution
Main article: Evolution of the eye
Diagram of eye evolution.svg

The common origin (monophyly) of all animal eyes is now widely accepted as fact based on 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.[8][9][10] The majority of the advancements in early eyes are believed to have taken only a few million years to develop, as the first predator to gain true imaging would have touched off an "arms race".[11] 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.




[edit] Types of eye

Nature has produced ten different eye layouts â€” indeed every way of capturing an image has evolved at least once in nature, with the exceptions of zoom and Fresnel lenses. Eye types can be categorized into "simple eyes", with one concave chamber, and "compound eyes", which comprise a number of individual lenses laid out on a convex surface.[1] Note that "simple" does not imply a reduced level of complexity or acuity. Indeed, any eye type can be adapted for almost any behaviour or environment. The only limitations specific to eye types are that of resolution â€” the physics of compound eyes prevents them from achieving a resolution better than 1Â°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures.[1] Eyes also fall into two groups on the basis of their photoreceptor's cellular construction, with the photoreceptor cells either being cilliated (as in the vertebrates) or rhabdomic. These two groups are not monophyletic; the cnidaira also possess cilliated cells, [15] and some annelids possess both.[16]
[edit] Simple eyes

Simple eyes are rather ubiquitous, and lens-bearing eyes have evolved at least seven times; in vertebrates, cephalopods, annelids, crustacea and cubozoa.[17]
[edit] Pit eyes

[edit] Pinhole eye
Nautiluses bear a pinhole eye

The pinhole eye is an "advanced" form of pit eye incorporating several improvements, most notably a small aperture (which may be adjustable) and deep pit. It is only found in the nautiloids.[1] Without a lens to focus the image, it produces a blurry image, and will blur out a point to the size of the aperture. Consequently, nautiloids can't discriminate between objects with an angular separation of less than 11Â°.[1] Shrinking the aperture would produce a sharper image, but let in less light.[1]
[edit] Spherical lensed eye

The resolution of pit eyes can be greatly improved by incorporating a material with a higher refractive index to form a lens, which may greatly reduce the blur radius encountered â€” hence increasing the resolution obtainable.[1] The most basic form, still seen in some gastropods and annelids, consists of a lens of one refractive index. A far sharper image can be obtained using materials with a high refractive index, decreasing to the edges â€” this decreases the focal length and thus allows a sharp image to form on the retina.[1] This also allows a larger aperture for a given sharpness of image, allowing more light to enter the lens; and a flatter lens, reducing spherical aberration.[1] Such an inhomogeneous lens is necessary in order for the focal length to drop from about 4 times the lens radius, to 2.5 radii.[1]

Heterogeneous eyes have evolved at least eight times â€” four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods.[1] No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown".[1]


The ocelli of insects bear a simple lens, but their focal point always lies behind the retina; consequently they can never form a sharp image. This capitulates the function of the eye. Ocelli (pit-type eyes of arthropods) blur the image across the whole retina, and are consequently excellent at responding to rapid changes in light intensity across the whole visual field â€” this fast response is further accelerated by the large nerve bundles which rush the information to the brain.[18] Focusing the image would also cause the sun's image to be focused on a few receptors, with the possibility of damage under the intense light; shielding the receptors would block out some light and thus reduce their sensitivity.[18] 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).[18]
[edit] Weaknesses

One weakness of this eye construction is that chromatic aberration is still quite high[1] â€” although for organisms without color vision, this is a very minor concern.

A weakness of the vertebrate eye is the blind spot at the optic disc where the optic nerve is formed at the back of the eye; there are no light sensitive rods or cones to respond to a light stimulus at this point. By contrast, the cephalopod eye has no blind spot as the retina is in the opposite orientation.
[edit] Multiple lenses

[edit] Refractive cornea
Further information: Mammalian eye

In the eyes of most terrestrial vertebrates (along with spiders and some insect larvae) the vitreous fluid has a higher refractive index than the air, relieving the lens of the function of reducing the focal length. This has freed it up for fine adjustments of focus, allowing a very high resolution to be obtained.[1] As with spherical lenses, the problem of spherical aberration caused by the lens can be countered either by using an inhomogeneous lens material, or by flattening the lens.[1] Flattening the lens has a disadvantage: the quality of vision is diminished away from the main line of focus, meaning that animals requiring all-round vision are detrimented. Such animals often display an inhomogeneous lens instead.[1]

As mentioned above, a refractive cornea is only useful out of water; in water, there is no difference in refractive index between the vitreous fluid and the surrounding water. Hence creatures which have returned to the water â€” penguins and seals, for example â€” lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong cornea.[1]
[edit] Reflector eyes



There is at least one vertebrate, the spookfish, whose eyes include reflective optics for focusing of light. Each of the two eyes of a spookfish collects light from both above and below; the light coming from the above is focused by a lens, while that coming from below, by a curved mirror composed of many layers of small reflective plates made of guanine crystals.[19]
[edit] Compound eyes
Portrait of a Housefly (Musca domestica)
An image of a house fly compound eye surface by using Scanning Electron Microscope at X450 magnification
Arthropods such as this carpenter bee have compound eyes

A compound eye may consist of thousands of individual photoreception units. 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.[20] Because the individual lenses are so small, the effects of diffraction impose a limit on the possible resolution that can be obtained. This can only be countered by increasing lens size and number â€” 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.[21] Compound eyes are common in arthropods, and are also present in annelids and some bivalved molluscs.[22]

Compound eyes, in arthropods at least, grow at their margins by the addition of new ommatidia.[23]
[edit] Apposition eyes




Structure of the ommatidia of appositon compound eyes.
[edit] Superposition eyes

[edit] Parabolic superposition

[edit] Other
The compound eye of a dragonfly




[edit] Relationship to lifestyle


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.[1]:23-4 Optical superposition eyes are constrained to a spherical shape, but other forms of compound eyes may deform to a shape where more ommatidia are aligned to, say, the horizon, without altering the size or density of individual ommatidia.[24] Eyes of horizon-scanning organisms have stalks so they can be easily aligned to the horizon when this is inclined, for example if the animal is on a slope.[25] 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.[24] 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 â€” or predators â€” 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.[24] Depth perception can be enhanced by having eyes which are enlarged in one direction; distorting the eye slightly allows the distance to the object to be estimated with a high degree of accuracy.[7]

Acuity is higher among male organisms that mate in mid-air, as they need to be able to spot and assess potential mates against a very large backdrop.[24] On the other hand, the eyes of organisms which operate in low light levels, such as around dawn and dusk or in deep water, tend to be larger to increase the amount of light that can be captured.[24]


[edit] Acuity
A hawk's eye


For a human eye with excellent acuity, the maximum theoretical resolution is 50 CPD[26] (1.2 arcminute per line pair, or a 0.35 mm line pair, at 1 m). A rat can resolve only about 1 to 2 CPD.[27] 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.

[edit] Color

All organisms are restricted to a small range of the electromagnetic spectrum; this varies from creature to creature, but is mainly between 400 and 700 nm[29]. 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.[30]

The most sensitive pigment, rhodopsin, has a peak response at 500 nm.[31] Small changes to the genes coding for this protein can tweak the peak response by a few nm;[2] pigments in the lens can also "filter" incoming light, changing the peak response.[2] Many organisms are unable to discriminate between colors, seeing instead in shades of "grey"; colour vision necessitates a range of pigment cells which are primarily sensitive to smaller ranges of the spectrum. In primates, geckos, and other organisms, these take the form of cone cells, from which the more sensitive rod cells evolved.[31] Even if organisms are physically capable of discriminating different colours, this does not necessarily mean that they can perceive the different colours; only with behavioral tests can this be deduced.[2]

Most organisms with colour vision are able to detect ultraviolet light. This high energy light can be damaging to receptor cells. With a few exceptions (snakes, placental mammals), most organisms avoid these effects by having absorbent oil droplets around their cone cells. The alternative, developed by organisms that had lost these oil droplets in the course of evolution, is to make the lens impervious to UV light â€” this precludes the possibility of any UV light being detected, as it does not even reach the retina.[31]:309
[edit] Rods and cones



Cones are responsible for color vision. They require brighter light to function than rods require. There are three types of cones, maximally sensitive to long-wavelength, medium-wavelength, and short-wavelength light (often referred to as red, green, and blue, respectively, though the sensitivity peaks are not actually at these colors). The color seen is the combined effect of stimuli to, and responses from, these three types of cone cells. Cones are mostly concentrated in and near the fovea. Only a few are present at the sides of the retina. Objects are seen most sharply in focus when their images fall on this spot, as when one looks at an object directly. Cone cells and rods are connected through intermediate cells in the retina to nerve fibers of the optic nerve. When rods and cones are stimulated by light, the nerves send off impulses through these fibers to the brain.[31]
[edit] Pigment

The pigment molecules used in the eye are various, but can be used to define the evolutionary distance between different groups, and can also be an aid in determining which are closely related â€“ although problems of convergence do exist.[31]


There are two types of opsin involved in vision; c-opsins, which are associated with ciliary-type photoreceptor cells, and r-opsins, associated with rhabdomeric photoreceptor cells.[32] 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.[32] 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.[32] 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.[32] Cnidaria, which are an outgroup to the taxa mentioned above, express c-opsins - but r-opsins are yet to be found in this group.[32] Incidentally, the melanin produced in the cnidaria is produced in the same fashion as that in vertebrates, suggesting the common descent of this pigment.[32]
[edit] See also
Wikipedia Books Wikipedia:Books has a book on: Eye

* Arthropod eye
* Human eye
* Mammalian eye
* Naked eye
* Sanpaku eyes

[edit] References

1. ^ a b c d e f g h i j k l m n o p q r s t u v w x y z aa ab ac ad ae af Land, M F; Fernald, R D (1992). "The Evolution of Eyes". Annual Review of Neuroscience 15: 1â€“29. doi:10.1146/ PMID 1575438.
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3. ^ Nairne, James (2005). Psychology. Belmont: Wadsworth Publishing. ISBN 049503150x. OCLC 61361417. ... uNMCCFDdHw.
6. ^ Who You Callin' "Shrimp"? â€“ National Wildlife Magazine
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13. ^ Eye-Evolution?
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23. ^ Mayer, G. (2006). "Structure and development of onychophoran eyes: What is the ancestral visual organ in arthropods?". Arthropod Structure and Development 35 (4): 231â€“245. doi:10.1016/j.asd.2006.06.003. PMID 18089073.
24. ^ a b
I like the Arcitc really, I do!
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Postby noonsun » Sun Dec 20, 2009 2:52 am

Well done, Sherlock
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Postby Eddie » Sun Dec 20, 2009 3:04 am

Elementary, my dear Watson.
I like the Arcitc really, I do!
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