Eye
Schematic diagram of the human eye.
Human Eye Anterior Segment - Magnified view seen on examination with a slit lamp under diffuse illumination showing conjunctiva overlying the white sclera, transparent cornea, pharmacologically dilated pupil and cataract
Latin	segmentum anterius bulbi oculi
Dorlands/Elsevier	s_07/13264628

Eyes are organs that detect light. Latin ( lingua Latīna, laˈtiːna is an Italic language, historically spoken in Latium and Ancient Rome. Elsevier, the world's largest Publisher of Medical and Scientific literature, forms part of the Reed Elsevier group Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Anisoptera redirects here For the genus of trees see Anisoptera (tree. Antarctic krill ( Euphausia superba) is a Species of Krill found in the Antarctic waters of the Southern Ocean. In Biology, an organ ( Latin: organum, "instrument tool" from Greek όργανον - organon "organ instrument Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 Different kinds of light-sensitive organs are found in a variety of animals. 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 and may allow the organism to seek out or avoid light, but hardly can be called vision. PLEASE TAKE NOTE************ In Chronobiology, entrainment of a Circadian system is the alignment of its own period and phase to the period and phase of an external rhythm

More complex eyes can distinguish shapes and colors. The visual fields of some such complex eyes largely overlap, to allow better depth perception (binocular vision), as in humans; and others are placed so as to minimize the overlap, such as in rabbits and chameleons. VisiCorp 's VisiOn was a short-lived but influential Graphical user interface -based Operating environment program for IBM PC compatible Depth perception is the visual ability to perceive the world in three Dimensions Although any animal capable of moving around its environment must be able to Binocular vision is vision in which both Eyes are used together Human beings, humans or man (Origin 1590–1600 L homō man OL hemō the earthly one (see Humus Rabbits are small Mammals in the family Leporidae of the order Lagomorpha, found in several parts of the world Chameleons (family Chamaeleonidae) are squamates that belong to one of the best-known Lizard families

The first proto-eyes evolved among animals 540 million years ago. ^[1] Almost all animals have eyes, or descend from animals that did. 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. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 The cell is the structural and functional unit of all known living Organisms It is the smallest unit of an organism that is classified as living and is often called The vertebrate retina is a light sensitive part inside the inner layer 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. Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type The visual signals are then transmitted to the brain via the optic nerve. The brain is the center of the Nervous system in animals All Vertebrates and the majority of Invertebrates have a brain The optic nerve, also called cranial nerve II, is the Nerve that transmits visual information from the Retina to the Brain. 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,^[2] and reducing aberrations when there is enough light. In Optics, transparency (also called pellucidity) is the Material property of allowing The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused The iris consists of Pigmented Fibrovascular tissue known as a stroma. The pupil is the hole that is located in the center of the iris of the eye and that controls the amount of light that enters the Eye. ^[3]

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. The cephalopods ( Greek plural (kephalópoda "head-feet" are the Mollusc class Cephalopoda characterized by Fish are aquatic Vertebrate animals that are typically ectothermic (previously Cold-blooded) covered with scales, and equipped with two Prehistoric amphibian Amphibians (class Amphibia such as Frogs Toads Salamanders Newts Gymnophiona, Sirens and A snake is an elongate Reptile of the suborder Serpentes Like all reptiles snakes are covered in scales. A camera is a device used to capture images either as still Photographs or as sequences of moving images ( Movies or Videos. ^[4]

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. Arthropods are Animals belonging to the Phylum Arthropoda (from Greek ἄρθρον arthron, " Joint " 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 multiple-image vision. The Strepsiptera (known in older literature as twisted-winged parasites) are an order of Insects with nine families making up about 600 species With each eye viewing a different angle, a fused image from all the eyes is produced in the brain, providing very wide-angle, high-resolution images.

Possessing detailed hyperspectral color vision, the Mantis shrimp has been reported to have the world's most complex color vision system. Hyperspectral imaging collects and processes information from across the Electromagnetic spectrum. Mantis shrimp or stomatopods are marine Crustaceans the members of the order Stomatopoda. ^[5] Trilobites, which are now extinct, had unique compound eyes. Trilobites ("three-lobes" are extinct Arthropods that form the class Trilobita. They used clear calcite crystals to form the lenses of their eyes. Calcite is a carbonate mineral and the most stable polymorph of Calcium carbonate ( Ca[[carbon C]] O 3 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. The largest eye ever to be reported measures 27 cm in diameter and belongs to a Colossal squid specimen. The Colossal Squid ( Mesonychoteuthis hamiltoni, from Greek mesos (middle onychos (claw and teuthis (squid sometimes called ^[6]

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. The jumping spider family ( Salticidae) contains more than 500 described genera and over 5000 Species, making it the largest family of Spiders with about The field of view (also field of vision) is the angular extent of the observable world that is seen at any given moment Peripheral vision is a part of vision that occurs outside the very center of gaze Some insect larvae, like caterpillars, have a different type of simple eye (stemmata) which gives a rough image. A larva ( Latin; plural larvae) is a juvenile form of Animal with indirect development, undergoing Metamorphosis (for example Caterpillars are the Larval form of a member of the order Lepidoptera (the Insect order comprising butterflies and Moths For information regarding Stemma in textual analysis see Stemmatics For "stoma" or "stomata" gas exchange pores on plant leaves see 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. An ocellus (plural ocelli) is a type of Photoreceptor organ in animals The word snail is a Common name that can be used for almost all members of the Molluscan class Gastropoda which have coiled shells in the They do have photosensitive cells, but no lens and no other means of projecting an image onto these cells. Photosensitivity is the amount to which an object reacts upon receiving Photons especially Visible light. They can distinguish between light and dark, but no more. This enables snails to keep out of direct sunlight. Sunlight, in the broad sense is the total spectrum of the Electromagnetic radiation given off by the Sun.

Contents 1 Evolution of eyes 2 Anatomy of the mammalian eye 2.1 Dimensions 2.2 Three layers 2.3 Anterior and posterior segments 2.4 Extraocular anatomy 3 Function of the mammalian eye 3.1 Retina 3.2 Accommodation 4 Acuity 5 Spectral response 6 Dynamic range 7 Eye movement 7.1 Extraocular muscles 7.2 Rapid eye movement 7.3 Saccades 7.4 Microsaccades 7.5 Vestibulo-ocular reflex 7.6 Smooth pursuit movement 7.7 Optokinetic reflex 7.8 Vergence movement 8 Diseases, disorders, and age-related changes 9 Eye injury and safety 9.1 Occupational eye injury 10 Cuisine 11 See also 12 References 13 External links

Evolution of eyes

Main article: Evolution of the eye

Biologists explain the origin and development of eyes, as well as of organs in general, by use of the principles of evolution. The evolution of the eye has been a subject of significant study as a distinctive example of a homologous organ present in a wide variety of taxa

The common origin (monophyly) of all animal eyes is established by shared anatomical and genetic features of all eyes; that is, all modern eyes, varied as they are, have their origins in a proto-eye evolved some 540 million years ago. A clade is a taxonomic group comprising a single Common ancestor and all the descendants of that ancestor ^[7][8][9]

The earliest "eyes", called eyespots, were light-sensitive proteins in unicellular organisms. eVolution is the third Album by eLDee, it was due to be released in 2008 The eyespot apparatus (or Stigma) is a photoreceptive Organelle found in the flagellate (motile cells of Green algae and other unicellular In multicellular organisms, simple patches of photoreceptor cells are physically similar to the receptor patches for taste and smell. Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron Eyespots and flat eye patches can only sense ambient brightness: they can distinguish light and dark, but not the direction of the lightsource. Ambient light (also available light or existing light) is a term used by photographers, Cinematographers and other practitioners of the visual ^[10] Thus, they are sufficient for synchronization of circadian rhythms and they enable a reaction such as turning toward or away from the light source, which from under water can mean the surface, for example. They are not sufficient for image-forming.

When the multicellular eyepatch depressed into a shallow "cup" shape, it achieved the ability to discriminate directional brightness by using the angle at which the light hit certain cells to identify the source. The pit deepened over time, the opening diminished in size, and the number of photoreceptor cells increased, forming an effective pinhole camera that was capable of distinguishing dim shapes (for example in the nautilus). A' pinhole camera' is a very simple Camera with no lens and a single very small Aperture. Nautilus (from Greek ναυτίλος, 'sailor' is the common name of any marine creatures of the Cephalopod family Nautilidae, the sole ^[11]

The thin overgrowth of transparent cells over the eye's aperture, originally formed to prevent damage to the photoreceptive cells, allowed the segregated contents of the eye chamber to specialize into a transparent humour that optimized color filtering, blocked harmful radiation, improved the eye's refractive index, and allowed functionality outside of water. The refractive index (or index of Refraction) of a medium is a measure for how much the speed of light (or other waves such as sound waves is reduced inside the medium The transparent protective cells eventually split into two layers, with circulatory fluid in between that allowed wider viewing angles and greater imaging resolution, and the thickness of the transparent layer gradually increased, in most species with the transparent crystallin protein. In Biology, a crystallin is a water-soluble structural Protein found in the lens of the Eye, accounting for the transparency of the structure ^[12]

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",^[13] or rather, a phylogenetic radiation from the species with that first proto-eye, among the descendents of which, there may well have been an "arms race". Prey animals and competing predators alike would be forced to rapidly match or exceed any such capabilities to survive. Hence multiple eye types and subtypes developed in parallel.

Vision in various animals shows adaptation to environmental requirements. For example, birds of prey have much greater visual acuity than humans, and some can see ultraviolet light. Birds of prey are Birds that hunt for food primarily on the wing using their keen senses especially vision Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays The different forms of eyes in, for example, vertebrates and mollusks are often cited as examples of parallel evolution, despite their distant common ancestry. Vertebrates are members of the Subphylum Vertebrata, Chordates with backbones or spinal columns The grouping sometimes includes Molluscs are animals belonging to the phylum Mollusca. There are around 250000 extant Species within the phylum with an estimated 70000 Parallel evolution is the independent evolution of similar traits starting from a similar ancestral condition due to similar environments or other evolutionary pressures

Anatomy of the mammalian eye

Dimensions

Dimensions vary only 1–2 mm among humans. The vertical diameter is 24 mm; the transverse being larger. At birth it is generally 16–17 mm, enlarging to 22. 5–23 mm by three years of age. Between then and age 13 the eye attains its mature size. It weighs 7. 5 grams and its volume is roughly 6. 5 milliliters.

Three layers

The structure of the mammalian eye can be divided into three main layers or tunics whose names reflect their basic functions: the fibrous tunic, the vascular tunic, and the nervous tunic. Mammals ( class Mammalia) are a class of Vertebrate Animals characterized by the presence of Sweat glands, including sweat glands The Sclera and Cornea form the fibrous tunic of the bulb of the eye; the sclera is opaque and constitutes the posterior five-sixths of the Tunic; the cornea The vertebrate retina is a light sensitive part inside the inner layer of the Eye. ^[14][15][16]

• The fibrous tunic, also known as the tunica fibrosa oculi, is the outer layer of the eyeball consisting of the cornea and sclera. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. The sclera, called the white or white of the eye, is the opaque (usually white though certain animals such as horses and lizards can have black sclera fibrous ^[17] The sclera gives the eye most of its white color. It consists of dense connective tissue filled with the protein collagen to both protect the inner components of the eye and maintain its shape. Connective tissue is one of the four types of tissue in traditional classifications (the others being epithelial, Muscle, and Nervous tissue) Collagen is the main Protein of Connective tissue in Animals and the most abundant protein in Mammals making up about 50% of the whole-body protein ^[18]

• The vascular tunic, also known as the tunica vasculosa oculi, is the middle vascularized layer which includes the iris, ciliary body, and choroid. The iris consists of Pigmented Fibrovascular tissue known as a stroma. The ciliary body is the circumferential tissue inside the Eye composed of the Ciliary muscle and Ciliary processes. The choroid, also known as the choroidea or choroid coat, is the vascular layer of the Eye lying between the Retina and the Sclera ^[17][19][20] The choroid contains blood vessels that supply the retinal cells with necessary oxygen and remove the waste products of respiration. The blood vessels are part of the Circulatory system and function to transport Blood throughout the body Oxygen (from the Greek roots ὀξύς (oxys (acid literally "sharp" from the taste of acids and -γενής (-genēs (producer literally begetteris the Cellular respiration is the set of the metabolic reactions and processes that take place in Organisms cells to convert biochemical energy from The choroid gives the inner eye a dark color, which prevents disruptive reflections within the eye. The iris is seen rather than the cornea when looking straight in one's eye due to the latter's transparency, the pupil (central aperture of iris) is black because there is no light reflected out of the interior eye. The pupil is the hole that is located in the center of the iris of the eye and that controls the amount of light that enters the Eye. If an ophthalmoscope is used, one can see the fundus, as well as vessels especially those crossing the optic disk—the point where the optic nerve fibers depart from the eyeball—among others ^[21]

• The nervous tunic, also known as the tunica nervosa oculi, is the inner sensory which includes the retina. The fundus of the Eye is the interior surface of the eye opposite the lens, and includes the Retina, Optic disc, Macula and The vertebrate retina is a light sensitive part inside the inner layer of the Eye. ^[17][20]
  • Contributing to vision, the retina contains the photosensitive rod and cone cells and associated neurons. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. To maximise vision and light absorption, the retina is a relatively smooth (but curved) layer. It has two points at which it is different; the fovea and optic disc. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. The optic disc or optic nerve head is the location where Ganglion cell axons exit the Eye to form the Optic nerve. The fovea is a dip in the retina directly opposite the lens, which is densely packed with cone cells. It is largely responsible for color vision in humans, and enables high acuity, such as is necessary in reading. Color vision is the capacity of an organism or machine to distinguish objects based on the Wavelengths (or frequencies) of the Light they reflect or emit The optic disc, sometimes referred to as the anatomical blind spot, is a point on the retina where the optic nerve pierces the retina to connect to the nerve cells on its inside. A blind spot, also known as a Scotoma, is an obscuration of the Visual field. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. The optic nerve, also called cranial nerve II, is the Nerve that transmits visual information from the Retina to the Brain. No photosensitive cells exist at this point, it is thus "blind".
  • In addition to the rods and cones, a small proportion (about 1-2% in humans) of the ganglion cells in the retina are themselves photosensitive through the pigment melanopsin. Melanopsin is a Photopigment found in specialized Photosensitive ganglion cells of the Retina that are involved in the regulation of Circadian rhythms They are generally most excitable by blue light, about 470–485 nm. Their information is sent to the SCN (suprachiasmatic nuclei), not to the visual center, through the retinohypothalamic tract which is formed as melanopsin-sensitive axons exit the optic nerve. The suprachiasmatic nucleus ( SCN) is a bilateral region of the brain located in the Hypothalamus, that is responsible for controlling endogenous Circadian rhythms The Retinohypothalamic tract (RHT is a photic input pathway involved in Circadian rhythms. It is primarily these light signals which regulate circadian rhythms in mammals and several other animals. ^[22] Many, but not all, totally blind individuals have their circadian rhythms adjusted daily in this way.

Anterior and posterior segments

The mammalian eye can also be divided into two main segments: the anterior segment and the posterior segment. The anterior segment is the front third of the Eye that includes the structures in front of the Vitreous humour: the Cornea, iris, Ciliary The posterior segment is the back two-thirds of the Eye that includes the Anterior hyaloid membrane and all of the optical structures behind it the Vitreous ^[23]

The human eye is not a plain sphere but is like two spheres combined, a smaller, sharper curved one and a larger lesser curved sphere. The former, the anterior segment is the front sixth ^[24] of the eye that includes the structures in front of the vitreous humour: the cornea, iris, ciliary body, and lens. The cornea is the transparent front part of the Eye that covers the iris, Pupil, and Anterior chamber. The iris consists of Pigmented Fibrovascular tissue known as a stroma. The ciliary body is the circumferential tissue inside the Eye composed of the Ciliary muscle and Ciliary processes. The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused ^{[19] [25]}

Within the anterior segment are two fluid-filled spaces:

• the anterior chamber between the posterior surface of the cornea (i. The anterior chamber is the fluid-filled space inside the Eye between the iris and the Cornea 's innermost surface the Endothelium. e. the corneal endothelium) and the iris. The corneal endothelium is a monolayer of specialized flattened mitochondria-rich cells that lines the posterior surface of the Cornea and faces the Anterior
• the posterior chamber between the iris and the front face of the vitreous. The posterior chamber is a narrow chink behind the peripheral part of the iris of the human eye and in front of the Suspensory ligament of the lens and the Ciliary ^[19]

Aqueous humor fills these spaces within the anterior segment and provides nutrients to the surrounding structures. The aqueous humor is a thick watery substance that is between the lens and the cornea

Some ophthalmologists specialize in the treatment and management of anterior segment disorders and diseases. Ophthalmology is the branch of Medicine which deals with the diseases and surgery of the visual pathways including the Eye, Brain ^[25]

The posterior segment is the back five-sixths ^[26] of the eye that includes the anterior hyaloid membrane and all of the optical structures behind it: the vitreous humor, retina, choroid, and optic nerve. The vitreous membrane (or Hyaloid membrane) is a layer of Collagen separating the Vitreous humour from the rest of the Eye. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. The choroid, also known as the choroidea or choroid coat, is the vascular layer of the Eye lying between the Retina and the Sclera The optic nerve, also called cranial nerve II, is the Nerve that transmits visual information from the Retina to the Brain. ^[27]

The radii of the anterior and posterior sections are 8 mm and 12 mm, respectively. The point of junction is called the limbus.

On the other side of the lens is the second humour, the aqueous humour, which is bounded on all sides: by the lens, ciliary body, suspensory ligaments and by the retina. The aqueous humor is a thick watery substance that is between the lens and the cornea The lens is a transparent biconvex structure in the Eye that along with the Cornea, helps to Refract Light to be focused The ciliary body is the circumferential tissue inside the Eye composed of the Ciliary muscle and Ciliary processes. It lets light through without refraction, helps maintain the shape of the eye and suspends the delicate lens. In some animals, the retina contains a reflective layer (the tapetum lucidum) which increases the amount of light each photosensitive cell perceives, allowing the animal to see better under low light conditions. The tapetum lucidum ( Latin: "bright tapestry" plural tapeta lucida) is a layer of tissue in the Eye of many Vertebrate animals that

Some ophthalmologists specialise in the treatment and management of posterior segment disorders and diseases. Ophthalmology is the branch of Medicine which deals with the diseases and surgery of the visual pathways including the Eye, Brain ^[28]

Extraocular anatomy

Lying over the sclera and the interior of the eyelids is a transparent membrane called the conjunctiva. The conjunctiva is a clear membrane that covers the Sclera (white part of the Eye) and lines the inside of the Eyelids It is made of lymphoid tissue It helps lubricate the eye by producing mucus and tears. In vertebrates mucus is a slippery secretion produced by and covering Mucous membranes It is a viscous Colloid containing Antiseptic enzymes (such as Tears are the liquid product of a process of lacrimation to clean and lubricate the Eyes The word lacrimation may also be used in a medical or literary sense It also contributes to immune surveillance and helps to prevent the entrance of microbes into the eye. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor A microorganism (also spelled micro organism or micro-organism and also called a microbe) is an Organism that is Microscopic (usually

In many animals, including humans, eyelids wipe the eye and prevent dehydration. An eyelid is a thin fold of skin that covers and protects an Eye. They spread tears on the eyes, which contains substances which help fight bacterial infection as part of the immune system. Tears are the liquid product of a process of lacrimation to clean and lubricate the Eyes The word lacrimation may also be used in a medical or literary sense An infection is the detrimental Colonization of a host Organism by a foreign Species. An immune system is a collection of mechanisms within an Organism that protects against Disease by identifying and killing Pathogens and Tumor Some aquatic animals have a second eyelid in each eye which refracts the light and helps them see clearly both above and below water. Most creatures will automatically react to a threat to its eyes (such as an object moving straight at the eye, or a bright light) by covering the eyes, and/or by turning the eyes away from the threat. Blinking the eyes is, of course, also a reflex. Blinking, known less commonly as nictating, is the rapid closing and opening of the Eyelid. A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus.

In many animals, including humans, eyelashes prevent fine particles from entering the eye. An eyelash or simply lash is one of the hairs that grow at the edge of the Eyelid. Fine particles can be bacteria, but also simple dust which can cause irritation of the eye, and lead to tears and subsequent blurred vision.

In many species, the eyes are inset in the portion of the skull known as the orbits or eyesockets. In Anatomy, the orbital bone is the cavity or socket of the Skull in which the Eye and its appendages are situated This placement of the eyes helps to protect them from injury.

In humans, the eyebrows redirect flowing substances (such as rainwater or sweat) away from the eye. The eyebrow is an area of thick delicate hairs above the Eye that follows the shape of the brow ridges.

Function of the mammalian eye

The structure of the mammalian eye owes itself completely to the task of focusing light onto the retina. Light, or visible light, is Electromagnetic radiation of a Wavelength that is visible to the Human eye (about 400–700 The vertebrate retina is a light sensitive part inside the inner layer of the Eye. This light causes chemical changes in the photosensitive cells of the retina, the products of which trigger nerve impulses which travel to the brain. A chemical substance is a Material with a definite chemical composition. Photosensitivity is the amount to which an object reacts upon receiving Photons especially Visible light. In Neurophysiology, the action potential is a self-regenerating Wave of Electrochemical activity that allows Nerve cells to carry a signal

In the human eye, light enters the pupil and is focused on the retina by the lens. Light-sensitive nerve cells called rods (for brightness), cones (for color) and non-imaging ipRGC (intrinsincally photosensitive retinal ganglion cells) react to the light. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. Photosensitive ganglion cells, or melanopsin-containing ganglion cells, are a recently discovered type of nerve cell in the Retina of the mammalian They interact with each other and send messages to the brain. The rods and cones enable vision. The ipRGCs enable entrainment to the earth's 24-hour cycle, resizing of the pupil and acute suppression of the pineal hormone melatonin. The pineal gland (also called the pineal body, epiphysis cerebri, or epiphysis) is a small endocrine gland in the vertebrate Brain Melatonin is a naturally occurring Hormone found in most animals including humans and some other living organisms including Algae.

Retina

The retina contains two forms of photosensitive cells important to vision—rods and cones—in addition to the photosensitive ganglion cells involved in circadian adjustment but probably not involved in vision. Rod cells, or rods, are Photoreceptor cells in the Retina of the Eye that can function in less intense Light than can the other type Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. Though structurally and metabolically similar, the functions of rods and cones are quite different. Rod cells are highly sensitive to light, allowing them to respond in dim light and dark conditions; however, they cannot detect color differences. These are the cells that allow humans and other animals to see by moonlight, or with very little available light (as in a dark room). Cone cells, conversely, need high light intensities to respond and have high visual acuity. Different cone cells respond to different wavelengths of light, which allows an organism to see color. In Physics wavelength is the distance between repeating units of a propagating Wave of a given Frequency. The shift from cone vision to rod vision is why the darker conditions become, the less color objects seem to have.

The differences between rods and cones are useful; apart from enabling sight in both dim and light conditions, they have further advantages. The fovea, directly behind the lens, consists of mostly densely-packed cone cells. The fovea, also known as the fovea centralis, is a part of the Eye, located in the center of the Macula region of the Retina. The fovea gives humans a highly detailed central vision, allowing reading, bird watching, or any other task which primarily requires staring at things. Its requirement for high intensity light does cause problems for astronomers, as they cannot see dim stars, or other celestial objects, using central vision because the light from these is not enough to stimulate cone cells. Historically Astronomy was more concerned with the classification and description of phenomena in the sky while Astrophysics attempted to explain these phenomena s are significant physical entities, associations or structures which current Science has confirmed to exist in Space. Because cone cells are all that exist directly in the fovea, astronomers have to look at stars through the "corner of their eyes" (averted vision) where rods also exist, and where the light is sufficient to stimulate cells, allowing an individual to observe faint objects. Averted vision is a controversial technique for viewing faint objects visually

Rods and cones are both photosensitive, but respond differently to different frequencies of light. They contain different pigmented photoreceptor proteins. Photoreceptor can refer to In anatomy/cell biology Photoreceptor cell: a photosensitive cell most commonly referring to a specialized type of neuron Proteins are large Organic compounds made of Amino acids arranged in a linear chain and joined together by Peptide bonds between the Carboxyl Rod cells contain the protein rhodopsin and cone cells contain different proteins for each color-range. Rhodopsin, also known as visual purple, is a Pigment of the Retina that is responsible for both the formation of the Photoreceptor cells and the The process through which these proteins go is quite similar — upon being subjected to electromagnetic radiation of a particular wavelength and intensity, the protein breaks down into two constituent products. Electromagnetic radiation takes the form of self-propagating Waves in a Vacuum or in Matter. Rhodopsin, of rods, breaks down into opsin and retinal; iodopsin of cones breaks down into photopsin and retinal. Opsins are a group of light-sensitive 35-55 kDa membrane-bound G protein-coupled receptors of the Retinylidene protein family found in Photoreceptor cells Retinal, technically called retinene1 or retinaldehyde, is a light-sensitive Retinene molecule found in the Photoreceptor cells of Photopsins (also known as iodopsins) are the Photoreceptor proteins found in the Cone cells of the Retina that are the basis of Color The breakdown results in the activation of Transducin and this activates cyclic GMP Phosphodiesterase, which lowers the number of open Cyclic nucleotide-gated ion channels on the cell membrane, which leads to hyperpolarization; this hyperpolarization of the cell leads to decreased release of transmitter molecules at the synapse. Transducin (also called Gt is a heterotrimeric G protein that is naturally expressed in Vertebrate Retina rods and cones (a different cGMP-specific phosphodiesterase type 5 is an Enzyme ( from the Phosphodiesterase class A cyclic nucleotide-gated (CNG ion channel is any Ion channel that opens in the presence of Cyclic nucleotides Mechanism The channels are gated by The cell membrane (also called the plasma membrane, plasmalemma, or "phospholipid bilayer" is a Selectively permeable Lipid bilayer Hyperpolarization any change in a cell's Membrane potential that makes it more polarized See Chemical synapse for an introduction to concepts and terminology used in this article Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands

Differences between the rhodopsin and the iodopsins is the reason why cones and rods enable organisms to see in dark and light conditions — each of the photoreceptor proteins requires a different light intensity to break down into the constituent products. Further, synaptic convergence means that several rod cells are connected to a single bipolar cell, which then connects to a single ganglion cell by which information is relayed to the visual cortex. A bipolar cell is a type of Neuron which has two extensions Bipolar cells are specialized sensory neurons for the transmission of special senses A ganglion cell (more correctly a retinal ganglion cell or RGC) is a type of Neuron typically located near the inner surface of the Retina The term visual cortex refers to the primary visual cortex (also known as striate cortex or This convergence is in direct contrast to the situation with cones, where each cone cell is connected to a single bipolar cell. This divergence results in the high visual acuity, or the high ability to distinguish detail, of cone cells compared to rods. If a ray of light were to reach just one rod cell, the cell's response may not be enough to hyperpolarize the connected bipolar cell. But because several "converge" onto a bipolar cell, enough transmitter molecules reach the synapses of the bipolar cell to hyperpolarize it. See Chemical synapse for an introduction to concepts and terminology used in this article Chemical synapses are specialized junctions through which Neurons signal to each other and to non-neuronal cells such as those in Muscles or Glands

Furthermore, color is distinguishable due to the different iodopsins of cone cells; there are three different kinds, in normal human vision, which is why we need three different primary colors to make a color space. Photopsins (also known as iodopsins) are the Photoreceptor proteins found in the Cone cells of the Retina that are the basis of Color Cone cells, or cones, are Photoreceptor cells in the Retina of the Eye which function best in relatively bright Light. Primary colors are sets of Colors that can be combined to make a useful range ( Gamut) of colors A Color model is an abstract mathematical model describing the way Colors can be represented as Tuples of numbers typically as three or four values or color components

A small percentage of the ganglion cells in the retina contain melanopsin and, thus, are themselves photosensitive. Melanopsin is a Photopigment found in specialized Photosensitive ganglion cells of the Retina that are involved in the regulation of Circadian rhythms The light information from these cells is not involved in vision and it reaches the brain not directly via the optic nerve but via the retinohypothalamic tract, the RHT. The Retinohypothalamic tract (RHT is a photic input pathway involved in Circadian rhythms. By way of this light information, the body clock's inherent approximate 24-hour cycling is adjusted daily to nature's light/dark cycle. Signals from these photosensitive ganglion cells have at least two other roles in addition. They exercise control over the size of the pupil, and they lead to acute suppression of melatonin secretion by the pineal gland. Melatonin is a naturally occurring Hormone found in most animals including humans and some other living organisms including Algae. The pineal gland (also called the pineal body, epiphysis cerebri, or epiphysis) is a small endocrine gland in the vertebrate Brain

Accommodation

Main article: Accommodation (eye)

The purpose of the optics of the mammalian eye is to bring a clear image of the visual world onto the retina. Accommodation is the process by which the:eye increases Optical power to maintain a clear image ( focus) on an object as it draws near the eye Because of limited depth of field of the mammalian eye, an object at one distance from the eye might project a clear image, while an object either closer to or further from the eye will not. In Optics, particularly as it relates to Film and Photography, the depth of field (DOF is the portion of a scene that appears sharp in the image To make images clear for objects at different distances from the eye, its optical power needs to be changed. This is accomplished mainly by changing the curvature of the lens. For distant objects, the lens needs to be made flatter, for near objects the lens needs to be made thicker and more rounded.

Water in the eye can alter the optical properties of the eye and blur vision. It can also wash away the tear fluid—along with it the protective lipid layer—and can alter corneal physiology, due to osmotic differences between tear fluid and freshwater. Osmosis is the Diffusion of a solvent (frequently water through a semi-permeable membrane, from a solution of low solute concentration (high water potential Osmotic effects are made apparent when swimming in freshwater pools, becase the osmotic gradient draws water from the pool into the corneal tissue (the pool water is hypotonic), causing edema, and subsequently leaving the swimmer with "cloudy" or "misty" vision for a short period thereafter. Tonicity is a measure of blood capacity or effective osmolality in cell Biology. Oedema (or Edema in American English formerly known as dropsy or hydropsy, is the increase of Interstitial fluid in any organ &mdash swelling The edema can be reversed by irrigating the eye with hypertonic saline which osmotically draws the excess water out of the eye. Tonicity is a measure of blood capacity or effective osmolality in cell Biology.

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. The Red-tailed Hawk ( Buteo jamaicensis) is a medium-sized Bird of prey, one of three species colloquially known in the United States as the " Eye examination Visual acuity (VA is acuteness or clearness of vision, especially form vision which is dependent on the sharpness of the retinal focus within the This article describes the unit of angle For other meanings see Degree. Angular resolution describes the resolving power of any image forming device such as an optical or Radio telescope, a Microscope, a Camera 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^[29] (1. 2 minute of arc per line pair, or a 0. 35 mm line pair, at 1 m). However, the eye can only resolve a contrast of 5%. Taking this into account, the eye can resolve a maximum resolution of 37 CPD, or 1. 6 minute of arc per line pair (0. 47 mm line pair, at 1 m). ^[30] A rat can resolve only about 1 to 2 CPD. ^[31] 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.

Spectral response

Human eyes respond to light with wavelength in the range of approximately 400 to 700 nm. Other animals have other ranges, with many such as birds including a significant ultraviolet (shorter than 400 nm) response. Ultraviolet ( UV) light is Electromagnetic radiation with a Wavelength shorter than that of Visible light, but longer than X-rays

Dynamic range

The retina has a static contrast ratio of around 100:1 (about 6 1/2 stops). The contrast ratio is a measure of a display system defined as the Ratio of the Luminance of the brightest color (white to that of the darkest color (black that As soon as the eye moves (saccades) it re-adjusts its exposure both chemically and by adjusting the iris. A saccade is a fast movement of an eye, head or other part of an animal's body or device Initial dark adaptation takes place in approximately four seconds of profound, uninterrupted darkness; full adaptation through adjustments in retinal chemistry (the Purkinje effect) are mostly complete in thirty minutes. The Purkinje effect (sometimes called the Purkinje shift, or dark adaptation and named after the Czech Anatomist Jan Evangelista Purkyně Hence, a dynamic contrast ratio of about 1,000,000:1 (about 20 stops) is possible. The contrast ratio is a measure of a display system defined as the Ratio of the Luminance of the brightest color (white to that of the darkest color (black that The process is nonlinear and multifaceted, so an interruption by light nearly starts the adaptation process over again. Full adaptation is dependent on good blood flow; thus dark adaptation may be hampered by poor circulation, and vasoconstrictors like alcohol or tobacco.

Eye movement

Main article: Eye movements

The visual system in the brain is too slow to process information if the images are slipping across the retina at more than a few degrees per second. ^[32] Thus, for humans to be able to see while moving, the brain must compensate for the motion of the head by turning the eyes. Another complication for vision in frontal-eyed animals is the development of a small area of the retina with a very high visual acuity. This area is called the fovea, and covers about 2 degrees of visual angle in people. To get a clear view of the world, the brain must turn the eyes so that the image of the object of regard falls on the fovea. Eye movements are thus very important for visual perception, and any failure to make them correctly can lead to serious visual disabilities.

Having two eyes is an added complication, because the brain must point both of them accurately enough that the object of regard falls on corresponding points of the two retinas; otherwise, double vision would occur. The movements of different body parts are controlled by striated muscles acting around joints. The movements of the eye are no exception, but they have special advantages not shared by skeletal muscles and joints, and so are considerably different.

Extraocular muscles

Main article: Extraocular muscles

Each eye has six muscles that control its movements: the lateral rectus, the medial rectus, the inferior rectus, the superior rectus, the inferior oblique, and the superior oblique. The extraocular muscles are the six Muscles that control the movements of the (human eye. Muscle (from Latin musculus, diminutive of mus "mouse" is contractile tissue of the body and is derived from the For the muscle of the neck see Rectus capitis lateralis muscle The lateral rectus muscle is a Muscle in the orbit. The medial rectus muscle is a Muscle in the orbit. As with most of the muscles of the orbit it is innervated by the inferior division of the Oculomotor The inferior rectus muscle is a Muscle in the orbit. Actions It depresses, adducts, Rotates laterally, and extorts The superior rectus muscle is a Muscle in the orbit. It is one of the Extraocular muscles. The Obliquus oculi inferior (inferior oblique is a thin narrow muscle placed near the anterior margin of the floor of the orbit. For the abdominal muscle see External oblique muscle The superior oblique muscle, or obliquus oculi superior, is a fusiform Muscle in the When the muscles exert different tensions, a torque is exerted on the globe that causes it to turn, in almost pure rotation, with only about one millimeter of translation. ^[33] Thus, the eye can be considered as undergoing rotations about a single point in the center of the eye. Once the human eye sustains damage to the optic nerve, the impulses will not be taken to the brain. Eye transplants can happen but the person receiving the transplant will not be able to see. As for the optic nerve, once it is damaged it cannot be fixed.

Rapid eye movement

Main article: Rapid eye movement sleep

Rapid eye movement, or REM for short, typically refers to the stage during sleep during which the most vivid dreams occur. Sleep is a Natural state of bodily rest observed throughout the animal kingdom During this stage, the eyes move rapidly. It is not in itself a unique form of eye movement.

Saccades

Main article: Saccade

Saccades are quick, simultaneous movements of both eyes in the same direction controlled by the frontal lobe of the brain. A saccade is a fast movement of an eye, head or other part of an animal's body or device

Microsaccades

Main article: Microsaccade

Even when looking intently at a single spot, the eyes drift around. Microsaccades are a kind of Fixational eye movement. Theyare small jerk-like involuntary Eye movements, similar tominiature versions of voluntary Saccades This ensures that individual photosensitive cells are continually stimulated in different degrees. Without changing input, these cells would otherwise stop generating output. Microsaccades move the eye no more than a total of 0. 2° in adult humans.

Vestibulo-ocular reflex

Main article: Vestibulo-ocular reflex

The vestibulo-ocular reflex is a reflex eye movement that stabilizes images on the retina during head movement by producing an eye movement in the direction opposite to head movement, thus preserving the image on the center of the visual field. The vestibulo-ocular reflex ( VOR) or oculovestibular reflex is a Reflex Eye movement that stabilizes images on the Retina during The vestibulo-ocular reflex ( VOR) or oculovestibular reflex is a Reflex Eye movement that stabilizes images on the Retina during The letter A is the first letter in the Latin alphabet. Its name in English is a (eɪ plural A reflex action, also known as a reflex, is an involuntary and almost instant movement in response to stimulus. The vertebrate retina is a light sensitive part inside the inner layer of the Eye. For example, when the head moves to the right, the eyes move to the left, and vice versa.

Smooth pursuit movement

Main article: Pursuit movement

The eyes can also follow a moving object around. Pursuit movement is the ability of the Eyes to smoothly follow a moving object This tracking is less accurate than the vestibulo-ocular reflex, as it requires the brain to process incoming visual information and supply feedback. Feedback is a circular causal Process whereby some proportion of a system's output is returned (fed back to the Input. Following an object moving at constant speed is relatively easy, though the eyes will often make saccadic jerks to keep up. The smooth pursuit movement can move the eye at up to 100°/s in adult humans.

It is more difficult to visually estimate speed in low light conditions or while moving, unless there is another point of reference for determining speed.

Optokinetic reflex

The optokinetic reflex is a combination of a saccade and smooth pursuit movement. When, for example, looking out of the window at a moving train, the eyes can focus on a 'moving' train for a short moment (through smooth pursuit), until the train moves out of the field of vision. At this point, the optokinetic reflex kicks in, and moves the eye back to the point where it first saw the train (through a saccade).

Vergence movement

Main article: Vergence

When a creature with binocular vision looks at an object, the eyes must rotate around a vertical axis so that the projection of the image is in the centre of the retina in both eyes. To look at an object closer by, the eyes rotate 'towards each other' (convergence), while for an object farther away they rotate 'away from each other' (divergence). In ophthalmology convergence is the simultaneous inward movement of both Eyes toward each other usually in an effort to maintain single Binocular vision when viewing In ophthalmology divergence is the simultaneous outward movement of both Eyes away from each other usually in an effort to maintain single Binocular vision when viewing Exaggerated convergence is called cross eyed viewing (focusing on the nose for example) . When looking into the distance, or when 'staring into nothingness', the eyes neither converge nor diverge.

Vergence movements are closely connected to accommodation of the eye. Under normal conditions, changing the focus of the eyes to look at an object at a different distance will automatically cause vergence and accommodation.

Diseases, disorders, and age-related changes

Main articles: List of eye diseases and disorders and List of systemic diseases with ocular manifestations

There are many diseases, disorders, and age-related changes that may affect the eyes and surrounding structures.

As the eye ages certain changes occur that can be attributed solely to the aging process. Most of these anatomic and physiologic processes follow a gradual decline. With aging, the quality of vision worsens due to reasons independent of aging eye diseases. While there are many changes of significance in the nondiseased eye, the most functionally important changes seem to be a reduction in pupil size and the loss of accommodation or focusing capability (presbyopia). Presbyopia (Greek word "presbys" (πρέσβυς meaning "old person" describes the condition where the Eye exhibits a progressively diminished ability The area of the pupil governs the amount of light that can reach the retina. The extent to which the pupil dilates also decreases with age. Because of the smaller pupil size, older eyes receive much less light at the retina. In comparison to younger people, it is as though older persons wear medium-density sunglasses in bright light and extremely dark glasses in dim light. Therefore, for any detailed visually guided tasks on which performance varies with illumination, older persons require extra lighting. Certain ocular diseases can come from sexually transmitted diseases such as herpes and genital warts. If contact between eye and area of infection occurs, the STD will be transmitted to the eye. ^[34]

With aging a prominent white ring develops in the periphery of the cornea- called arcus senilis. Aging causes laxity and downward shift of eyelid tissues and atrophy of the orbital fat. These changes contribute to the etiology of several eyelid disorders such as ectropion, entropion, dermatochalasis, and ptosis. Ectropion is a Medical condition in which the lower Eyelid turns outwards Entropion is a Medical condition in which the Eyelids fold inward Dermatochalasis is a medical condition It is defined as an excess of Skin in the upper or lower Eyelid. Ptosis is an abnormally low position (drooping of the upper Eyelid. The vitreous gel undergoes liquefaction (posterior vitreous detachment or PVD) and its opacities—visible as floaters—gradually increase in number. A posterior vitreous detachment (PVD is a condition of the eye in which the Vitreous humour separates from the Retina. Floaters are deposits of various size shape consistency Refractive

Various eye care professionals, including ophthalmologists, optometrists, and opticians, are involved in the treatment and management of ocular and vision disorders. An eye care professional is an individual who provides a service related to the Eyes or vision. Ophthalmology is the branch of Medicine which deals with the diseases and surgery of the visual pathways including the Eye, Brain Optometry is a health care profession concerned with Eyes and related structures as well as vision, Visual systems and vision information Scientific Equipment OpticianAn optician is an Eye care professional who provides corrective lenses based on a prescription for the correction of a Refractive A Snellen chart is one type of eye chart used to measure visual acuity. A Snellen chart is an Eye chart used by Eye care professionals and others to measure Visual acuity. An eye chart is a chart used to measure Visual acuity. Types of eye charts include the Snellen chart, Landolt C, and the Lea test. Eye examination Visual acuity (VA is acuteness or clearness of vision, especially form vision which is dependent on the sharpness of the retinal focus within the At the conclusion of an eye examination, an eye doctor may provide the patient with an eyeglass prescription for corrective lenses. An eye examination is a battery of tests performed by an Optometrist or Ophthalmologist assessing vision and ability to focus on and discern An eyeglass prescription is a written order by an Optometrist or Ophthalmologist to an Optician for Eyeglasses. A corrective lens is a lens worn in front of the Eye, mainly used to treat Myopia, Hyperopia, astigmatism, and adjustable focus Some disorders of the eyes for which corrective lenses are prescribed include myopia (near-sightedness) which affects one-third of the population, hyperopia (far-sightedness) which affects one quarter of the population, and presbyopia, a loss of focusing range due to aging. Myopia (from Greek: μυωπία myopia "near-sightedness" also called near- or short-sightedness, is a refractive defect Hyperopia, also known as farsightedness or longsightedness, is a Defect of vision caused by an imperfection in the Eye (often when Presbyopia (Greek word "presbys" (πρέσβυς meaning "old person" describes the condition where the Eye exhibits a progressively diminished ability

Eye injury and safety

Accidents involving common household products cause 125,000 eye injuries each year in the U. S. ^[35] More than 40,000 people a year suffer eye injuries while playing sports. ^[35] Sports-related eye injuries occur most frequently in baseball, basketball and racquet sports. ^[35]

Occupational eye injury

Each day about 2000 U. S. workers have a job-related eye injury that requires medical treatment. ^[36] About one third of the injuries are treated in hospital emergency departments and more than 100 of these injuries result in one or more days of lost work. ^[36] The majority of these injuries result from small particles or objects striking or abrading the eye. Examples include metal slivers, wood chips, dust, and cement chips that are ejected by tools, wind blown, or fall from above a worker. Some of these objects, such as nails, staples, or slivers of wood or metal penetrate the eyeball and result in a permanent loss of vision. Large objects may also strike the eye/face causing blunt force trauma to the eyeball or eye socket. Chemical burns to one or both eyes from splashes of industrial chemicals or cleaning products are common. Thermal burns to the eye occur as well. Among welders, their assistants, and nearby workers, UV radiation burns (welder’s flash) routinely damage workers’ eyes and surrounding tissue. A welder (also weldor, which term distinguishes the Tradesman from the equipment used to make welds is a Tradesman who specialises in Welding Arc eye, also known as welder's flash, bake eyes, corneal flash burns, or flash burns, is a painful ocular condition sometimes experienced by

In addition to common eye injuries, health care workers, laboratory staff, janitorial workers, animal handlers, and other workers may be at risk of acquiring infectious diseases via ocular exposure. An infectious disease is a clinically evident Disease resulting from the presence of Pathogenic microbial agents including Pathogenic viruses Pathogenic ^[36]

Cuisine

In some countries, stuffed cow's eyes are considered a delicacy. Cattle, colloquially referred to as cows, are domesticated Ungulates a member of the Subfamily Bovinae of the family They are made by first removing the vitreous humor, lens, cornea, and iris, then are usually boiled. Cow eyes are often stuffed with varieties of coleslaw, beef, and even cream cheese. Coleslaw (or cole slaw) is a Salad consisting primarily of shredded raw Cabbage. Beef is the Culinary name for Meat from Bovines especially domestic Cattle (cows Cream cheese is a sweet soft mild-tasting white Cheese, defined by the US Department of Agriculture as containing at least 33% Milkfat (as marketed

Seal eyes are eaten by the Inuit, providing a source of zinc in their diet. Inuit (plural the singular Inuk, means "man" or "person" is a general term for a group of culturally similar Indigenous peoples inhabiting Zinc (ˈzɪŋk from Zink is a Metallic Chemical element with the symbol Zn and Atomic number 30 ^[37]

A delicacy in western Norwegian cuisine is the singed head of a sheep or lamb, smalahovud, where the eyes are also eaten. Smalahove (also called Smalehovud or Skjelte) is a Norwegian traditional dish usually eaten around and before Christmas

See also

• Ophthalmology
• Eye exam
• Eye contact
• Eyelid
• Eyespot (mimicry)
• Infant vision
• James Elkins
• Annulus of Zinn
• Conjunctiva
• Macula
• Nictitating membrane
• Schlemm's canal
• Trabecular meshwork

References

• Anatomy. History of Ophthalmology. Retrieved on 23 April 2005.
• Kandel ER, Schwartz JH, Jessell TM. Eric Richard Kandel (born November 7, 1929) is a Psychiatrist, a neuroscientist and Professor of Biochemistry Principles of Neural Science, 4th ed. First published in 1981 by Elsevier, Principles of Neural Science is a Neuroscience Textbook edited by Eric R McGraw-Hill, New York (2000). ISBN 0-8385-7701-6

External links

• DJO | Digital Journal of Ophthalmology
• Glossary of Eye Conditions
• Evolution of the Eye
• Diagram of the eye
• Webvision. The organisation of the retina and visual system.
• VisionSimulations.com | Images and vision simulators of various diseases and conditions of the eye
• Eyes and computers.
• Eyeatlas online (ophthalmological images) by Umberto Benelli, MD, PhD
• ClarkVision's estimation of the resolution of the eye
• Video: Vision and How Our Eyes Work
• Summary of eye diseases and disorders
• Your Baby's Eyes.
• National Institute for Occupational Safety and Health - Eye Safety
• Eye strips images of all but bare essentials before sending visual information to brain, UC Berkeley research shows
• Anything related to eyes

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