ratio of word probabilities predicted from brain for cow and eye

cow	eye
top 10 words in brain distribution (in article): species animal female male breed human food live cat wild	top 10 words in brain distribution (in article): water form surface land region cause time type world zone
top 10 words in brain distribution (not in article): bird egg fish hunt wolf insect prey body eat bear	top 10 words in brain distribution (not in article): ice rock river sea ocean wind soil flow lake occur
times more probable under cow 30 20 10 6 4 2.5 1.25 1 1.25 2.5 4 6 10 20 30 times more probable under eye (words not in the model)
Cattle'", colloquially referred to as "'cows'", are domesticated ungulates, a member of the subfamily Bovinae of the family Bovidae. They are raised as livestock for meat (called beef and veal), dairy products (milk), leather and as draft animals (pulling carts, plows and the like). In some countries, such as India, they are honored in religious ceremonies and revered. It is estimated that there are 1.3 billion cattle in the world today. Species of cattle. Cattle were originally identified by Carolus Linnaeus as three separate species. These were "Bos taurus", the European cattle, including similar types from Africa and Asia; "Bos indicus", the zebu; and the extinct "Bos primigenius", the aurochs. The aurochs is ancestral to both zebu and European cattle. More recently these three have increasingly been grouped as one species, with "Bos primigenius taurus", "Bos primigenius indicus" and "Bos primigenius primigenius" as the subspecies. Complicating the matter is the ability of cattle to interbreed with other closely related species. Hybrid individuals and even breeds exist, not only between European cattle and zebu but also with yaks (called a dzo), banteng, gaur, and bison ("cattalo"), a cross-genera hybrid. For example, genetic testing of the Dwarf Lulu breed, the only humpless "Bos taurus"-type" cattle in Nepal, found them to be a mix of European cattle, zebu and yak. Cattle cannot successfully be bred with water buffalo or African buffalo. The aurochs originally ranged throughout Europe, North Africa, and much of Asia. In historical times, their range was restricted to Europe, and the last animals were killed by poachers in Masovia, Poland, in 1627. Breeders have attempted to recreate cattle of similar appearance to aurochs by crossing of domesticated cattle breeds, creating the Heck cattle breed. (See also aurochs and zebu articles.) Word origin. "Cattle" did not originate as a name for bovine animals. It derives from the Latin "caput", head, and originally meant movable property, especially livestock of any kind. The word is closely related to "chattel" (a unit of personal property) and "capital" in the economic sense. Older English sources like King James Version of the Bible refer to livestock in general as cattle (as opposed to the word deer which then was used for wild animals). Additionally other species of the genus "Bos" are sometimes called wild cattle. Today, the modern meaning of "cattle", without any other qualifier, is usually restricted to domesticated bovines. Terminology of cattle. In general, the same words are used in different parts of the world but with minor differences in the definitions. The terminology described here contrasts the differences in definition between the United States and other British influenced parts of world such as Canada, Australia, New Zealand, Ireland, and the United Kingdom. Singular terminology dilemma. "Cattle" can only be used in the plural and not in the singular: it is a plurale tantum. Thus one may refer to "three cattle" or "some cattle", but not "one cattle". There is no universally used singular equivalent in modern English to "cattle", other than the gender and age-specific terms such as cow, bull, steer and heifer. Strictly speaking, the singular noun for the domestic bovine was "ox", however, "ox" today is rarely used in this general sense. An ox today generally denotes a draft beast, most commonly a castrated male (but is not to be confused with the unrelated wild musk ox). "Cow" has been in general use as a singular for the collective "cattle" in spite of the objections of those who say that it is a female-specific term, so that that phrases such as "that cow is a bull" would be absurd from a lexicographic standpoint. However, it is easy to use when a singular is needed and the gender is not known, as in "There is a cow in the road". Further, any herd of fully mature cattle in or near a pasture is statistically likely to consist mostly of cows, so the term is probably accurate even in the restrictive sense. Other than the few bulls needed for breeding, the vast majority of male cattle are castrated as calves and slaughtered for meat before the age of three years. Thus, in a pastured herd, any calves or herd bulls usually are clearly distinguishable from the cows due to distinctively different sizes and clear anatomical differences. The Oxford English Dictionary lists the use of "cows" as a synonym for "cattle" as an American usage. Merriam-Webster, a U.S. dictionary, recognizes the non-gender-specific use of "cow" as an alternate definition, whereas Collins, a UK dictionary, does not. Colloquially, more general non-specific terms may denote cattle when a singular form is needed. Australian, New Zealand and British farmers use the term "beast" or "cattle beast". "Bovine" is also used in Britain. The term "critter" is common in the western United States and Canada, particularly when referring to young cattle. In some areas of the American South (particularly the Appalachian region), where both dairy and beef cattle are present, an individual animal was once called a "beef critter", though that term is becoming archaic. Other terminology. Cattle raised for human consumption are called "beef cattle". Within the beef cattle industry in parts of the United States, the term "beef" (plural "beeves") is still used in its archaic sense to refer to an animal of either gender. Cows of certain breeds that are kept for the milk they give are called "dairy cows" or "milking cows" (formerly "milch cows" – "milch" was pronounced as "milk"). Most young male offspring of dairy cows are sold for veal, and may be referred to as "veal calves." The term "dogies" was once used to describe calves and young steers in the context of ranch work in the American west, as in "Keep them dogies moving," but in modern use is considered archaic unless used in a humorous context. In some places, a cow kept to provide milk for one family is called a "house cow". Other obsolete terms for cattle include "neat" (this use survives in "neatsfoot oil", extracted from the feet and legs of cattle), and "beefing" (young animal fit for slaughter). An onomatopoeic term for one of the commonest sounds made by cattle is "moo", and this sound is also called "lowing". There are a number of other sounds made by cattle, including calves "bawling", and bulls "bellowing" (a high-pitched yodeling call). The bullroarer makes a sound similar to a territorial call made by bulls. Anatomy. Cattle have one stomach with four compartments. They are the rumen, reticulum, omasum, and abomasum, the rumen being the largest compartment. Cattle sometimes consume metal objects which are deposited in the reticulum, the smallest compartment, and this is where hardware disease occurs. The reticulum is known as the "Honeycomb." The omasum's main function is to absorb water and nutrients from the digestible feed. The omasum is known as the "Many Plies." The abomasum is like the human stomach; this is why it is known as the "true stomach". Cattle are ruminants, meaning that they have a digestive system that allows use of otherwise indigestible foods by repeatedly regurgitating and rechewing them as "cud". The cud is then reswallowed and further digested by specialised microorganisms in the rumen. These microbes are primarily responsible for decomposing cellulose and other carbohydrates into volatile fatty acids that cattle use as their primary metabolic fuel. The microbes inside of the rumen are also able to synthesize amino acids from non-protein nitrogenous sources such as urea and ammonia. As these microbes reproduce in the rumen, older generations die and their carcasses continue on through the digestive tract. These carcasses are then partially digested by the cattle, allowing it to gain a high quality protein source. These features allow cattle to thrive on grasses and other vegetation. The gestation period for a cow is nine months. A newborn calf weighs. The world record for the heaviest bull was a Chianina named Donetto, when he was exhibited at the Arezzo show in 1955. The heaviest steer was eight year old ‘Old Ben’, a Shorthorn Hereford cross weighing in at in 1910. Steers are generally killed before reaching. Breeding stock usually live to about 15 years (occasionally as much as 25 years). A common misconception about cattle (particularly bulls) is that they are enraged by the color red (something provocative is often said to be "like a red flag to a bull"). This is incorrect, as cattle are red-green color-blind. The myth arose from the use of red capes in the sport of bullfighting; in fact, two different capes are used. The capote is a large, flowing cape that is magenta and yellow. The more famous muleta is the smaller, red cape, used exclusively for the final, fatal segment of the fight. It is not the color of the cape that angers the bull, but rather the movement of the fabric that irritates the bull and incites it to charge. Although cattle cannot distinguish red from green, they do have two kinds of color receptors in their retinas (cone cells) and so are theoretically able to distinguish some colors, probably in a similar way to other red-green color-blind or dichromatic mammals (such as dogs, cats, horses and up to ten percent of male humans). Domestication and husbandry. Cattle occupy a unique role in human history, domesticated since at least the early Neolithic. They are raised for meat (beef cattle), dairy products and hides. They are also used as draft animals and in certain sports. Some consider cattle the oldest form of wealth, and cattle raiding consequently one of the earliest forms of theft. Cattle are often raised by allowing herds to graze on the grasses of large tracts of rangeland. Raising cattle in this manner allows the use of land that might be unsuitable for growing crops. The most common interactions with cattle involve daily feeding, cleaning and milking. Many routine husbandry practices involve ear tagging, dehorning, loading, medical operations, vaccinations and hoof care, as well as training for agricultural shows and preparations. There are also some cultural differences in working with cattle- the cattle husbandry of Fulani men rests on behavioural techniques, whereas in Europe cattle are controlled primarily by physical means like fences. Breeders utilise cattle husbandry to reduce M. bovis infection susceptibility by selective breeding and maintaining herd health to avoid concurrent disease. Cattle are farmed for beef, veal, dairy, leather and they are less commonly used simply to maintain grassland for wildlife- for example, in Epping Forest, England. They are often used in some of the most wild places for livestock. Depending on the breed, cattle can survive on hill grazing, heaths, marshes, moors and semi desert. Modern cows are more commercial than older breeds and, having become more specialized, are less versatile. For this reason many smaller farmers still favor old breeds, like the dairy breed of cattle Jersey. In Portugal, Spain, Southern France and some Latin American countries, bulls are used in the activity of bullfighting; a similar activity, Jallikattu, is seen in South India; in many other countries this is illegal. Other activities such as bull riding are seen as part of a rodeo, especially in North America. Bull-leaping, a central ritual in Bronze Age Minoan culture (see Bull (mythology)), still exists in southwestern France. In modern times, cattle are also entered into agricultural competitions. These competitions can involve live cattle or cattle carcasses. In terms of food intake by humans, consumption of cattle is less efficient than of grain or vegetables with regard to land use, and hence cattle grazing consumes more area than such other agricultural production. Nonetheless, cattle and other forms of domesticated animals can sometimes help to utilize plant resources in areas not easily amenable to other forms of agriculture. These factors were not as important in earlier times prior to the Earth's large human population. Environmental impact. A 400-page United Nations report from the Food and Agriculture Organization (FAO) states that cattle farming is "responsible for 18% of greenhouse gases." The production of cattle to feed and clothe humans stresses ecosystems around the world, and is assessed to be one of the top three environmental problems in the world on a local to global scale. The report, entitled "Livestock's Long Shadow", also surveys the environmental damage from sheep, chickens, pigs and goats. But in almost every case, the world's 1.5 billion cattle are cited as the greatest adverse impact with respect to climate change as well as species extinction. The report concludes that, unless changes are made, the massive damage reckoned to be due to livestock may more than double by 2050, as demand for meat increases. One of the cited changes suggests that intensification of the livestock industry may be suggested, since intensification leads to less land for a given level of production. Some microbes respire in the cattle gut by an anaerobic process known as methanogenesis (producing the gas methane). Cattle emit a large volume of methane, 95% of it through eructation or burping, not flatulence. As the carbon in the methane comes from the digestion of vegetation produced by photosynthesis, its release into the air by this process would normally be considered harmless, because there is no net increase in carbon in the atmosphere — it's removed as carbon dioxide from the air by photosynthesis and returned to it as methane. Methane is a more potent greenhouse gas than carbon dioxide, having a warming effect 23 to 50 times greater, and according to Takahashi and Young "even a small increase in methane concentration in the atmosphere exerts a potentially significant contribution to global warming". Further analysis to the methane gas produced by livestock as a contributor to the increase in greenhouse gases is provided by Weart. Research is underway on methods of reducing this source of methane, by the use of dietary supplements, or treatments to reduce the proportion of methanogenetic microbes, perhaps by vaccination. Cattle are fed a concentrated high-corn diet which produces rapid weight gain, but this has side effects which include increased acidity in the digestive system. When improperly handled, manure and other byproducts of concentrated agriculture also have environmental consequences. Grazing by cattle at low intensities can create a favourable environment for native herbs and forbs; however, in most world regions cattle are reducing biodiversity due to overgrazing driven by food demands by an expanding human population. Oxen== Oxen'" (singular "'ox'") are large and heavyset breeds of "Bos taurus" cattle trained as draft animals. Often they are adult, castrated males. Usually an ox is over four years old due to the need for training and to allow it to grow to full size. Oxen are used for plowing, transport, hauling cargo, grain-grinding by trampling or by powering machines, irrigation by powering pumps, and wagon drawing. Oxen were commonly used to skid logs in forests, and sometimes still are, in low-impact select-cut logging. Oxen are most often used in teams of two, paired, for light work such as carting. In the past, teams might have been larger, with some teams exceeding twenty animals when used for logging. An ox is nothing more than a mature bovine with an "education." The education consists of the animal's learning to respond appropriately to the teamster's (ox driver's) signals. These signals are given by verbal commands or by noise (whip cracks) and many teamsters were known for their voices and language. In North America, the commands are (1) "get up", (2) "whoa", (3) "back up", (4) "gee" (turn right) and (5) "haw" (turn left). Oxen must be painstakingly trained from a young age. Their teamster must provide as many as a dozen yokes of different sizes as the animals grow. A wooden yoke is fastened about the neck of each pair so that the force of draft is distributed across their shoulders. From calves, oxen are chosen with horns since the horns hold the yoke in place when the oxen lower their heads, back up, or slow down (particularly with a wheeled vehicle going downhill). Yoked oxen cannot slow a load like harnessed horses can; the load has to be controlled downhill by other means. The gait of the ox is often important to ox trainers, since the speed the animal walks should roughly match the gait of the ox driver who must work with it. U.S. ox trainers favored larger breeds for their ability to do more work and for their intelligence. Because they are larger animals, the typical ox is the male of a breed, rather than the smaller female. Females are potentially more useful producing calves and milk. Oxen can pull harder and longer than horses, particularly on obstinate or almost un-movable loads. This is one of the reasons that teams drag logs from forests long after horses had taken over most other draft uses in Europe and North America. Though not as fast as horses, they are less prone to injury because they are more sure-footed and do not try to jerk the load. An "ox" is not a unique breed of bovine, nor have any "blue" oxen lived outside the folk tales surrounding Paul Bunyan, the mythical American logger. A possible exception and antecedent to this legend is the Belgian Blue breed which is known primarily for its unusual musculature and at times exhibits unusual white blue, blue roan, or blue coloration. The unusual musculature of the breed is believed to be due to a natural mutation of the gene that codes for the protein Myostatin, which is responsible for normal muscle atrophy. Many oxen are used worldwide, especially in developing countries. Ox is also used for various cattle products, irrespective of age, sex or training of the beast – for example, ox-blood, ox-liver, ox-kidney, ox-heart, ox-hide. Hindu tradition. Cows are venerated within the Hindu religion of India. According to Vedic scripture they are to be treated with the same respect 'as one's mother' because of the milk they provide; "The cow is my mother. The bull is my sire." They appear in numerous stories from the Puranas and Vedas, for example the deity Krishna is brought up in a family of cowherders, and given the name Govinda (protector of the cows). Also Shiva is traditionally said to ride on the back of a bull named Nandi. Bulls in particular are seen as a symbolic emblem of selfless duty and religion. In ancient rural India every household had a few cows which provided a constant supply of milk and a few bulls that helped as draft animals. Many Hindus feel that at least it was economically wise to keep cattle for their milk rather than consume their flesh for one single meal. Gandhi explains his feelings about cow protection as follows: "The cow to me means the entire sub-human world, extending man's sympathies beyond his own species. Man through the cow is enjoined to realize his identity with all that lives. Why the ancient rishis selected the cow for apotheosis is obvious to me. The cow in India was the best comparison; she was the giver of plenty. Not only did she give milk, but she also made agriculture possible. The cow is a poem of pity; one reads pity in the gentle animal. She is the second mother to millions of mankind. Protection of the cow means protection of the whole dumb creation of God. The appeal of the lower order of creation is all the more forceful because it is speechless." In heraldry. Cattle are typically represented in heraldry by the bull'". Present status. The world cattle population is estimated to be about 995,838,000 head. India is the nation with the largest number of cattle, about 281,700,000 or 28.29% of the world cattle population, followed by Brazil: 187,087,000, 18.79%; China: 139,721,000, 14.03%; the United States: 96,669,000, 9.71%; EU-27: at 87,650,000, 8.80%; Argentina: 51,062,000, 5.13%; Australia: 29,202,000, 2.93%; South Africa: 14,187,000, 1.42%; Canada: 13,945,000, 1.40% and other countries: 49,756,000 5.00%. Africa has about 20,000,000 head of cattle, many of which are raised in traditional ways and serve partly as tokens of their owner's wealth. Cattle today are the basis of a multi-billion dollar industry worldwide. The international trade in beef for 2000 was over $30 billion and represented only 23 percent of world beef production. (Clay 2004). The production of milk, which is also made into cheese, butter, yogurt, and other dairy products, is comparable in economic size to beef production and provides an important part of the food supply for many of the world's people. Cattle hides, used for leather to make shoes and clothing, are another widespread product. Cattle remain broadly used as draft animals in many developing countries, such as India.	Eyes'" are organs that detect light, and send signals 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. Image-resolving eyes are present in cnidaria, mollusks, chordates, annelids and arthropods. The simplest "eyes", in even unicellular organisms, do nothing but detect whether the surroundings are light or dark, which is sufficient for the entrainment of circadian rhythms. From more complex eyes, retinal photosensitive ganglion cells send signals along the retinohypothalamic tract to the suprachiasmatic nuclei to effect circadian adjustment. Overview. 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. The first proto-eyes evolved among animals 540 million years ago, about the time of the so-called Cambrian explosion. 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-something main phyla. In most vertebrates and some mollusks, 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, and reducing aberrations when there is enough light. The eyes of cephalopods, fish, amphibians and snakes usually have fixed lens shapes, and focusing vision is achieved by telescoping the lens — similar to how a camera focuses. Compound eyes are found among the arthropods and are composed of many simple facets which, depending on the details of anatomy, may give either a single pixelated image or multiple images, per eye. Each sensor has its own lens and photosensitive cell(s). Some eyes have up to 28,000 such sensors, which are arranged hexagonally, and which can give a full 360-degree field of vision. Compound eyes are very sensitive to motion. Some arthropods, including many Strepsiptera, have compound eyes of only a few facets, each with a retina capable of creating an image, creating vision. With each eye viewing a different thing, a fused image from all the eyes is produced in the brain, providing very different, high-resolution images. Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. 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. In contrast to compound eyes, simple eyes are those that have a single lens. 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 larvae, 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 of 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. Evolution. Visual pigments appear to have a common ancestor and were probably involved in circadian rhythms or reproductive timing in simple organisms. Complex vision, associated with dedicated visual organs, or eyes, evolved many times in different lineages. Types of eye. Nature has produced ten different eye layouts — indeed every way of capturing an image has evolved at least once in nature, with the exception 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. 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 — the physics of compound eyes prevents them from achieving a resolution better than 1°. Also, superposition eyes can achieve greater sensitivity than apposition eyes, so are better suited to dark-dwelling creatures. 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, 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. Found in about 85% of phyla, these basic forms were probably the precursors to more advanced types of "simple eye". They are small, comprising up to about 100 cells covering about 100 µm. The directionality can be improved by reducing the size of the aperture, by incorporating a reflective layer behind the receptor cells, or by filling the pit with a refractile material. Pinhole eye. The pinhole eye is an "advanced" form of pit eye incorporating these improvements, most notably a small aperture (which may be adjustable) and deep pit. It is only found in the nautiloids. 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 11°. Shrinking the aperture would produce a sharper image, but let in less light. 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 — hence increasing the resolution obtainable. 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 — this decreases the focal length and thus allows a sharp image to form on the retina. 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. 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. Heterogeneous eyes have evolved at least eight times — four or more times in gastropods, once in the copepods, once in the annelids and once in the cephalopods. No aquatic organisms possess homogeneous lenses; presumably the evolutionary pressure for a heterogeneous lens is great enough for this stage to be quickly "outgrown". This eye creates an image that is sharp enough that motion of the eye can cause significant blurring. To minimize the effect of eye motion while the animal moves, most such eyes have stabilizing eye muscles. 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 — this fast response is further accelerated by the large nerve bundles which rush the information to the brain. Focussing the image would also cause the sun's image to be focussed 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. 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). Weaknesses. One weakness of this eye construction is that chromatic aberration is still quite high — although for organisms without color vision, this is a very minor concern. A weakness of the vertebrate eye is the blind spot which results from a gap in the retina where the optic nerve exits at the back of the eye; the cephalopod eye has no blind spot as the retina is in the opposite orientation. Multiple lenses. Some marine organisms bear more than one lens; for instance the copeopod "Pontella" has three. The outer has a parabolic surface, countering the effects of spherical aberration while allowing a sharp image to be formed. "Copilla'"s eyes have two lenses, which move in and out like a telescope. Such arrangements are rare and poorly understood, but represent an interesting alternative construction. An interesting use of multiple lenses is seen in some hunters such as eagles and jumping spiders, which have a refractive cornea (discussed next): these have a negative lens, enlarging the observed image by up to 50% over the receptor cells, thus increasing their optical resolution. Refractive cornea. 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. 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. 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. 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 — penguins and seals, for example — lose their refractive cornea and return to lens-based vision. An alternative solution, borne by some divers, is to have a very strong cornea. Reflector eyes. An alternative to a lens is to line the inside of the eye with mirrors", and reflect the image to focus at a central point. The nature of these eyes means that if one were to peer into the pupil of an eye, one would see the same image that the organism would see, reflected back out. Many small organisms such as rotifers, copeopods and platyhelminths use such organs, but these are too small to produce usable images. Some larger organisms, such as scallops, also use reflector eyes. The scallop "Pecten" has up to 100 millimeter-scale reflector eyes fringing the edge of its shell. It detects moving objects as they pass successive lenses. 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. 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 — 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. Some annelids and bivalves also have apposition eyes. They are also possessed by "Limulus", the horseshoe crab, and there are suggestions that other chelicerates developed their simple eyes by reduction from a compound starting point. (Some caterpillars appear to have evolved compound eyes from simple eyes in the opposite fashion.) Apposition eyes work by gathering a number of images, one from each eye, and combining them in the brain, with each eye typically contributing a single point of information. The typical apposition eye has a lens focusing light from one direction on the rhabdom, while light from other directions is absorbed by the dark wall of the ommatidium. In the other kind of apposition eye, found in the Strepsiptera, lenses are not fused to one another, and each forms an entire image; these images are combined in the brain. This is called the schizochroal compound eye or the neural superposition eye. Because images are combined additively, this arrangement allows vision under lower light levels. Superposition eyes. The second type is named the superposition eye. The superposition eye is divided into three types; the refracting, the reflecting and the parabolic superposition eye. The refracting superposition eye has a gap between the lens and the rhabdom, and no side wall. Each lens takes light at an angle to its axis and reflects it to the same angle on the other side. The result is an image at half the radius of the eye, which is where the tips of the rhabdoms are. This kind is used mostly by nocturnal insects. In the parabolic superposition compound eye type, seen in arthropods such as mayflies, the parabolic surfaces of the inside of each facet focus light from a reflector to a sensor array. Long-bodied decapod crustaceans such as shrimp, prawns, crayfish and lobsters are alone in having reflecting superposition eyes, which also has a transparent gap but uses corner mirrors instead of lenses. Parabolic superposition. This eye type functions by refracting light, then using a parabolic mirror to focus the image; it combines features of superposition and apposition eyes. Other. Good fliers like flies or honey bees, or prey-catching insects like praying mantis or dragonflies, have specialized zones of ommatidia organized into a fovea area which gives acute vision. In the acute zone the eye are flattened and the facets larger. The flattening allows more ommatidia to receive light from a spot and therefore higher resolution. There are some exceptions from the types mentioned above. Some insects have a so-called single lens compound eye, a transitional type which is something between a superposition type of the multi-lens compound eye and the single lens eye found in animals with simple eyes. Then there is the mysid shrimp "Dioptromysis paucispinosa". The shrimp has an eye of the refracting superposition type, in the rear behind this in each eye there is a single large facet that is three times in diameter the others in the eye and behind this is an enlarged crystalline cone. This projects an upright image on a specialized retina. The resulting eye is a mixture of a simple eye within a compound eye. Another version is the pseudofaceted eye, as seen in Scutigera. This type of eye consists of a cluster of numerous ocelli on each side of the head, organized in a way that resembles a true compound eye. The body of "Ophiocoma wendtii", a type of brittle star, is covered with ommatidia, turning its whole skin into a compound eye. The same is true of many chitons. Relationship to lifestyle. Eyes are generally adapted to the environment and lifestyle 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. 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. 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. 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. 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. 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. 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. It is not only the shape of the eye that may be affected by lifestyle. Eyes can be the most visible parts of organisms, and this can act as a pressure on organisms to have more transparent eyes at the cost of function. Eyes may be mounted on stalks to provide better all-round vision, by lifting them above an organism's carapace; this also allows them to track predators or prey without moving the head. Acuity. Visual acuity 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 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 would be 50 CPD (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. 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 mm pupil to about 3 arcminutes per line pair. At a pupil diameter of 3 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. 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. 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 nm. Small changes to the genes coding for this protein can tweak the peak response by a few nm; pigments in the lens can also "filter" incoming light, changing the peak response. Many organisms are unable to discriminate between colors, seeing instead in shades of "grey"; color 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. Even if organisms are physically capable of discriminating different colors, this does not necessarily mean that they can perceive the different colors; only with behavioral tests can this be deduced. Most organisms with color 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 — this precludes the possibility of any UV light being detected, as it does not even reach the retina. Rods and cones. The retina contains two major types of light-sensitive photoreceptor cells used for vision: the rods and the cones. Rods cannot distinguish colors, but are responsible for low-light black-and-white (scotopic) vision; they work well in dim light as they contain a pigment, visual purple, which is sensitive at low light intensity, but saturates at higher (photopic) intensities. Rods are distributed throughout the retina but there are none at the fovea and none at the blind spot. Rod density is greater in the peripheral retina than in the central retina. 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. 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 – although problems of convergence do exist. Opsins are the pigments involved in photoreception. Other pigments, such as melanin, are used to shield the photoreceptor cells from light leaking in from the sides. The opsin protein group evolved long before the last common ancestor of animals, and has continued to diversify since. 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. 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.